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?
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Single Phase Electrical Service 5
- Thread starter axd38
- Start date
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.
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.
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- Thread starter
- #4
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.
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.
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.
K2ofKeyLargo
K2ofKeyLargo
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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.
steelerfan28655
Electrical
- May 6, 2004
- 40
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.
Happiness is a way of travel, not a destination.
itsmoked,
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.
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.
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PowerfulStuff
Electrical
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?
Thanks.
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?
Thanks.
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.
PowerfulStuff
Electrical
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
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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.
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.
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RalphChristie
Electrical
SWER
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
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
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 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.
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 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.
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.
K2ofKeyLargo
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.
K2ofKeyLargo
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