Generator Question.

[Mvillines]

steve -

I agree, I was just thinking out loud.
The tap tules are for Feeder Taps, and for it to be a feeder, it has to have upstream OCP.

If the generator is not equipped with an OCPD then the conductors from the generator to the first OCPD would be feeders. This does bring up the idea of feeder taps. Then there?s that dreaded 445.13.

Definitions ? Feeder: All conductors between the service equipment, the source of separately derived system, or other power supply source and the final branch-circuit overcurrent device.

 

[templdl]

This is all fine and good. But the fact remains is the generator capabl of putting out enough overcurrent for a great enough length of time to even trip the breaker?
It is interesting how this question always is ignored because nobod
dy want to find out whay the answer may be.
Generators a sized with the load in mind. They have a limited capacity to provide much more than that. Why would one thing that they would then have the capability of generating enough overcurrent to trip a breaker?? Now, let's see now......we even are sizing the breaker at 115% of the generators capacity?
Here's a good article:
http://ecmweb.com/mag/electric_protecting_generators_overload/index
Here a a few excerpts:
"Many installers don't think about sizing generator protection nearly as much as they think about the protective arrangements for the conductors the generator supplies. In terms of electrical design, this is a classic example of ignoring the forest for the trees. Although we certainly don't want to damage the conductors from overloading, the Code also requires overload protection for generators. In addition, a large generator represents a major capital expenditure."

"But is that enough? Does that circuit breaker (or set of fuses) provide effective generator protection? We need to know the thermal overload capability of the generator. For this purpose, the best available tool is a generator damage curve. This is, in effect, the manufacturer's engineering judgment as to the level of overload current versus time that the generator can sustain without significant damage."

 

[dbuckley]

For a genset the protection against serious overload comes from the underspeed protect. Assuming a reasonably matched prime mover / generator combination, a significant overload will slow the set and the engine protection will stop the genset altogether.

The difficulty is small overloads, which wont trip the breaker, nor trigger the underspeed protect, yet stress the genset, sometimes beyond redemption.

 

[hillbilly]

templdl and dbuckley, thanks for the posts.

That is certainly food for thought, and something that I hadn't considered....until now.:smile:

I intend to fuse the output of this generator at <125A.
Proper fusing should remedy this problem....or no?

Any thoughts?

steve

 

[iwire]

You are supplying two different systems/supplies to the building wiring, I do not see this as a permitted installation. There are conditions that permit more than one source, I do not see the residential dwelling you are working on as complying with those conditions.

Steve
Sorry, I thought this was a dwelling.

230.2(D) is for services, a generator is not a utility provided service.

I would still find out if this is permitted by your local AHJ.

As far as I know the NEC says nothing about supplying a single building with both a service and a feeder.

There are rules about multiple feeders, and rules about multiple services but not a feeder and a service. :smile:

 

[Pierre C Belarge]

I agree that there are no specific requirements in regards to a service and feeder supply serving the same building.

 

[templdl]

Hillbilly,
The question still remains, can the generator produce enough overcurrent to cause the fuse to blow? I highly doubt is. Even fuse at simply 100% it may not.
Remember that utilities are different than generators and the NEC isn't concerned about protecting the generator and approaches circuit protection in the same way as the would if supplied by a utility.