Derating generator output conductors run with control wiring?

[mark32]

I have a generator (Generac 22kw) installation coming up and was trying to size my conductors and raceway(s). Initially I was going to run separate conduits for power and the control wiring but I see that Generac now claims all the wiring can be run in the same raceway. I just happen to see a local inspector, not the same inspector on this job however, but he gave me thumbs up on pulling all the conductors together. My question is when it comes to derating these conductors, more importantly, exactly how many of these control wires are considered CC? According to their wiring diagram, there are six conductors, and just in case the homeowner wants to add an oil pan heater, I'll throw in another two conductors making it a total of 12 conductors. (Two ungrounded, one grounded, one EGC, two for the heater, six control) I wanted to pull #3 cu but I may have to bump it up.

For the heck of it, can I reduce the size of the neutral?

 

[don_resqcapt19]

See the second paragraph in 310.15(B)(3)(a).

 

[bsmith8691]

Without having a code book in front of me, I believe the feeder needs to be sized 115 percent for starters. Then you can adjust from there. Unless it's one of Generacs pre made whips, I run the "control" wires in a separate conduit.

Sent from my XT1254 using Tapatalk

 

[Smart $]

The heater wires, when and if used, will push you up to not less than four ccc's, requiring an 80% ampacity adjustment.

The neutral may be reduced in size to match the ampacity minimum determined in accordance with 220.61.

 

[Smart $]

Without having a code book in front of me, I believe the feeder needs to be sized 115 percent for starters. Then you can adjust from there. Unless it's one of Generacs pre made whips, I run the "control" wires in a separate conduit.

Let's just call them generator conductors.... so 115% from generator terminals to first [distribution] ocpd. A feeder requires load-based sizing per 215.2.

 

[JFletcher]

Make sure you check the lugs on the Generac before upsizing conductors to the point the lugs wont fit the wire.

 

[mark32]

Thanks to all for the input. Don, I only had my '05 nec with me when I posted so I didn't get to read that article until I got home, thanks for the info.

If anyone wants to play along, I have another question. Because I have to go up a size for the feeder's ungrounded conductors, do I not have to go up a size for the EGC? This is what I'm looking to pull for the feeder:

Two #2's (ungrounded), one #3 (Grounded), and one #6 (EGC)

Again, this is for 22kw generator. The generator's output using NG is 19500/240 = 81.25 amps. I'm choosing #3 (110*.8 = 88a) for the neutral because the generator can only put out 81.25a. Not sure my thinking is entirely sound or correct.

 

[Smart $]

...
If anyone wants to play along, I have another question. Because I have to go up a size for the feeder's ungrounded conductors, do I not have to go up a size for the EGC? This is what I'm looking to pull for the feeder:

Two #2's (ungrounded), one #3 (Grounded), and one #6 (EGC)

Again, this is for 22kw generator. The generator's output using NG is 19500/240 = 81.25 amps. I'm choosing #3 (110*.8 = 88a) for the neutral because the generator can only put out 81.25a. Not sure my thinking is entirely sound or correct.

As noted earlier, 115% for generator conductors...

81.25*115%/.8=116.8A

...so #2 is actually too small if these are generator conductors.

If these are feeder conductors, the size is based on load or ocpd rating, whichever triggers the larger conductor. You have not provided or set those details in order for us to make any assessment.

 

[mark32]

Hello Smart, I appreciate your help. Indeed these are generator conductors, the ocpd on the generator is 100a. You lost me though on your calculation. 81.25*115/.8 = 116.8a. What does the 116.8a represent? I'm looking at 81.25*115 = 93.53a. So 80% of #2cu 130*.8 = 104. I know you're Smart ? so I must be doing something wrong.

 

[Smart $]

Hello Smart, I appreciate your help. Indeed these are generator conductors, the ocpd on the generator is 100a. You lost me though on your calculation. 81.25*115/.8 = 116.8a. What does the 116.8a represent? I'm looking at 81.25*115 = 93.53a. So 80% of #2cu 130*.8 = 104. I know you're Smart ������ so I must be doing something wrong.

There seems to be a general misunderstanding here. The 115% for generator conductors only applies between the generator terminals and the first ocpd. In stating this, keep in mind that not all generators have an integral ocpd. Generator conductors are analogous to transformer secondary conductors. If the terminals you are connecting to are on the load side of that 100A ocpd, then you are dealing with feeder conductors.

As to the calculation, I'm just putting the 80% derate factor into the compare-to-table value rather than like you factoring it into the compare-to-ampacity value... and your assessment is (would have been?) correct if you are using 90°C conductors.

 

[Smart $]

... Because I have to go up a size for the feeder's ungrounded conductors, do I not have to go up a size for the EGC? ...

As a feeder protected by a 100A breaker, you can feasibly say #2 Cu is the smallest conductor that has sufficient size for the load for 250.122(B) purposes. Technically, you could use #3 if your calculated load is below 92A (90°C@115A×80%), which is greater than the next smaller standard ocpd rating of 90A for 240.4(B) compliance. However, if your calculated load is not more than 92A and the inspector questions it, just say you are allowing for future expansion to greater than 92A calculated load and refer him to 90.1(B).

 

[JFletcher]

#2Cu derated to 80% for 4-6 CCC = 115A x .8 = 92A, which is less than the max listed output of a Guardian 22kW generator (91.3A on LP, 81.3A on NG).

If the calculated load is <80A, would#3 CU derated to 80% (80A) be good, or would the breaker at the unit need to be downsized from 100 to 80A (or an 80A fused disco put in)?

To throw another wrench in the works: if it's cold enough to warrant the block heater, you may want the battery warmer as well (Cold Weather Kit)... 2 more CCC perhaps.

I retract what I previously wrote about checking the lugs to see if #2 would fit. Specs say up to 2/0.

 

[mark32]

Hi Smart, thanks for clearing that up with regard to generator conductors vs feeders. I see what you did in your calculation now, that's a neat trick.

I guess I'll stick with a #8cu egc.

Fletcher and others, I should have mentioned up front that I will be using thhn. A battery heater, hmmm, that's something to consider, although I've yet to personally see a standby gen here in NJ with one.