Design Input

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Alwayslearningelec

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I’m trying to do a design for the temp wiring/power to these trailers.LOL

Trailer panels are single phase 120/208v

2 trailers with 60A main breaker panel -Might feed them with 40A. One of the trailers is ours and was told we always feed it with 40A and not a problem.

1 trailer with 125A main breaker.

I am going to have a 250A 277/480v existing panel that we will use for power that will feed a xfrmr. This xfrmr will feed sub panel. This subpanel will feed the trailers.



Working backwards.

  1. The subpanel feeding the trailers can be a single phase panel, correct? I will have (1) 125A and (2) 60A branch breakers( could possibly be 40A) in the panel. What size main would I have in this subpanel. 200A?
  2. I could use a 75kva xfrmr as that would output 208 amps on the secondary 120/208v side?
  3. This xfrmr would require a 125A breaker in the existing panel to feed the 277/480v side?


Hope this makes some sort of sense. Thanks for the help.
 
It would make more sense to use a 3 phase subpanel... the 125 would be correct for the 75kva 3 phase transformer primary and 200 amp M/B would be acceptable.
 
Last edited:
horsegoer said:
Working backwards.

  1. The subpanel feeding the trailers can be a single phase panel, correct? I will have (1) 125A and (2) 60A branch breakers( could possibly be 40A) in the panel. What size main would I have in this subpanel. 200A?
  2. I could use a 75kva xfrmr as that would output 208 amps on the secondary 120/208v side?
  3. This xfrmr would require a 125A breaker in the existing panel to feed the 277/480v side?
Click to expand...
1) Yes 1Ø feeders are fine. Typically we use 125 amp primary and 225 amp secondary on a 75 KVA.
2) Yes.
3) 125 amps is fine.
 
according t my math.. 45 kva would be adequate.. perhaps I made a mistake.
 
I was thinking the same thing for temp set up.

Largest phase connected 125+60=185

185 x .80= 148

45000/208/1.732= 124.91x 1.25=156.13



(125+40+60)*208=46800
46800/208/1.732= 129.91
46800*.80=37440




46800/480/1.732=56.29

45000/480/1.732= 54.12

54.12*1.25= 67.66, 70 amp OCD



Just spit balling.

All temp trailer I have done at 208 single phase usually are 100 amp with electric heat.

Most 125 amp I have hooked up we're on 240 single phase.

He did say 208.
 
Well now I’m confused.
Tulsa Electrician said:
I was thinking the same thing for temp set up.

Largest phase connected 125+60=185

185 x .80= 148

45000/208/1.732= 124.91x 1.25=156.13



(125+40+60)*208=46800
46800/208/1.732= 129.91
46800*.80=37440




46800/480/1.732=56.29

45000/480/1.732= 54.12

54.12*1.25= 67.66, 70 amp OCD



Just spit balling.

All temp trailer I have done at 208 single phase usually are 100 amp with electric heat.

Most 125 amp I have hooked up we're on 240 single phase.

He did say 208.
Click to expand...
Ok, I need to be walked through this calculation a little bit.

I'd love to understand it for future reference/use.
Since there are (2) 60A loads don't you have to add 125+60+60?
Why do you take 80%?
45000 is the 45kva xfrmr?
208 is voltage obviously.
1.732 is power factor? Do you only use this for 3 phase calcs?
What does the final result of 156.13 represent? Total load?
What is this calc? (125+40+60)*208=46800! It's different than the 125+60 = 185*.80.

Thanks.
 
The trailers are 'single phase' but the supply transformer is 3 phase. If you use a single phase panel coming off the transformer then you don't use all 3 legs and are wasting 1/3 of your capacity. If you use a single phase panel for transformer secondary protection and distribution to the several trailers, then you need 125 + 60 + 40 = 225A panel, and a transformer that can supply 225A, or a bit more than 81kVA.

If you use all three phases, then 1 trailer is connected to phases A and B, one to B and C, and one to C and A. So any give leg of the transformer only 'sees' 2 of the 3 trailers. That is where the 125+60 rather than 125+60+60 comes from.

The 125 and the 60 are not in phase, so the total current on the 'A' terminal supplying 125A to 'B' and 60A to 'C' is about 164A, not 185A. If you select based on 164A/phase then you need a 164*208*1.732 = 59kVA transformer.

Approximating from a different direction, assuming _balanced_ loading: (125 + 60 + 40) * 208 = 47kVA Of course you know from the start that the loading will not be balanced.

You also know from the start that it is very likely that none of the trailers will be loaded to their full main breaker rating.

So best you can do is say '75kVA more than sufficient if I connect the trailers to use all 3 phases of the transformer, and is probably enough if I connect the trailers to use only 2 legs of the transformer. 45kVA is probably enough if I connect the trailers to use all 3 phases of the transformer, but I don't have sufficient information to be sure of this'

-Jon
 
So in this situation if my subpanel is single
infinity said:
1) Yes 1Ø feeders are fine. Typically we use 125 amp primary and 225 amp secondary on a 75 KVA.
2) Yes.
3) 125 amps is fine.
Click to expand...
So your saying the subpanel feeding the trailers single phase panels can be single phase even though fed from 3P xfrmr?
 
augie47 said:
It would make more sense to use a 3 phase subpanel... the 125 would be correct for the 75kva 3 phase transformer primary and 200 amp M/B would be acceptable.
Click to expand...
Can't I use a single phase sub panel? It is feeding single phase trailer panels but it is fed from a 3P xfrmr.
 
Also with a 2P 125A breaker, 2P 40A breaker and 2P 60a breaker what size main breaker should I have in the sub panel? 225A?
 
horsegoer said:
Also with a 2P 125A breaker, 2P 40A breaker and 2P 60a breaker what size main breaker should I have in the sub panel? 225A?
Click to expand...
The main is typically based on the size of the transformer. For a 75 kva 225 amps would be a standard size panel main.
 
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