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Single Phase Electrical Service

Single Phase Electrical Service

Single Phase Electrical Service

The local electrical company drew up site diagram for a new single phase service that is suppose to feed an existing single phase 1200A 240V MDP panel. His drawing indicates 100KW  pad mounted transformer, with 3 sets of 350 AL kcmil wiring coming in. Is the 100KW size of pad mounted transformer size big enough? My calculations give me that the tranformer would have to be around 390KW?

RE: Single Phase Electrical Service

Utilities will usually downsize their transformers, as they know that services are rearely, if ever, loaded up to their capacities for enough time to heat up the transformer to its thermal limit.

They are willing to save a substantial amount on each transformer, and then upsize the ones that actually do approach the limits.  It is hard to beat the advantage of having several dozen units in their storeyards.

BTW, I get 80%*1200A*240V = 230kVA.

RE: Single Phase Electrical Service

I have an old commercial and industrial demand estimating guide from SDG&E that uses a percent of service panel size ranging from 25% to 48%, depending on the type of customer, to estimate demands on transformers.

RE: Single Phase Electrical Service

I know that based on customer they derate the transformer. But in my calculations they gave me KW not KVA. So that is what I used. Still not sure.

RE: Single Phase Electrical Service

Transformers are rated in kVA not kW.  Heating depends on the current magnitude not the phase angle.

It's not that the utility derates the transformer.  They assume based on experience that the actual demand will be far lower than the panel rating.  They may also deliberately overload the transformer for economic reasons.

Have the loads on the existing panel been metered?  If so, this information should be provided to the utility to help them determine the proper transformer size.

RE: Single Phase Electrical Service

I work for an electric utility.  We ususally look at the loads connected to the panel, not the rating of the panel itself.  My experience is that most architects way overdesign panels.  We often size transformers much smaller than the capacity of a panel connected to it.


RE: Single Phase Electrical Service

I am a consulting engineer, we have to submit our drawings to local and state authorities whom check them against applicable codes.  We are required by said codes to size things for connected loads.  Everyone knows these panels will most likely never see loads of that size.  We, unlike public utility companies must show that the equipment and feeders are protected, even if every single light is on, motor is starting and outlet is used.  We are able to use some demand factors as specified in the NEC/CEC, but these still result in switchgear that is never going to be more than 20-40% loaded.  Utility companies know this, so they size their transformation and distribution systems accordingly.  Some will even charge the customer additional fees if the requested service is excessively oversized and they furnish a transformer which is not sufficiently loaded.  Your installation is fairly typical and not outside industry standards.

RE: Single Phase Electrical Service

I also work for an electric utility. It is common practice for us to undersize transformers based on the calculated demand. For single phase, residential loads it is common to overload a pole mounted transformer by 125%. This means that not only do we not base our transformers on the main breaker size, we do not even size our transformers to match the calculated load of the service. Most transformers are required by ANSI to withstand loading of up to 140% for short periods of time. By overloading each transformer, we can reduce the amount of no load losses on our system. Remember, we pay for the leakage current in each and every transformer on the system. That is why we must charge a facilities charge to customers that insist on having a specific size of transformer even though they don't need it.

Happiness is a way of travel, not a destination.

RE: Single Phase Electrical Service

steelerfan:  So that's why I see 195V on my 208!!!

Interesting stuff there.

So how can one get the utility to upgrade a transformer without charging the customer? (Presuming something like excessive drops)

RE: Single Phase Electrical Service


If you are getting only 195V at your 208V service, you should complain to the utility.  They should increase the transformer size, service cable size, or transformer taps to get you to at least 198V (95%) of nominal.  Utilities like to have customers with adequate voltage - it reduces complaints and increases revenue.

RE: Single Phase Electrical Service

I dont understand American standards, in Europe nobody would think of installing 1 phase 1200 Amp.

In Denmark almost every house has at least 3x400 V 20 Amp supply.

Greetings Aksel

RE: Single Phase Electrical Service

A star for akselj! The distribution system where I work (Northern Territory, Australia) has transformers down to 50kVA and all three phase. This means that most motors of any reasonable size will be three phase for better starting and reliability and allows for increased power transfer over lines and cables compared to single phase.

Do any of the contributors from USA know a particular reason for such large single phase installations? I could guess that the theory behind three phase distribution is good but that the impossibility of balancing loads right down to domestic level doesn't realise all the possible benefits. Increase installation costs may be another point- any thoughts?


RE: Single Phase Electrical Service

In the US, most utilities will run 3-phase medium voltage power 4.16kV, 7.2kV, 12.47kV, (there are a lot of different systems installed over the years), on their distribution networks, as they run through the neighborhoods, either on overhead pole lines or underground duct banks they will utilize two of the three phases plus the neutral from a transformer to feed individual dwelling units.  The most common voltage for residences is 120/240V,1-phase,3-wire 125A.  As it is rare for a residence to use 3-phase power.  Commercial services can use a variety of systems.  Older systems include 240V, 3-phase delta, some with a center tapped transformer to provide 120V, 120/240V,1-phase, 3-wire, 480y/277V, 3ph/4w, etc....  It is unusuall to see a large single phase service, however, in some older areas there is simply no 3-phase infrastructure to do anything else.  I don't know the conditions, but I imagine that the reason for a 1200A, 120/240v, single phase service would be something like that.

RE: Single Phase Electrical Service

Isn't Australia the land of SWER?

RE: Single Phase Electrical Service

Yes, we've got SWER, although I think we got it from someone else! It's a rarity that I've never come across but makes sense where you want to install power over long distances for only small loads. I'm pretty sure that it's only used in remote localities where there are single residences separated by may kilometres - populations densities less than a few people/square km

RE: Single Phase Electrical Service

jghirst:  Thanks for the answer!

akselj:  Are you saying all houses have 400V 3phase?

stevenal: Come on... What the heck is SWER?

&#&$&$ FLA!

RE: Single Phase Electrical Service

Here in the U.S. about 2/3 of electrical utilities will not put 3-phase into houses nowdays. This is partly because single phase motor technology improved quite a bit in the late 1960s and partly because of zoning board thought police. A lot of local governments try to prohibit a using a building for use as both a residence and a business because of influence from the real estate interests read real estate snots and thieves.

However, Duquesne Light Company in Pittsburgh has a regulation that if a a house needs more than 400 amps single phase in a 14,400Y24,940 volt distribution area the house must be wired with 120Y208 volts three phase, 240 volts ungrounded three phase, or 277Y480 volts 3-phase. If over 200 amps single phase in a 2,400Y4,160 volt distribution are the house must be wired 3-phase. This basically says that a US$250,000 or larger house gets 3-phase for the amount of air conditioning that it runs.

West Penn Power will also run 120/240 volts 4-wire delta 3-phase into your house if you have a geothermal heat pump. For how much electricity that a heat pump uses it might as well be 3-phase.

The original way that noise abatement was done along Interstate 71 ( a 60 Mile per Hour highway ) in Columbus, Ohio was to install 3-phase air conditioners in all of the houses. The voltage was 120/240 volts 4-wire delta 3-phase. However, this voltage has a bad rap because of the use of delta breakers that grafted 3-phase into single phase panels in a way that was dangerous. These were outlawed in 1974 which means that any new or replacement service had to have a single phase subpanels running off of a genuine 3-phase panel.

RE: Single Phase Electrical Service


Single wire earth return systems.

We use it in South Africa. (Northern Cape - Kalahari desert) Also used in Australia and New Zealand and maybe a few other countries

See also http://www.ruralpower.org/

RE: Single Phase Electrical Service

United States rules prohibit single wire earth return systems.

For a genuine 1200 amp single phase underground duct bank you need four 500 KCM copper per phase with 3 conductors in each of 4 PVC conduits. The laws of heat transfer make National Electrical Code annex B mandantory. A free air duct bank can use four 350 KCM copper per phase with 3 conductors in each of 4 conduits.

A bit of apples and oranges is that an ordinary 1200 amp circuit breaker can only be loaded to 960 amperes but a heavy industrial breaker is allowed to carry the full 1200 amps.

I would be careful about saying that most 1200 amp electrical services will only be asked to carry 600 amps of load. If you have plastics machines or electric thermal storage space heat you could very easily be pulling more 800 or 900 amps. In this case you would need a 250 KVA single phase transformer.

One consideration is that when most people build a heavy circuit copper they only have 1/2 to 3/4 of the wire strands conducting when the service is new. This is because they will not fan out the wire strands, clean them with #220 silicon carbide abrasive paper, and then apply antioxidant. There are some other issues such as cutting off the sharp tips before scrubbing with abrasive paper but you get the idea. There is a reason why there are 2 anitoxidant compounds on the market that are ONLY for use with copper wire and brass conduit threads. For detailed directions go to my web site http://home dot earthlink dot net/~mc5w .

A 250 KVA or 333 KVA single phase padmount transformer only comes in the type 1 style with 2 vertically hinged doors. If you really need a 250 KVA, go to one of the same w#@re#@uses where the municipal utilites buy their stuff, buy yourself a padmount transformer, stick it on the pad and tell your utility that they are going to like it.

If you have a heat pump instead of air conditioning or any other combination of electric heat and air conditioning then deliberate overloading of transformers is less acceptable. If you have a demand management system or off peak heating such as water heaters or thermal storage space heat deliberate overloading is even less acceptable. Depends on how many transformer fires you want.

RE: Single Phase Electrical Service

response to akselj regarding why do we run 1200A single phase transformers:  I think that the reason is the cost of building the power lines.  In the USA, (I'm in south Florida), we build a three phase distribution line (4kV to 25kV usually) out of a substation and branch off of this line with single phase lines to that run into residential neighborhoods.
 It is less costly and easier to build a single phase line than a three phase line.  All houses are fed off of a single phase and a neutral, with the transformer secondary being a center tapped to provide 120v and 240v.  The larger loads (air conditioner, hot water heater, stoves and ovens) run off 240V and the wall plugs and lights run off 120V.
The trade off of having power lines with a bunch of single phase loads is that the lines need to have the load evenly distributed on each of the phases to keep losses down.
Something else we do here is that when we have a small three phase load we will run two phases plus a neutral and get three phase by connecting two transformers in an "open delta" configuration.  All because it is cheaper to build a pole line with two phases rather than three.


RE: Single Phase Electrical Service

Ralph: Thanks for the definition.. wow. Bet that comes with some trade offs.

RE: Single Phase Electrical Service

Danish house installations are normally 3 Phase 400 V +N +Gnd. European standard voltage.

Light and outlets are 230V (Phase +N) divided into 1 phase 13 A groups.
Stoves and many washing mashines (mainly older types) are 3 phase (1 phase for motor, 2 for heating) often 16 A groups

The installation is normally protected by HPFI (a switch that breaks the mains by current leaks over 30 mA)

The local HV supply is 10 kV with 10/0,4 KV transformers each covering some 50 houses, both 10 and 0,4KV distribution nets are normally in underground cables.

Greetings Aksel

RE: Single Phase Electrical Service

Very interesting akselj, sounds like a nice system.  Around here ALL the local residential distribution is 21kV single phase. (Calif,USA)  Often wish I had 3ph.

RE: Single Phase Electrical Service


SWER-lines have it pros and cons. This technology consists of a single overhead HV conductor and the earth is used as the current return path. Thus, it is much cheaper than the conventional three-phase and single phase (2 wire) networks in use - ideal for the electrification of rural areas. Savings are possible due to the fact that less material and labour are required to construct the network. However, the feasibility of a SWER-scheme is dependent on the earthing costs involved (due to the fact that there is a continuous flow of current in the earth electrode compared with the conventional networks where current will only flow in the earth electrode under fault conditions) and this is mainly determined by the ground potential rise (GPR) of 20V (safety limit)

Simplicity - simple design allows speed in the construction of the system. No equalising of sags is necessary as in the case of 3phase lines
Maintenance - Reduced maintenance cost, as the possibility of conductor-to-conductor fault on single-wire lines is removed
Low capital cost - Using only one conductor which results in longer span lengths, thus less structures and material
Metering - Load growth can be easily checked by inserting low-voltage instruments directly in the earth lead at the isolating transformer.
Voltage - A unique advantage of SWER is that there is a voltage rise of 1% to 2% at the receiving end (light loads only) instead of a voltage drop whereas voltage drop invariably occurs in three-phase systems.
Power factor - not less than 0.9 to 0.95, the reason given that inductive loads are run in conjunction with static-phase converters which make use of capacitors as one of the components.

Single phase - supply only single phase
Conversion - The full advantage of the long design spans cannot be utilised if three-phase conversion is ever desirable. This is because span lengths are optimised for a single conductor
Interferences on telephone lines - The system, when operating under high density load conditions, increase the degree of interference with telephone and telegraph lines. If this necessitates conversion of telephone lines, considerable costs can be involved.
Earthing - System necessitates a periodic check on earth electrode resistance in order to ensure that no hazard exists from voltage gradients across the surface of the ground.
Isolating transformer - Unit introduces additional system losses, construction and material cost.
Load balance of primary line - In common with all single-phase systems, the efficiency of the three-phase primary distribution line is reduced when large loads are to be supplied. Max. load that can be supplied is largely dependent upon the ability of the three-phase primary distributor to supply the unbalanced single-phase loading. This factor is the greatest disadvantage of SWER (and other single phase) systems, but it can be overcome if arrangements are made to supply three SWER networks from a common point on the three-phase distributor.
protection - protection against high impedance faults are difficult

From: An earthing design guide for SWER-systems in the Northern Cape region by C.H.L Sander

Hope this extra information is helpful


RE: Single Phase Electrical Service

Must be odd seeing one line trapsing across land.

Poor worms. :)

Does this cause chickens to sprout extra eyes?

Seriously, thanks, interesting stuff there.

I take it the current travels kinda direct back to the generator.  Does it travel near the surface? Would it be detectable say 100 miles from the point of use?

RE: Single Phase Electrical Service

Virtually you can use it anywhere where you have a ground return path. I think the way how it travels back to the source through the earth-mass would depend on the characteristics of the soil. (resistivity of soil, moisture content of soil, how deep the water-table is, etc.) In Australia is one SWER-system of 107 consumers with 1.37MW load, fed through a network of 2300km. The most remote consumer is 150km from the source.

The SWER-system were first used in New Zealand, but is also used in Australia (biggest networks), South Africa, India and Canada.

RE: Single Phase Electrical Service

Thanks for the very interesting information.... wow, I'm thinking like "itsmoked"... what is the voltage drop thru a cow standing near a transformer?  Seems like you could have some tingling current if you were standing in the wrong place... Is it possible to stick a pair of electrodes in the ground some distance apart and measure a voltage, maybe even light a small lamp?  


RE: Single Phase Electrical Service

The entire SWER line current must be carried by the source electrode, resulting in a voltage across this electrode that is equal to the product of the line current and the electrode resistance. The maximum ground potential rise (GPR) of the earth electrode is limited to 20V so as to prevent dangerous step-and-touch potentials. SWER lines are unlikely to be constructed in areas where the soil resistivity is higher than 300ohm metre. The maximum SWER line current is limited in accordance with the economical achievable source electrode resistance.

The steady state voltage gradients at which stock experience discomfort were found by experiments to be as follows:
Cows     - 45 volts per metre of distance
Lambs    - 25 volts per metre of distance

This evidence points to the possibility of risk with earthing system voltage drop in excess of 40V. The application of a safety factor of 2 over the voltage gradients at which stock feel discomfort indicates that SWER earthing systems should be designed with a maximum permissible voltage rise on the earth system of 20V under normal operating conditions. With such a limitation, the risk and discomfort are negligible.

From: An earthing design guide for SWER-systems in the Northern Cape region by C.H.L Sander

My chance (forced?) to read through my own books....


RE: Single Phase Electrical Service

I still say wow!
Thanks for that info(& hitting the books) Ralph.


RE: Single Phase Electrical Service

Back in the days of telegraphs that used ground returns the preferred grounding electrode was a 3 foot by 4 foot metal plate buried below the water table. If a line was more than about 14 miles using a ground return presented less loop resistance than using a return wire. However, as the number of circuits increased a certain amount of crosstalk was introduced by the common return path. At a certain point ground returns had to be abandoned because of interference from both alternating current power systems and DC streetcar power.

The Spiral4 telephone carrier set that were used during and after World War 2 could be configured to carry 4 voice circuits, 3 voice + 3 teletypwriter channels, or 12 teletypwriter channels. The usual military configuration was 3 voice + 3 teletypewriter channels. Teletypwriter channels were done with a 3 channel modem. The 12 teletypwriter channel configuration using base 6 ( rather than binary ) modems was used for the sigsaly voice encryption system.

Part of how Alexander Graham Bell invented the telephone was that he was trying to build a modem that would carry say a dozen telegraph signals using audio tones for frequency division multiplexing. Turns out that it worked better for carrying voice.

RE: Single Phase Electrical Service

I would just like to thank all of you for this amazing range of information.  I would never have known anything about SWER or it's history except for this forum.  A star for Eng-tips in general.

RE: Single Phase Electrical Service

EEJaime;  I agree.

Man, mc5w, where do you dig this stuff up?
I get the feeling that *today* you are a 90 year old guy driving a 34 pickup, with replacement insulators rolling around on the dash.


RE: Single Phase Electrical Service

That is a pretty good image, I couldn't have put it better. I've often wondered, what did it smoke?
Just kidding, good day all.

RE: Single Phase Electrical Service

Just to extend the thread a bit more!

The link from jghrist regarding safety of Powerlines was very thorough. One suggestion that has come up recently to explain possible slight increase in health problems when living around HV powerlines is not related to field exposure. The proposal is that the high electric fields around the line cause the conglomeration and then precipitation of existing air pollutants, so if you live next to a line you have a higher exposure to them. Unfortunately I'm not sure of any testing to verify it but it shows a different path to look at rather than focussing entirely on the direct effects of E and M fields on health.

RE: Single Phase Electrical Service

Part of my knowhow was reading old technology electrical books just as a matter of understanding some of the ways that people licked difficult problems. Turns out that most of the circuits that are used with transistors were originally used with VACUUM TUBES. Quadruplex telegraph essentially worked on the pulse amplitude modulation principle - Thomas Edison realized that by using 4 voltage levels instead of 2 he could transmit 2 signals in the same direction over the same circuit. Adding in hybrids at each end allowed 1 circuit to carry 2 or 4 signals in opposite directions at the same time.

Also, I have been subscribing to Mike Holt's National Electrical Code newsletters and some of the more recent ones were about how some NEC grounding rules date back to telephone practice before the U.S. Civil War. A 25 Ohm grounding electrode gives resonably good loop efficiency on say a 100 mile long circuit that uses a ground return.

It also turned out that telegraph systems eventually standardized to 60 volt batteries and this was perhaps why Thomas Edison used 120 volts for lighting.

If you really think about it the only big difference between fiber optic and an old style telegraph line is speed. You still have issues such as how to encode information, signal amplifiers, blah, blah blah.

The automatic district telegraph loops for fire alarms were in 1 sense a low speed version of Ethernet. There had to be protocol as to which sending machines would have priority if 2 senders tried to use the same loop at the same time.

RE: Single Phase Electrical Service

To get a concept.
In the US 60 years ago (about, no AC), I think 40%
was the num for homes.
Around 1965 to 1975 a decision was made
(no more "Y"), after ac.
Many things went into the decissions.
Center tap the 220 delta to get 115.
Rotation of the phases is easy at 2500, 3 wire
Less pf, gnd, phase problems, and, 3/4 wires
for current in an ideal system.



RE: Single Phase Electrical Service

If you look in the General Electric Blue Book you will see a wiring diagram for using a 3 stator meter to meter house current that is 120 volts 2 wire single phase plus 240 volts corner grounded 3-phase. By interposing current transformers you could use a form 9s meter as both a 3s ( for 3-wire single phase ) and a 5s meter ( for the 3-phase ) in the same box.

Eventually, the 120/240 volts 4-wire delta 3-phase idea won out as well as 120Y208 volts and 277Y480 volts.

The original reason why buildings had seperate single phase lighting and 3 phase power services was that design A motors had to be on their own transformer bank so that the motors would not dim the lights when starting. Design B motors fixed that problem which was partly a lamination varnish that made it possible to die cast aluminum conductors of any shape in th rotor slots.

RE: Single Phase Electrical Service


I can only repeat what was given at the time.
Spoke to a person later, same same.
The last GE book I have is 1950 and is a quick ref.

In about 1972 I tried to get 3Phase "y" installed, at 2000A.
Houston lighting and power would only install "delta".
We had existing 2000amps 3 phase (to us) about 1000 in lghts, and aux and the other 1000 in motors (AC,etc).  

I do not know about the first part of your reply.

I wish you could have told TI, FAirchild, Sig, Ray,
and a few others about the second part, in 72 to 82.
I could have spent about 3yrs home instead of on travel.

In a mfg complex. I have dimmed lights in a 32 km (20 miles) sq area, (made it dark). Seperation ???
One transformer one out put.
In the building I am at now, we have three, adding two.
Each is 300a 3 phase. I could shut the gen down.
if the subs were too slow.

I cannot understand In the second part about seperation or about insulation.
Sleep By


RE: Single Phase Electrical Service

Up until the late 1990s Cleveland Electric Illuminating Company would only install 277Y480 volts for large commercial buildings. For some reason, factories and machine shops could only get 240 ungrounded, 480 ungrounded, and 120/240 volts single phase grounded.

When CEI mergered with Ohio Edison to form First Energy the dictate from Akron, Ohio was that all new 3-phase services had to be 4-wire solidly grounded or be convertible from corner grounded to 4-wire wye or delta. New 3-phase meter sockets have to be form 16s or 9s.

In Shaker Heights, Ohio CEI has separate 4,400 volt distribution circuits for motors and lights so that the power quality problems from apartment elevators will not mess up the lights. This is even for apartment buildings that are close to the substation that steps down from 34,500 volts to 4,400 volts.

There is also an air compressor testing shop in Akron, Ohio that knocks out all of the HID bulbs, streetlights, cash registers and computers every time they test a 50 HP or larger air compressor, even if the unit uses wye-delta starting. The wires on the primary circuit are too small and First Energy wants something like $15,000 to run a tap off of the 13,200Y23,000 industrial distribution that is 1 block away.

Up here in Cleveland we still have a lot of people who are 30 or 40 years behind on National Electrical Code. In 2004 somebody in Independence, Ohio put in a 277Y480 volt service with both the attachment insulators and the splices above the weatherheads. The requirement for the splices and point of attachment below the weatherheads was enacted in 1978 or 1981 ton prevent stranded conductor from acting as a poor excuse for a water hose. When I get the pictures developed I wil post them on my website and let you know.

Getting back to the original topic, the efficiency of modern capacitor start capacitor run motors is very much better than 30 years ago so the cost balance is in favor of single phase unless you use lots of juice or it is right out front on the street. You can also get static and rotary phase conveters that use a tapped autotransformer to match the amount of phase conversion to the load.

You can also get a combination static phase converter/ Harmonic filter from Mirus International that will run a 3-phase input vbariable frequency drive off of single phase power with harmonics equivalent to an 18 pulse rectifier.

Also, Marwell makes 2 different meter socket adapters that will allow you to build a service that is single phase upgradeable to 4-wire delta or 4-wire wye when your load grows. One of those adapters and well as 2 other can be used to build a service that is 3-wire open delta upgradable to 4-wire wye.

RE: Single Phase Electrical Service

axd38  mc5w  any one else

Your question is should you confront your sup.
 With being a small shop (60 people ) yes.
I have been terminated and promoted for the same thing.
Being eng mag, I try to make it easy to all to voice expresions.
 In the MILITARY business there is not much room for error
(it kills people).  The only reason I can get some sleep
is I know I have done my best to protect the 18 yr olds
and the rest of the country, that must use the equiptment I design.
It is all com gps, jammers, (defense).

To me it is not 100 or 350, it is 350 *1.4.
No questions no doubts make it last forever.
If someone else tells you different than it makes you
cry at night "RUN".
Only you have the answer.


RE: Single Phase Electrical Service

axd38 Sorry
But it is not about an error in concept but about a
personal decision.
All the other answers did was give advice on some of the mabeys that should have been considerted.
I should not have gone into the personal aspect
but that is the best way to relate.

again sorry

RE: Single Phase Electrical Service

I have also found out that if a house with a larger yard needs to desalinate water using reverse osmosis they could easily be using a unit the uses a 7.5 HP 480 volt 3-phase motor. A larger unit wouod need to be sized so that the utility can turn it off during peak periods.

A reverse osmosis desalinator that uses a 1/2 HP motor will put out 600 gallons per day if running CONTINUOUSLY. Oviously, a unit that is large enough to water a lawn and/or garden would need to use a much heftier motor.

RE: Single Phase Electrical Service


Is your answer related to something, or did you get
mixed up.

Back to the original questiom.
If axd38 has the wrong answer, the reason must be
pointed out. If the only person that can do this is
"his", the question still must be asked.

The tables "numbers" not an issue,
the basies for the numbers are the issue.
How I got there, How he got there, is the question.


RE: Single Phase Electrical Service


As long as he builds his 1200 amp single phase service correctly he should have no problems. However, I would be a bit concered about the matter that he should consider a higher voltage such as 240/480 single phase or consider using a 3-phase voltage such 240Y416 or 277Y480 volts.

You can run a single phase 2-wire 240 volt load such as an electric heater off of a 277 volt branch circuit using a 240 volt by 32 volt buck boost transformer. This would be an autotransformer configuration that step s down a small amout. However, when using a step down autotransformer the primary side and secondary side equipment grounding conductors need to be based on the primary overcurrent protection multiplied by the primary to secondary voltage ratio. Any current carrying conductor on the primary side also needs to be the same size as the equipment ground or larger.

RE: Single Phase Electrical Service

Traditionally, single phase meters are cheaper than 3-phase meters. Besides more parts, the stators in a mechanical 3-phase meter have to be adjusted to produce equal torque.

However, electronic meters have changed that criteria. In the case of the General Electric KV and later meters the meters use the same boards regardless of the number of phases. If you look in publication GEI-52590A.pdf you will find that a form 16s 7-prong meter can be programmed to act as a form 2s single phase meter - if you remove or insulate the 3 unused prongs you can stick that 16s meter into a 2s socket!

RE: Single Phase Electrical Service

Thanks to everyone for contributing to this thread - and a star for mc5w!  I've learned some things I didn't even know I didn't know!


RE: Single Phase Electrical Service

At my utility, we will not supply single phase past 800A, and only then in special cases.
1)For overhead transformation, the secondary cables will be very large and heavy, and place awkward forces on the pole.

1)This amount of unbalanced load can begin to mess up our ability to conveniently phase balance the currents on the distribution system (and therefore balance the voltages).

2) Utilities don't stock that many 100kVA (or 167kVA) transformers compared to the three 50kVA units that a 3-phase service would require (if overhead),

3)1200A services are definitely going to require external current transformers at the metering point, whereas 400A services (320A from the utilities' POV) are sometimes handled via self-contained meters in standard meter bases.

4)Have you spec'ed the required secondary cables yet? 1200A needs multiple runs of some pretty bulky cables (I am not going to get specific as local codes vary), particularly if you have to go any great distance (120/240V systems reach a 3% voltage drop quite quickly.).  

Using a 400A / three phase system is usually CHEAPER to install than a 1200A single phase.

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