75 KW Generator

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Little Bill

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
I have a customer needing a cord to feed his power distribution box. He/they usually rent generators. They don't always get the same size genny but said that a 75kw is the most common one they get. The voltage is 480V 3?. I had already sized the cord he needed to his load which is 160A before I was told about the generator. The cord is 4/0, 4-wire.
He said the generator had a 250A breaker on it.

My question is how the breaker was sized 250A for a 75kw generator?
I looked at a chart for sizing generators that said for a 75kw 480V unit it would be around 112A. Their formula was: kw/(voltage x 1.732)/pf.
So I did the calcs. and it seems to match.

75000/(480 x 1.732)/.80 *assuming pf
75000/831.36/.80 = 112.76A

Where did the 250A breaker come from?
Am I missing something in the calculations?
 
Little Bill said:
75000/(480 x 1.732)/.80 *assuming pf
75000/831.36/.80 = 112.76A

Where did the 250A breaker come from?
Am I missing something in the calculations?
Click to expand...

Dual voltage generators run the numbers using 208. Often there is a way to change the trip setting on the breaker when you change voltages on the rental generator.

I would plan on using 4/0 single conductor portable cord with cam locs. You can buy them premade in 50' or 100' lengths.
 
Little Bill said:
I have a customer needing a cord to feed his power distribution box. He/they usually rent generators. They don't always get the same size genny but said that a 75kw is the most common one they get. The voltage is 480V 3?. I had already sized the cord he needed to his load which is 160A before I was told about the generator. The cord is 4/0, 4-wire.
He said the generator had a 250A breaker on it.

My question is how the breaker was sized 250A for a 75kw generator?
I looked at a chart for sizing generators that said for a 75kw 480V unit it would be around 112A. Their formula was: kw/(voltage x 1.732)/pf.
So I did the calcs. and it seems to match.

75000/(480 x 1.732)/.80 *assuming pf
75000/831.36/.80 = 112.76A

Where did the 250A breaker come from?
Am I missing something in the calculations?
Click to expand...
Sometimes the alternator coupled to the prime mover is over sized to cater to the non-linear loads. Check the name plate of the alternator.
 
iwire said:
Dual voltage generators run the numbers using 208. Often there is a way to change the trip setting on the breaker when you change voltages on the rental generator.

I would plan on using 4/0 single conductor portable cord with cam locs. You can buy them premade in 50' or 100' lengths.
Click to expand...

Do you have a link to the place that sells the pre-made cords?
 
Little Bill said:
Do you have a link to the place that sells the pre-made cords?
Click to expand...

I use Trystar Industries in NM, or google "event electric distribution" or something like that. If it's a long run, spend the extra $$ on multiple sets of 50' cables, you really don't want to move 100' 4/0 ones :happyno:. (And certainly not 4c 4/0...) Otherwise, most companies will make them up in whatever length you want.
 
Little Bill said:
I have a customer needing a cord to feed his power distribution box. He/they usually rent generators. They don't always get the same size genny but said that a 75kw is the most common one they get. The voltage is 480V 3?. I had already sized the cord he needed to his load which is 160A before I was told about the generator. The cord is 4/0, 4-wire.
He said the generator had a 250A breaker on it.

My question is how the breaker was sized 250A for a 75kw generator?
I looked at a chart for sizing generators that said for a 75kw 480V unit it would be around 112A. Their formula was: kw/(voltage x 1.732)/pf.
So I did the calcs. and it seems to match.

75000/(480 x 1.732)/.80 *assuming pf
75000/831.36/.80 = 112.76A

Where did the 250A breaker come from?
Am I missing something in the calculations?
Click to expand...

Bill -
I'm seeing a few things that don't fit

Customer load is 480V, 160A, 3ph, pf unknown. That is 133KVA. Generally speaking one would not run the gen flat out, so figuring on loading to 80%, that requires 166KVA. A 75kw gen is not going to do this.

Your 75kw gen calculation is good. The gen will be rated for .8 pf. Nameplate of 112.8A is highly likely. The CB I would expect is 112.8 x 1.25 = 141A, next size 150A. No way will it have a 250A CB. Even if the gen were capable of connection for 208V, that takes a 350A CB. That would be one really odd 75kw gen to have that big of a CB.

Now, if the gen were a 125KW, that would have 250A CB.

In either case, a 75KW gen is too small if the load is in fact 160A. Maybe they are loading sheding to where a 75Kw will run essential loads.

I'd say it is time for some phone calls. Maybe:
Verify the loading. Verify loading under generation
Verify the generator rental - as in check the invoices
And I wouldn't take a "he said" for the cb size. Consider calling the rental outfit.​

This looks screwy enough - I'd check.

edit to cleaning up phrasing

ice
 
iceworm said:
Bill -
I'm seeing a few things that don't fit

Customer load is 480V, 160A, 3ph, pf unknown. That is 133KVA. Generally speaking one would not run the gen flat out, so figuring on loading to 80%, that requires 166KVA. A 75kw gen is not going to do this.

Your 75kw gen calculation is good. The gen will be rated for .8 pf. Nameplate of 112.8A is highly likely. The CB I would expect is 112.8 x 1.25 = 141A, next size 150A. No way will it have a 250A CB. Even if the gen were capable of connection for 208V, that takes a 350A CB. That would be one really odd 75kw gen to have that big of a CB.

Now, if the gen were a 125KW, that would have 250A CB.

In either case, a 75KW gen is too small if the load is in fact 160A. Maybe they are loading sheding to where a 75Kw will run essential loads.

I'd say it is time for some phone calls. Maybe:
Verify the loading. Verify loading under generation
Verify the generator rental - as in check the invoices
And I wouldn't take a "he said" for the cb size. Consider calling the rental outfit.​

This looks screwy enough - I'd check.

edit to cleaning up phrasing

ice
Click to expand...

Oh I'm just doing some checking before I order anything. I always double/triple check before taking the "leap" into ordering something.

First, I made a mistake in my OP. I had meant to say 140A but after further checking even that is not correct. I was told the total load but wasn't told that all is not used at the same time. But if it is, they usually rent a 2nd generator. So total load at the most on a single genny is 90A. So that seems to fit the 80% of the 112A rating of the genny.

My main concern was the 250A breaker on the genny and how that was determined. The customer checked and verified that is what is on the genny. But as I said, they rent and don't always get it from the same place or get the same generator. I suppose they are limited to availability sometimes.

On the plus side, looks like I don't have to get the 4/0 elephant cord!
 
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