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step up, step down transformer requirements

step up, step down transformer requirements

step up, step down transformer requirements

I have a 100 amp, 3 phase 208 volt service panel, and I need to provide electricity to 6 locations.  The first location requires 60 amps, 3 phase, 208 volt service and is located 200 feet away.  What size AWG do I need?  The next five locations are located 1300 feet away and I need to provide a seperate 15 amp, single phase service on a single receptacle to each location. I was going to use a 208/480 3 phase step up transformer and a 480/208 step down transformer and then provide three locations with 20 amp, single phase service.  What should the specs be on the step up and step down transformer and what should the AWG be?  For the next two locations can I use a 3 phase step up step down transformer for only two locations with one phase unused or should I just use a single phase step up  step down transformer for each location?

Thanks Chris1957

RE: step up, step down transformer requirements

1. 60A service will need:
1a. #6 AWG copper conductor with temperature rating 75°C(167°F) of conductor wherever the aligned devices accept it, e.g. circuit breakers. It is normally rated at 65A.
1b. #6 AWG copper conductor with temperature rating 90°C(194°F) of conductor wherever the aligned devices accept it, e.g. circuit breakers. It is normally rated at 75A.
1c. #4 AWG copper conductor with temperature rating 60°C(140°F) of conductor wherever the aligned devices accept it, e.g. circuit breakers. It is normally rated at 70A.
2. It appears that the step-up and step-down transformers for 1300ft run are not necessary. This could be reconsidered. It is sufficient and more reliable to design higher size of conductor.
2a. Please, clarify the single-phase circuit voltage. Is it 208V single-phase or 120V single-phase, 20A receptacle?
For 120V 3 single phase 20A receptacle you may need 3 x 120V x 20A = 7200VA or 7.5kVA 208delta/480Vstar 3phase 4wire with neutral solidly grounded transformer, and 480Vdelta/208star 3phase 4wire with neutral solidly grounded transformer.

RE: step up, step down transformer requirements

Please clarify the following:

- For the 208V 60A service, what it the method of installation for the conductors (ie, in conduit, raceway, buried....?

- For the 5 locations at 1300ft,
  a) method of installation as described above?
  b) distance between the five locations...if they are relatively close to each other but distant from the source then it would be better to have 1 step up/step down transformer set with the 208V distributed from the distant step down location, perhaps from a subpanel.

Keep in mind that for any 3 phase 4 wire system feeding single phase loads that a high degree of load imbalance can be expected between phases if the loads are intermittent. I do not know what you intend to use the receptacles for. When dealing with receptacles I think you should consider intermittent and unbalanced as the norm rather than the exception. For example, a given 3 phase set might have one leg with a digital clock, one with a space heater, and the last with a television.

I would think that most industrial type transformers should be able to operate with imbalance up to the point of "single phasing" without ill effects, although that case is quite inefficient. However, with this in mind the neutral should be sized as large as the phase conductors. Follow the applicable electrical codes but keep in mind that these are minimum values.  

RE: step up, step down transformer requirements

1a-c.  Thanks for the info. Can you tell me the formula for deciding cable size?

2.  I was wondering if it would work without the step up step down converters and just increasing the conducter size.

2a. My mistake.  120V, single-phase, 20A.

a. Both questions:  conducter installed in 3 inch conduit.

All five locations will have the same device installed, a satellite communications dish.  4 of the location are on 10 ft X 10 ft pads separated by 20 ft between each pad.  For the fifth location:  at the 700 ft mark of the 1300 ft run I have to take 90 degree right turn and go about 300 feet to its location.

When discussing unbalanced load does that have to do with the different loads applied by each device on the different phases or do I have an unbalanced load if I have only two of the same devices using a phase each and nothing on the third phase?  Somebody had told me that the load had to be balanced on the step up and step down tranformer but he told me that when I was thinking about using two three-phase step up step down transformers and having nothing on the last phase.

Thanks for all your help.

RE: step up, step down transformer requirements

With such long runs of cable, you'll want to consider the effect of voltage drop along the cable, particularly if you have any motor loads, which might stall due to reduced terminal voltage during starting inrush current.

If you use the transformers, you may be able to set the tap ratio's to provide higher voltage at the load during normal operation which would help performance during motor starting.  But the transformer impedance will also figure into the voltage drop.

RE: step up, step down transformer requirements


First, I will say that what follows is meant in a helpful way as my advice to you and should not be construed as anything else...
You should probably get a copy of the NEC or whatever standard is applicable for your installation and location and familiarize yourself with the appropriate standards before you get too far into this. I say this because of the impression that you are not familiar with doing this type of work. This is based on, for example, the fact that you have selected a conduit size (and quite large at that) before knowing the size and number of conductors it will contain and the fact that you are not familiar with sizing conductors. My advice is to develop enough understanding to be sure that whatever methods and materials are used for your application are safe, reliable, and economical since you are ultimately responsible for the outcome regardless of whose recommendations you follow in the design. The need for this will become very apparent when you realize that you may get a different recommendation from eachperson you ask...someone else may post here to disagree with me before it is all done, it wouldn't be the first time. Anyway, you need to know for yourself what is right or take recommendations from someone who you trust who also has the credentials to back it up. Sorry for the preaching but I felt it had to be said.  

I will give you some things to get started. I will also tell you that I am not an expert on electrical construction nor the NEC so you should (always!) make sure you understand and agree with what is presented before acting on it.

- Voltage drop is the first big concern for your applications. The NEC allows 3% drop at the end of the line but your equipment may require tighter voltage regulation. There may be an easier way to address this, but the method I know is (1)select a wire size based on ampacity of the load using NEC ampacity tables for your conductor. Next, (2) calculate voltage drop for the distance using wire impedance from NEC tables. Finally, (3) select larger wire size and repeat if first result is unacceptable.

For an example, use a 60A service at 200ft. The NEC shows that a 90C conductor size AWG 6 is rated for 75A as stated by jbartos above. The NEC also shows that three AWG 6 conductors in a single steel conduit have an impedance of 0.45 ohms/1000ft. V=I*R, so 60A * .09 ohms = 5.4V or 2.6% voltage drop at 200ft for a 208V service. An acceptable answer, just make sure that it is 200ft 'as the conduit runs' and not 'as the crow flies'.

- For the distant locations, I would recommend using one large service to the center of the 1300ft location, then a subpanel to distribute the power to the four pads. This is based on the fact that 4 large conductors (3phase/4wire, 208V/120Y) will usually be cheaper than 8 small ones (single phase 120V). (Again, I am not an expert on NEC!) You may be able to feed the outlets at the pads directly from the single subpanel at the center, but I would check that. The one location at 700ft can be fed from either end or it may be possible to tap the center of the 1300ft feed. I will warn you on the center tap idea that you should definitely check the NEC before doing so as that practice may not be allowed for your application.  

The decision on whether to use transformers at that distance will be based on the wire costs. For the 208V service, perform the calculations for voltage drop, select a wire size, and calculate the cost for the wire and conduit. (will be less than 3 in.). Next, for 480V determine the required transformer size (maybe 7.5kva) using a 480/208 delta-delta for step up and 480/208Y for step down, then the required wire size and conduit size. Calculate the cost of the transformers, wire, and conduit for 480V service and compare to the 208V service. Of course, a decision to feed the service at 480V may affect your plans for the 700ft service so you will need to calculate the cost changes, if any, for that as well.   

- Finally, I do not think that the unbalanced load is an issue. I did address my thoughts on that in my previous post but will add that if you are concerned you should contact the manufacturer for the transformer you intend to use and ask for a specification for acceptable unbalance.

I hope this helps.

RE: step up, step down transformer requirements

Suggestion: Provide
1. Watts at  Recept#1        #2       #3      #4      #5
2. Run of smaller sized copper conductors for 1300ft could potentially be a better solution than the transformers.
3. Delta-delta winding on the first transformer and Delta-Star winding on the second transformer is not recommended because of possible oscillations and harmonics between those two delta windings, i.e. secondary delta of 1st transformer and primary delta of 2nd transformer.
4. The formula you need to size conductors for the long runs is a voltage drop formula for cable/conductors. The voltage drop is normally calculated via software. Sometimes, tables and charts are used. Many cable/wire distributors will tell you size of conductor/cable if you tell them length of cable/conductor run and load or amperes and power factor, or motor nameplate data.
5. It is not so complicated as it is posted. Any electrical contractor will have this estimated, sketched and material selected in a relatively short time, a few hours at most.
6. Crucial point is to decide between transformers or direct runs, consisting of suitably sized conductors, from your 100A panel.
7. Quick estimate from my tables is:
14 amp single-phase circuit will need two 1/0 copper conductors or perhaps you could use four #2 AWG if they are less expensive in bulk. This installation will have about 5% voltage drop at 1300ft distance which is tolerable if you have 208V stable at your 100A panel. If you have less than 208V at your 100A panel than some voltage adjustments will be needed, e.g. a change of transformer taps upstream or usage of autotransformer to increase voltage several percent. This autotransformer, about 35kVA, would not be needed if you select the transformer concept, as you mentioned it in the first posting.

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