Inspector's comments re generator output feeder sizing

[TimWA]

I just met a WA inspector on a 12Kw standby generator inspection, and I quizzed him on generator feeder/breaker sizing. A popular topic right now. This is a synopsis of his comments, is this representative view?

"The generator on-board breaker is sized by the manufactirers UL listing, it is to protect the generator only. Generator output feeders are protected by the backfed breaker in the load panel. Thus the panel breaker and feeder conductors can be sized by the generator output rating (at 125% for continuous duty), not the generator onboard breaker. Eg. if generator is 12Kw @ 240V, then output is 50A (continuous), so size output feeders at #6 THHN and 60A breaker in the panel. Even though that generator has onboard breaker of 70A. This provides selective coordination also."

 

[Joethemechanic]

if generator is 12Kw @ 240V, then output is 50A (continuous)

One problem, he forgot to figure PF into the equation. For a typical 0.8 PF you need to multiply by 1.25


so we would have 12,000 / 240 x 1.25 = 62.5 amps on a single phase generator

That is what your amperage typically would be when you are at your full output of 12KW

With typical PF of 0.8 a 12KW machine is at 15KVA

 

[augie47]

I believe the Code answer would be 240.21.
OCP shall be provided at the point where the conductors receive their suppy (with exceptions, of course).

In this case you have a 70 amp breaker on 65 amp cable so an argument can be made ref: 240.4(B), but, in general, the supply OCP is the circuit protector.

 

[ceb58]

One problem, he forgot to figure PF into the equation. For a typical 0.8 PF you need to multiply by 1.25


so we would have 12,000 / 240 x 1.25 = 62.5 amps on a single phase generator

That is what your amperage typically would be when you are at your full output of 12KW

With typical PF of 0.8 a 12KW machine is at 15KVA

That would correct if it were a 3 phase unit. A single phase is rated with a 1.0 PF. Generac rates their 12Kw unit as 50 amp/LP 45.8/ nat.

 

[Joethemechanic]

That would correct if it were a 3 phase unit. A single phase is rated with a 1.0 PF. Generac rates their 12Kw unit as 50 amp/LP 45.8/ nat.

What does the number phases have to do with PF?

Single phase loads have PF just the same as 2 or 3 phase loads. The only way you can be at a pf of 1.0 is if everything is a resistive load.


Or are you saying what they advertise as a 12KW generator is really only a 12KVA generator?

BTW, the single phase Kohler that I have for my house has 0.8 PF on the nameplate

 

[Joethemechanic]

I just looked at product specs. So they use 1.0 pf so they can artificially inflate their KW rating. What a bunch of marketing garbage. So what they sell as a 12 KW is really a 9.6 KW unit.


I guess it will run 12kw worth of incandescent bulbs. Man, everything today is a con job

 

[ceb58]

What does the number phases have to do with PF?

Single phase loads have PF just the same as 2 or 3 phase loads. The only way you can be at a pf of 1.0 is if everything is a resistive load.


Or are you saying what they advertise as a 12KW generator is really only a 12KVA generator?

BTW, the single phase Kohler that I have for my house has 0.8 PF on the nameplate

Watts​

? LP/NG 10,000W (LP)9,000W (NG)12,000W (LP)11,000W (NG)Fuel Type Propane (LP Vapor),Natural GasPropane (LP Vapor),Natural GasVoltage 120/240V AC, SinglePhase, 1.0 pf120/240V AC, SinglePhase, 1.0 pf​

Amps LP/NG 41.7/37.5 50.0/45.8

This is from Generac's spec. sheet. You are correct even though it is single phase it can have a lower PF. I am just use to working with Generac units.

 

[Joethemechanic]

Watts​

? LP/NG 10,000W (LP)9,000W (NG)12,000W (LP)11,000W (NG)Fuel Type Propane (LP Vapor),Natural GasPropane (LP Vapor),Natural GasVoltage 120/240V AC, SinglePhase, 1.0 pf120/240V AC, SinglePhase, 1.0 pf​

Amps LP/NG 41.7/37.5 50.0/45.8

This is from Generac's spec. sheet. You are correct even though it is single phase it can have a lower PF. I am just use to working with Generac units.

Yeah it occurred to me what they did right after I posted so I looked. I guess it looks good to the home depot shoppers

I wonder if it will perform as good as one of these Onan JB units

I know it won't even come near a JC

 

[ceb58]

I believe the Code answer would be 240.21.
OCP shall be provided at the point where the conductors receive their suppy (with exceptions, of course).

In this case you have a 70 amp breaker on 65 amp cable so an argument can be made ref: 240.4(B), but, in general, the supply OCP is the circuit protector.

First its good to see you back.


240.21 Location in Circuit.

(G) Conductors from Generator Terminals​

​

​

​

​

Conductorsfrom generator terminals that meet the size requirement in445.13 shall be permitted to be protected against overloadby the generator overload protective device(s) required by​

445.12.

By this wording if the gen. has OCP then 445.13 would not come in. But by the wording I would take it the conductors would be sized to the OCD.

 

[ceb58]

Yeah it occurred to me what they did right after I posted so I looked. I guess it looks good to the home depot shoppers

I wonder if it will perform as good as one of these Onan JB units

I know it won't even come near a JC

We have surplused several of these units from our old communication buildings. They are good units.

 

[Joethemechanic]

We have surplused several of these units from our old communication buildings. They are good units.

About half the police districts in Philly still have the JC units. I'm thinking it was the 17th district that lost power. The JC in the basement started and transferred the load. A month later, I show up to do the PM and it's running and carrying the building load. (winter time no ac). Well there were some undercover detectives with an office next to the generator/utility room, and they told me they had been complaining about the noise for at least a month.

I couldn't believe it lasted, it was hot in that room like an oven.