Generator cable sizing

[hhsting]

I have separately derived generator that has no main breaker. Conductors call it A from the generator to the first overcurrent protection are 25 feet apart. I know the equipment grounding conductor is same as supply side bonding and size it per Table 250.102(C)(1).

However I have the following questions:

1. I dont see phase and neutral of Conductors A to be service conductors since they are not from utility. I dont see them as being feeders either since between generator and first overcurrent protection there is no overcurrent protection. How do you size phase and neutral Conductors A? Its confusing. Their is nothing like this 240.21 for generator.

2. Does NEC 2017 require mandatory to have generator main breaker at the generator? If yes then where is it in code.


Thank you

 

[zbang]

2. Does NEC 2017 require mandatory to have generator main breaker at the generator? If yes then where is it in code.

See section 445.12.

 

[Fred B]

check article 705 it reference article 445.12 requiring overcurrent protection for the generator

445.12 Overcurrent Protection.​

(A) Constant-Voltage Generators.​

Constant-voltage generators, except ac generator exciters, shall be protected from overload by inherent design, circuit breakers, fuses, protective relays, or other identified overcurrent protective means suitable for the conditions of use.

 

[LarryFine]

And yes, the generator conductors are indeed a feeder.

 

[hhsting]

And yes, the generator conductors are indeed a feeder.

How so they are feeders? I dont follow. Their is no overcurrent protection at the generator. First overcurrent protection is 25 feet from the generator.

 

[hhsting]

check article 705 it reference article 445.12 requiring overcurrent protection for the generator

445.12 Overcurrent Protection.​

(A) Constant-Voltage Generators.​

Constant-voltage generators, except ac generator exciters, shall be protected from overload by inherent design, circuit breakers, fuses, protective relays, or other identified overcurrent protective means suitable for the conditions of use.

Ok my doesnt have any overcurrent protection. First overcurrent protection is 25 feet from the generator. How would one size the conductors phase/neutral between generator and the first overcurrent protection?

 

[Joethemechanic]

Ok my doesnt have any overcurrent protection. First overcurrent protection is 25 feet from the generator. How would one size the conductors phase/neutral between generator and the first overcurrent protection?

What is the size of the generator? How many phases, what voltage? KW?

 

[don_resqcapt19]

How so they are feeders? I dont follow. Their is no overcurrent protection at the generator. First overcurrent protection is 25 feet from the generator.

How are they not feeders?

Feeder.
All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device. (CMP-10)

Just a special case of feeder, just like transformer secondary conductors are.

 

[Joethemechanic]

The available fault current on an engine driven generator is going to be limited by engine HP and inertia from the rotating components. For a residential unit it will be quite low.

 

[hhsting]

What is the size of the generator? How many phases, what voltage? KW?

480V, 3 phase, 900kva

 

[hhsting]

How are they not feeders?

Just a special case of feeder, just like transformer secondary conductors are.

So then I am confused. Would one use NEC 2017 Article 445.13(A) or 240.21(B) to size conductors between generator and first overcurrent protection that is 25 feet away from the generator?

 

[Joethemechanic]

480V, 3 phase, 900kva

And there is no OCPD on it? I've never seen one that big without OCPD on the generator or in the enclosure. Who manufactured this? Don't they give any kind of guidance? We're talking close to 1,100 amps

 

[Strathead]

Ok my doesnt have any overcurrent protection. First overcurrent protection is 25 feet from the generator. How would one size the conductors phase/neutral between generator and the first overcurrent protection?

So, assuming that the designer did their job properly, the generator must be protected by inherant design. So, you would size your conductors to the breaker 25 feet away. And you would size your ground wire to table 250.102.

 

[hhsting]

And there is no OCPD on it? I've never seen one that big without OCPD on the generator or in the enclosure. Who manufactured this? Don't they give any kind of guidance? We're talking close to 1,100 amps

No. Its now confusing what to apply NEC 2017 Article 445.13(A) or 240.21(B)?

 

[zbang]

Be careful that "25 feet away" doesn't mean straight-line between devices, it's the length of the conductors; wouldn't be hard to clock up 25 conductor feet when the generator and OCPD are only 10 feet apart if the conduit has to go up to the ceiling and back down.

Start with 445.13 and pay attention to the exception, then see if that conforms to 240.21(B)(2).

Also notice that 445.2 refers you to 430.14 Location of Motors (probably doesn't affect anything, but check anyway).

 

[don_resqcapt19]

So then I am confused. Would one use NEC 2017 Article 445.13(A) or 240.21(B) to size conductors between generator and first overcurrent protection that is 25 feet away from the generator?

The minimum conductor size is found in 445.13(A) and is 115% of the generator output current. The load end OCPD would be at or below the conductor ampacity per 240.21(B)(2)(2) if they are 25' long.

 

[hhsting]

The minimum conductor size is found in 445.13(A) and is 115% of the generator output current. The load end OCPD would be at or below the conductor ampacity per 240.21(B)(2)(2) if they are 25' long.

What about equipment grounding conductor size should it be 250.122(G) tap conductors or Table 250.102(C)(1) supply side bonding jumper between generator and first overcurrent protection?

 

[ron]

What about equipment grounding conductor size should it be 250.122(G) tap conductors or Table 250.102(C)(1) supply side bonding jumper between generator and first overcurrent protection?

Table 250.102(C)(1) (similar to the SSBJ for a transformer secondary to its first disconnect).