240.21(B) and generators

[don_resqcapt19]

A recent thread points out that the rules for generators that do not have factory installed OCPD are not completely clear. I am thinking of submitting the following:

Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The tap shall be permitted at any point on the load side of the feeder overcurrent protective device. Section 240.4(B) shall not be permitted for tap conductors. Where the tap conductor originates at the terminals of a generator, the full load current of the generator shall be used as the rating of the line side overcurrent protective device for the application of 240.21(B).

 

[jim dungar]

Because a generator is a source not a feeder (with some type of upstream protection) would it be more appropriate to use the rules in 240.21(C)?

 

[ron]

It would be great if you could integrate into your change the maximum distance permitted between the alternator output and the first OCPD. I use 10'

 

[jaggedben]

By definition, it's not a tap conductor if there isn't an upstream OCPD.

Also the code doesn't do this for services, so why should it for generators?

 

[don_resqcapt19]

Because a generator is a source not a feeder (with some type of upstream protection) would it be more appropriate to use the rules in 240.21(C)?

I see both the generator conductors and the transformer secondary conductors as special cases of feeders, and choose 240.21(B) because of the references to the transformer primary OCPD in 240.21(C).

 

[don_resqcapt19]

It would be great if you could integrate into your change the maximum distance permitted between the alternator output and the first OCPD. I use 10'

If this change would make it into the code, both the 10' and 25' tap rules as well as the unlimited outside tap rule would apply. As with the other taps it would be a design choice.

 

[don_resqcapt19]

By definition, it's not a tap conductor if there isn't an upstream OCPD.

Also the code doesn't do this for services, so why should it for generators?

You are correct that it is not actually a tap, so maybe Jim's suggestion to put this in 240.21(C) would be better.

There are service overcurrent protection rules in Part VII of Article 230.

Is it your opinion that the protection of generator conductors is adequately covered in the current NEC?

Note that many generators have factory installed OCPDs and this would have no effect on those. If you have a factory OCPD, it is the same as running conductors from any other OCPD.

 

[jaggedben]

...

Is it your opinion that the protection of generator conductors is adequately covered in the current NEC?

...

Not necessarily but at least some of what you seem to be concerned with is already addressed in 445.13(A) and (B). 445 seems like the place to make changes.

My point about services is that we allow the OCPD at the load end of the conductor to provide overload protection and I don't see why generators (with less fault current, most likely) should be treated differently.

One thing I don't see in 445 is anything like what's in 230.70(A)(1), (or the rest of 230.70 and 230.71). Adding some similar requirements to 445 (or referring to those I just mentioned), would make more sense to me, for generators without OCPD.
Not sure I've ever seen such a generator myself, but I haven't looked closely at many.

 

[jim dungar]

I see both the generator conductors and the transformer secondary conductors as special cases of feeders, and choose 240.21(B) because of the references to the transformer primary OCPD in 240.21(C).

Do we need 240.21(D) for other sources like generators and ???

 

[wwhitney]

A recent thread points out that the rules for generators that do not have factory installed OCPD are not completely clear.

445.13(A) and 240.21(G) seem to cover it; what's the problem?

They allow unlimited length conductors between the generator and the first OCPD.

Cheers, Wayne

 

[don_resqcapt19]

Do we need 240.21(D) for other sources like generators and ???

That might be a better idea, but not sure I want to write a complete first level subdivision.

 

[don_resqcapt19]

445.13(A) and 240.21(G) seem to cover it; what's the problem?

They allow unlimited length conductors between the generator and the first OCPD.

Cheers, Wayne

thanks

 

[Joethemechanic]

I never really put much thought into incident energy from gensets because all of them I have worked on had manufacturer supplied and installed OCPD within a few feet from the generator head. But I am thinking that a unit close to a megawatt has an engine of close to 1,500 HP and lots of rotating mass. It seems like it could deliver bolted fault current for at least a few seconds.

I just don't seem to remember reading anything in specifications about it

 

[Birken Vogt]

A recent thread points out that the rules for generators that do not have factory installed OCPD are not completely clear. I am thinking of submitting the following:

Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The tap shall be permitted at any point on the load side of the feeder overcurrent protective device. Section 240.4(B) shall not be permitted for tap conductors. Where the tap conductor originates at the terminals of a generator, the full load current of the generator shall be used as the rating of the line side overcurrent protective device for the application of 240.21(B).

Forgive me, I don't understand the reason or application here. I have always used the idea that the 115% rule applied in this situation. With your rule, would it become effectively 100% ?

Most generators of the size discussed in the other thread have CTs on their output, run to their controller, and will shut down the engine if they operate above nameplate for very long. If that makes any difference for this discussion.

Generator cable sizing

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...

forums.mikeholt.com

 

[don_resqcapt19]

Forgive me, I don't understand the reason or application here. I have always used the idea that the 115% rule applied in this situation. With your rule, would it become effectively 100% ?

Most generators of the size discussed in the other thread have CTs on their output, run to their controller, and will shut down the engine if they operate above nameplate for very long. If that makes any difference for this discussion.

Generator cable sizing

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...

forums.mikeholt.com

I missed a code section that applies to this so will not be making a PI.

Just the fact that we must use conductors rated at 115% of the generator output does not address the overcurrent protection of those conductors and that was where I was going, but as Wayne pointed out the is in in 240.21(G).

 

[ron]

We still do not have guidance from the NEC regarding how far the OCPD can be from the alternator terminals.

 

[wwhitney]

We still do not have guidance from the NEC regarding how far the OCPD can be from the alternator terminals.

As 240.21(G), which provides permission for the OCPD to be located other than at the point of supply of the conductors, is silent on the matter, the implication is an unlimited distance. 240.21(B) and (C) all specify distance limits where applicable, so the contrast is clear.

Cheers, Wayne

 

[Birken Vogt]

So in your opinion, if a genset has a CT that runs to the controller and will shut down the engine if overloaded, could we run 100% sized wire?

I went back through the 240 and 445 sections but it is not clear to me that that is what they are saying.

I am just glad that most generators come with breakers installed.

 

[don_resqcapt19]

So in your opinion, if a genset has a CT that runs to the controller and will shut down the engine if overloaded, could we run 100% sized wire?

I went back through the 240 and 445 sections but it is not clear to me that that is what they are saying.

I am just glad that most generators come with breakers installed.

The language in 445.13(B) is poorly written. It clearly requires a listed overcurrent protective device, but also permits a CT and over current relay without requiring that combination of devices to be listed. Not sure that the controller would qualify as overcurrent relay.

 

[Birken Vogt]

Not sure that the controller would qualify as overcurrent relay.

Why would it not? It is very accurate and effective.