Generator power distribution. Separation of branches

[mshields]

EXHIBIT 700.2 illustrates how feeders from a standby generator need to be isolated. It also shows a single breaker at the generator.

EXHIBIT 700.3 and 700.4 show arrangements with no main breaker ahead of the three breakers feeding Articles 700, 701 and 702 loads respectively.

I believe the use of a main breaker is allowed but not required and that it is true for all three scenarios shown do you agree? Do you think the use of a MCB on the first and not on the second and third Exhibits was just random, OR is there something about the first Exhibit that drives a requirement for having the main breaker?

Thanks,

Mike

PS: The bottom line here is that I plan on having a system exactly like Exhibit 700.2 but WITHOUT A MAIN CB and I want to make sure it's kosher.

 

[petersonra]

I am not sure I like the idea of having a MCB. Even if it is well coordinated, there is still some chance that a downstream fault will trip the main before the feeder CB trips.

 

[mshields]

I agree

I agree

I agree and I'd prefer to go without the main breaker. After all, it has to be coordinated to .01 for Emergency but only .1 for the other two branches. Therefore, it represents a weak link in the Emergency system.

I suppose one could coordinate it to .01 but who wants to apply that head ache to branches where it's not required and besides it's still an OCPD that is not dedicated to the Emergency system and to your point could trip due to a non emergency branch trip.

I just want to make sure they're not trying to tell me something in Exhibit 700.2 by putting in the main on that one but not on the other two Exhibits.

 

[Pharon]

I believe the use of a main breaker is allowed but not required and that it is true for all three scenarios shown do you agree?

700.10(B)(5) Wiring from an emergency source to supply emergency
and other loads in accordance with 700.10(B)(5)a, b, c,
and d as follows:
a. Separate vertical switchgear sections or separate vertical
switchboard sections, with or without a common
bus, or individual disconnects mounted in separate
enclosures shall be used to separate emergency loads
from all other loads.
b. The common bus of separate sections of the switchgear,
separate sections of the switchboard, or the individual
enclosures shall be permitted to be supplied by single
or multiple feeders without overcurrent protection at
the source.

Exception to (5)b: Overcurrent protection shall be permitted at
the source or for the equipment, provided that the overcurrent
protection complies with the requirements of 700.28.

As long as the main breaker at the gen is selectively coordinated, it's permitted (but not required).

Do you think the use of a MCB on the first and not on the second and third Exhibits was just random, OR is there something about the first Exhibit that drives a requirement for having the main breaker?

I think it's random. Also, Code Handbook exhibits are not enforceable. They're just supplementary explanatory info to help understand the Code text.

PS: The bottom line here is that I plan on having a system exactly like Exhibit 700.2 but WITHOUT A MAIN CB and I want to make sure it's kosher.

Sounds legit to me.

 

[charlie b]

I believe the use of a main breaker is allowed but not required and that it is true for all three scenarios shown do you agree?

I do not agree. I think the absence of a generator breaker in Exhibits 700.3 and 700.4 is a requirement. The Exception to 700.(B)(5)(b) is what can allow us to use a generator breaker, but only if we demonstrate selective coordination.

I also think the presence of a generator breaker on Exhibit 700.2 is optional. However, since the overcurrent protection of the conductors between the generator and the switchboard will be protected by the breakers on the switchboard, then you need to follow the tap rules.

PS: The bottom line here is that I plan on having a system exactly like Exhibit 700.2 but WITHOUT A MAIN CB and I want to make sure it's kosher.

Again, I think that can be acceptable.

 

[mshields]

since you bring up the tap rule

since you bring up the tap rule

The tap rule pertains to tapped conductors off a feeder protected by an OCPD (correct me if I'm wrong). If you have a system such as that shown in 700.3 is it a legitimate application of the tap rule?

 

[charlie b]

There doesn't need to be an OCPD upstream of a tap. Transformer taps do not have them. But it is a bit foggy as to how the conductors between the generator and the main disconnect (or the set of disconnects) are protected against overcurrent. The conductors are not from a utility, so they don't fall under article 230. The conductors are not feeder taps, as you correctly pointed out. The generator article does not really address this issue. So I am not sure how to answer this question.

 

[Pharon]

Generator power distribution. Separation of branches

Doesn't every generator have internal thermal overload protection? I always considered the prime function of gen main breakers to be for disconnecting means.

 

[mshields]

thermal protection

thermal protection

Yes - but not short circuit protection.

 

[Pharon]

The short circuit protection at the switchgear compartments should be adequate for clearing faults, though.

 

[Pharon]

The short circuit protection at the switchgear compartments should be adequate for clearing faults, though.

Edit: nevermind -- that's not true -- brain freeze for a minute there... a fault on that feeder would not be cleared without a main breaker at the gen. So it sounds like tap rule would have to apply in this case, even though it's not technically a tap. Maybe a Code clarification is needed for this kind of thing.

 

[mshields]

Not necessarily

Not necessarily

what if you get a short in the feeder from the generator to the branch OCPD's. The OCPD's won't even see the fault.

 

[Pharon]

That's what I said in my last post. Which is why I think it makes sense to follow the tap rules, even though it doesn't seem to be a requirement in this application.