310.15 Revisted...But w/Pics

[sharnrock]

What's wrong with 2/0 feeding a 200A panel? Isn't it rated for 200 amps in residential? I believe you can even downsize the neutral to a 1/0.

As far as the generator. That house probably only pulls 50 amps with the A/C on and the stove going. As long as it's 12kw or more it should be alright.

Although it's not part of the OP's question... Ya those control wires need their own pipe. Even if they're high voltage, now you have to derate your gen. feeders 70% per 310.16

 

[Dennis Alwon]

The nipples from ATS to house panels, actually, all of them, are PVC. The one coming from meter to ATS does not have a bushing because it would not catch with the sealing locknut.

Thom, I don't believe a sealing locknut is required in that installation.

 

[ptonsparky]

...I believe Bob was talking about low voltage cable in with high voltage going to the genny.

Yea, that's what I figured, I just didn't reference the rest of my reply to the post on bushings.

 

[Dennis Alwon]

What's wrong with 2/0 feeding a 200A panel? Isn't it rated for 200 amps in residential? I believe you can even downsize the neutral to a 1/0.

Well T 310.15(B)(6) allows 2/0 for 200 amp but we don't have a 200 amp service we have a 400 amp service. T. 310.15(B)(6) does not make exception for 2- 200 amp panel because it needs to carry the entire load of the dwelling. Diversity of the load is the key here but I believe we will see some changes in 2011.

 

[360Youth]

I believe Bob was talking about low voltage cable in with high voltage going to the genny.

I have been revisting this issue in my head for some time. Part of the reason I do as I do now is when I first started with the installs we were using the factory package deal that has all of the system wiring in one flex. So, from the very begining I was accustomed to seeing control wires and gen feeds in the same raceway in a listed application. I have learned over the years that apparently a manufacturer can get most anything listed and it doesn't mean a thing to what a field worker can do. :roll: The other reason is that I may have, out of my own bias, misread 702.9 which allows system standby wiring in the same raceway as other general wiring. I was reading system wiring as the complete sytem and not just feeder wiring.

 

[macmikeman]

The nipples from ATS to house panels, actually, all of them, are PVC. The one coming from meter to ATS does not have a bushing because it would not catch with the sealing locknut.

I couldn't tell if it was metal or pvc. I should have put that in somplace in my question. Yes the one from the meter pan to the ats was the one I was talking about, not the two other bushed conduits. I failed an inspection while using sealing locknuts on the outside of the nipple - unable to get a plastic bushing on the inside cause of it. I had to remove everything and re-thread. My bad.....

 

[macmikeman]

By the way, those two cans in the picture appear to be right next to each other. On my planet, that would mean using a metal chase nipple or a running thread. It must be the angle of the photo:wink:

 

[mark32]

Well T 310.15(B)(6) allows 2/0 for 200 amp but we don't have a 200 amp service we have a 400 amp service. T. 310.15(B)(6) does not make exception for 2- 200 amp panel because it needs to carry the entire load of the dwelling. Diversity of the load is the key here but I believe we will see some changes in 2011.

Just so I follow you Dennis, the installer should have paralleled 3/0 correct? Would a single 400 be okay as it states in 310.15(B)(6)?

 

[Dennis Alwon]

Just so I follow you Dennis, the installer should have paralleled 3/0 correct? Would a single 400 be okay as it states in 310.15(B)(6)?

You got it. The table is a much debated item in the NEC but it is clear to me that T.310.15(B)(6) makes no allowances for using 2/0 in the case of 2- 200 amp amps. Obviously, 400KCM would be fine for a single panel at 400 amps.

Do I think there is a real issue with using 2- 2/0 copper conductors-- not really but it is not code compliant as I see it.

 

[lefty08]

You got it. The table is a much debated item in the NEC but it is clear to me that T.310.15(B)(6) makes no allowances for using 2/0 in the case of 2- 200 amp amps. Obviously, 400KCM would be fine for a single panel at 400 amps.

Do I think there is a real issue with using 2- 2/0 copper conductors-- not really but it is not code compliant as I see it.

Don't see how you think it is not code compliant? 2/0 copper is rated for 200 amps as per 310.15(B)(6)? If you run 2- 2/0 copper conductors in parallel, wouldn't that now be rated for 400 amps? Are you derating the cables because they are run in parallel? Don't get your logic that 2- 3/0 cooper conductors rated at 225 amps each as per 310.15(B)(6) run in parallel is acceptable, but 2- 2/0 copper conductors rated at 200 amps each are a violation. Please enlighten me.

 

[kbsparky]

... Ya those control wires need their own pipe. Even if they're high voltage, now you have to derate your gen. feeders 70% per 310.16....

I disagree with the notion that derating is necessary here. Those are control wires as you have mentioned, not current carrying conductors as 310.16 would assume.

 

[Twoskinsoneman]

Don't see how you think it is not code compliant? 2/0 copper is rated for 200 amps as per 310.15(B)(6)? If you run 2- 2/0 copper conductors in parallel, wouldn't that now be rated for 400 amps? Are you derating the cables because they are run in parallel? Don't get your logic that 2- 3/0 cooper conductors rated at 225 amps each as per 310.15(B)(6) run in parallel is acceptable, but 2- 2/0 copper conductors rated at 200 amps each are a violation. Please enlighten me.

Are we talking about parallel runs? I thought the issue was two separate feeders feeding different panels.

 

[hurk27]

Don't see how you think it is not code compliant? 2/0 copper is rated for 200 amps as per 310.15(B)(6)? If you run 2- 2/0 copper conductors in parallel, wouldn't that now be rated for 400 amps? Are you derating the cables because they are run in parallel? Don't get your logic that 2- 3/0 cooper conductors rated at 225 amps each as per 310.15(B)(6) run in parallel is acceptable, but 2- 2/0 copper conductors rated at 200 amps each are a violation. Please enlighten me.

2/0 is only rated 175 amps each which would be only good for 350 amps of load as per table 310.16 @75?C which this would be in line with the allowance for the 400Kcmil which only has a 335 amp rating in the same table. H'mmmm guess I just cornfused myself of my own answer:-?

I have always thought table 310.15(B)(6) only downsized one size, but now I see that 400Kcmil is two sizes smaller than the 310.16 table requirement?
Inspectors here have always required 500Kcmil, for a 400a dwelling service, and 600Kcmil on commercial, guess I have never questioned it as it was a reduction in one size:-?

Boy I stumped myself on my own answer:-?

 

[Twoskinsoneman]

I disagree with the notion that derating is necessary here. Those are control wires as you have mentioned, not current carrying conductors as 310.16 would assume.

If the control wires are class 2/3 they can't be in the raceway anyway so it is moot as far as derating. If they are class 1 than 725.50(B) explains derating.

 

[hurk27]

I do see one thing that is a manufacture instructions violation:

The control wire you ran look to be common class2 thermostat cable 18/2 I think. I know that in the manufacture instructions for Generic these conductors have to be class 1, and should be no smaller then a #14

N-1 and N-2 are the utility monitoring feed back to tell the control board to start the transfer cycle, they are connected through two 2 amp glass fusses to the utility line side of the transfer switch and have 240 volts between them, the other three are 192, 15A and I believe 23, these are 12 volts DC and supply control and battery charging (in some units) and can have up to 5 amps on them. these can be run in just a class 2 cable with 3 #14 stranded conductors, and should not be in the counduit as others have said.

 

[Dennis Alwon]

Don't see how you think it is not code compliant? 2/0 copper is rated for 200 amps as per 310.15(B)(6)? If you run 2- 2/0 copper conductors in parallel, wouldn't that now be rated for 400 amps?

I will try again to explain a different way. Where in T. 310.15(B)(6) does it allow us to use parallel conductors? The Table does not address parallel setup so it cannot be used.

We were really talking about 2- 200 amp panels which is not the same as a parallel setup. T. 310.15(B)(6) is based on the diversity of a residential load. When you break the service up with 2- 200 amp panels you have changed the diversity that was meant for that Table.

Read art. 310.15(B)(6) closely and it allows feeders to use T. 310.15(B)(6) as long as the feeder carries the entire load of the dwelling. That is not the case with 2-200 amp panels.

 

[weressl]

I know this topic has seen its share of discuusion recently but I was wondering how many feel the installation of an ATS I installed yesterday would pass where ever you are.

The meter base feeds a 400 amp SE rated ATS with parallel 2/0 and the ATS feeds two 200 amp house panels. Each panel is fed with 2/0 runs. To me the new wording of 310.15 allows reduction of feeder cable only if that feeder serves the entire load. In this case, neither of the 2/0 runs feed the "entire load" of the house, so I was all set to run 3/0 from ATS to each house panel. But I had a converstion with a different inspector the previous day and he reads it as still allowable in this scenario so I "took a chance" that it would fly and ran the 2/0. (I always have 2/0 on hand but only get 3/0 as I need, plus it obviously less expensive). Based on the pics below, does this still fall under allowable residential reductions, or should the house feeders been 3/0. The install passed with no questions.

Interesting or maybe even odd.

Are the utility supply wires are the same size as the load side? 50% neutral on the utility?

The standby supply, presumably the generator, wire seem to be the same size as the load side, except there is only one on the supply and two for the load. Somebody is on for a surprise.

 

[360Youth]

Interesting or maybe even odd.

Are the utility supply wires are the same size as the load side? 50% neutral on the utility?

The standby supply, presumably the generator, wire seem to be the same size as the load side, except there is only one on the supply and two for the load. Somebody is on for a surprise.

I have yet to see a house that pulls a load equivalent to the service size. The wires you see are sized according to the equipment installed. I did not calculate the generator size, just wired according to OCP of genset and ATS.

 

[weressl]

I have yet to see a house that pulls a load equivalent to the service size.

I don't disagree. However, in the future loads can be added to each tennant's subpanel that can overload the generator - without automatic load shedding - and then they can come screaming to the Owner that their load was interrupted, while they are paying for uninterrupted supply.

The wires you see are sized according to the equipment installed. I did not calculate the generator size, just wired according to OCP of genset and ATS.

I understand, but see the resultant scenario above. Remember that the details suppied here is very limited and even you may not know all that was decided behind the scene, so to speak. Assuming that those decisions were addressed can only be an assumption....and you know what happens when one assumes.

 

[hurk27]

I don't disagree. However, in the future loads can be added to each tennant's subpanel that can overload the generator - without automatic load shedding - and then they can come screaming to the Owner that their load was interrupted, while they are paying for uninterrupted supply.



I understand, but see the resultant scenario above. Remember that the details supplied here is very limited and even you may not know all that was decided behind the scene, so to speak. Assuming that those decisions were addressed can only be an assumption....and you know what happens when one assumes.

I think this is a single dwelling we are talking about.
I have never had a problem with installing a 40kw or even a 20kw generator on a 400 amp service for a dwelling, I could see a problem if it was all electric and no gas, but around here its almost all gas heat and cooking.

I did have a 15Kw Generac balk at two AC units trying to fire up at the same time of transfer on a 200 amp service, even with the built in time delays in the thermostat they would still both try to fire up at the same time, a couple of time delays set to fire up the 5 ton first then the 3 ton work great, but when we did 10 Kw ones we would always do load shedding by installing a separate 100 amp panel and just placing the needed loads in it and no AC's.

I would say if you use a 20kW on a 400 amp service and there are multiple AC units, just have time delays installed on the unit so they each fire up one minute apart.