Design input - Revised

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Alwayslearningelec

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Sorry for new thread but other one got a little messy and loads/calcs changed.
I think this is a little cleaner and easier to follow.


Following loads to be fed:

(3) single phase 120/208v panels.

(1) w/ 125A MCB

(2) w/ 60A MCB


Material I will use for these power requirements.

3P-70A breaker in existing 277/480v panel. This will feed 3P 45kva xfrmr.

I will run 3/c #3 aluminum MC to the primary side of 45kvs xfmr(277/480v) from the 3P 70A breaker.

I will run 4/c#250 from the secondary side 45kva xfrmr to a 225A MCB sub panel which will have 2P-125A breaker, (2) 2P 60 breakers. Can this suble panel be SINGLE PHASE? If so 3W?

I will run 1/0 aluminum SER cable from sub panel 125A breaker to trailer 125A breaker panel.

I will run #4 aluminum SER cable from sub panel 60A breakers to trailer 60A breaker panel. This is for 2 trailers.

Big question is if 3 phase 45KVA xfrmr will provide enough power for the (3) single phase trailer panels.

Thanks.
 
I'm gonna answer in pieces, rather than writing an essay

horsegoer said:
I will run 4/c#250 from the secondary side 45kva xfrmr to a 225A MCB sub panel which will have 2P-125A breaker, (2) 2P 60 breakers. Can this suble panel be SINGLE PHASE? If so 3W?
Click to expand...

As a practical matter, _NO_.

There is no rule that says this panel needs to be 3 phase, but if you install a single phase panel here then you are only using 2 of the 3 legs of the transformer, effectively giving you a 30kVA transformer for the price of a 45.

-Jon
 
horsegoer said:
I will run 4/c#250 from the secondary side 45kva xfrmr to a 225A MCB sub panel which will have 2P-125A breaker, (2) 2P 60 breakers. Can this suble panel be SINGLE PHASE?
Click to expand...
No in practice. Implicitly your transformer will be 208Y/120V on the secondary. You will need a 3 phase panel with 3 busses and a neutral bar. You will feed one trailer from A-B phases, one from B-C phases, and one from C-A phases. [Plus what Jon said.]

horsegoer said:
Big question is if 3 phase 45KVA xfrmr will provide enough power for the (3) single phase trailer panels.
Click to expand...
If the 60A and 125A panels have only 120V L-N loads, then your mostly heavily loaded leg on the transformer could be up to 185A. While your 45 kVA transformer has a rating of 45 / (3*0.120) = 125A.

If the 60A and 125A panels have only 208V L-N, then your most heavily loaded leg on the transformer will be less, due to the 60 degree phase angle between them. The law of cosines gives sqrt(602+1252+2*60*125*cos(60 degrees)) = 163A as the maximum possible load if the panels aren't overloaded.

With a real world mixture of loads, the worst case is somewhere in between those. But really what you need is a load calc for each trailer, rather than going by the main breaker rating.

Cheers, Wayne
 
winnie said:
I'm gonna answer in pieces, rather than writing an essay



As a practical matter, _NO_.

There is no rule that says this panel needs to be 3 phase, but if you install a single phase panel here then you are only using 2 of the 3 legs of the transformer, effectively giving you a 30kVA transformer for the price of a 45.

-Jon
Click to expand...
Got it. So i'm clear on the sub panel. 3P4W it is.
 
wwhitney said:
No in practice. Implicitly your transformer will be 208Y/120V on the secondary. You will need a 3 phase panel with 3 busses and a neutral bar. You will feed one trailer from A-B phases, one from B-C phases, and one from C-A phases. [Plus what Jon said.]


If the 60A and 125A panels have only 120V L-N loads, then your mostly heavily loaded leg on the transformer could be up to 185A. While your 45 kVA transformer has a rating of 45 / (3*0.120) = 125A.

If the 60A and 125A panels have only 208V L-N, then your most heavily loaded leg on the transformer will be less, due to the 60 degree phase angle between them. The law of cosines gives sqrt(602+1252+2*60*125*cos(60 degrees)) = 163A as the maximum possible load if the panels aren't overloaded.

With a real world mixture of loads, the worst case is somewhere in between those. But really what you need is a load calc for each trailer, rather than going by the main breaker rating.

Cheers, Wayne
Click to expand...
Thank you. I was told the total actual load on the 60A panels would be more like 35-40A and the 125A panel about 90A.
 
The use of SER to feed the trailers is at best questionable. Art 550 is for mobile./manufactured homes (dwellings) but 550.4 notes office type units must meet the basic criteria which requires an INSULATED equipment ground which excludes SER
 
horsegoer said:
Thank you. I was told the total actual load on the 60A panels would be more like 35-40A and the 125A panel about 90A.
Click to expand...
Well, if that's all 120V loads, that puts you at 125A - 130A. Since some of the loading is likely 208V, the sum will be a bit less. So 45 kVA is about right to within the accuracy of your statement above and my understanding. : - )

Cheers, Wayne
 
horsegoer said:
Thank you. I was told the total actual load on the 60A panels would be more like 35-40A and the 125A panel about 90A.
Click to expand...

If the actual loads are 35, 35, 90 and these are all 120V loads, then you just hit the 125A limit of the transformer. If there are l-l loads then the current will be lower.

As you've designed the system, the transformer could be somewhat overloaded without anything tripping, and since you don't have an actual load calculation (just 'I was told'), you don't know if this will be an issue.

-Jon
 
winnie said:
What is the OCPD for the 4/c#250?
Click to expand...
We we're going right from secondary side xfrmr to the sub panel. No OCPD. Need one I do believe.
Now as far as the size in the scheme I propose above, I'm not sure. 175A disconnect. THat would feed the 225A sub panel.
 
wwhitney said:
Well, if that's all 120V loads, that puts you at 125A - 130A. Since some of the loading is likely 208V, the sum will be a bit less. So 45 kVA is about right to within the accuracy of your statement above and my understanding. : - )

Cheers, Wayne
Click to expand...
Thanks. 120v loads are loads feed from 1P breaker obviously and all/any 208v loads would be those feed from 2P breaker??
 
Someone took a picture of the trailer with the 125A main breaker.
 

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horsegoer said:
We we're going right from secondary side xfrmr to the sub panel. No OCPD. Need one I do believe.
Now as far as the size in the scheme I propose above, I'm not sure. 175A disconnect. THat would feed the 225A sub panel.
Click to expand...
Can I have the OCPD in my existing panel feeding the primary xfrmr serve as the protection for my secondary side conductors/sub panel?
 
horsegoer said:
Thanks. 120v loads are loads feed from 1P breaker obviously and all/any 208v loads would be those feed from 2P breaker??
Click to expand...
Almost. A 2P breaker serving a 2-wire circuit without a neutral would be a straight 208V load. But there may be some 120/208V 3-wire loads that use a neutral, too, and they would be a mix of 208V and 120V loading, often primarily 208V.

Cheers, Wayne
 
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