Feeder Conductor ampacity

[electro7]

I have a solar installation that is a bit more complicated.

I have 100A of solar OCPD and I want to tie into a 100A rated sub-panel (B2) in a recreation building. That 100A rated sub-panel is fed from another sub-panel (B1) in the house. It has a 100A feeder breaker with #4 AWG wire. Panel B1 is fed from the main service with #2 and is protected by a 125A rated breaker. The service has a single 240V 400A breaker, single phase.

Here is my proposed design to make this work and my one question I have:

Upgrade the B2 and B1 panel to a 200A for the 120% busbar rule keeping the feeder conductors and feeder breakers the same. Downsize the main breaker to 350A.

My one question is the existing B2 feeder conductors are #4. Am I reading correctly in the 2014 NEC that the feeder conductors can be 83% of the rated ampacity? So in other words #4 AWG is okay with 100A of OCPD?
Is my proposed design within NEC rules on that #4 AWG feeder conductors? There would be 100A of OCPD of solar. 2 inverters, one at 40A the other 60A of OCPD.

Thanks

 

[electro7]

Nevermind. I found the answer to my own question in Annex D7 example.

 

[Carultch]

I have a solar installation that is a bit more complicated.

I have 100A of solar OCPD and I want to tie into a 100A rated sub-panel (B2) in a recreation building. That 100A rated sub-panel is fed from another sub-panel (B1) in the house. It has a 100A feeder breaker with #4 AWG wire. Panel B1 is fed from the main service with #2 and is protected by a 125A rated breaker. The service has a single 240V 400A breaker, single phase.

Here is my proposed design to make this work and my one question I have:

Upgrade the B2 and B1 panel to a 200A for the 120% busbar rule keeping the feeder conductors and feeder breakers the same. Downsize the main breaker to 350A.

My one question is the existing B2 feeder conductors are #4. Am I reading correctly in the 2014 NEC that the feeder conductors can be 83% of the rated ampacity? So in other words #4 AWG is okay with 100A of OCPD?
Is my proposed design within NEC rules on that #4 AWG feeder conductors? There would be 100A of OCPD of solar. 2 inverters, one at 40A the other 60A of OCPD.

Thanks

The 83% rule is specific to service conductors of single phase residential applications, and is extended to the feeder conductors that carry the full service load. I.e. a direct continuity of the service conductors from the service disconnect to other equipment, prior to any branches, taps, or panelboards to split the load.

Feeders in general need at least as much terminal ampacity (110.14(C)) and corrected wire ampacity per 310.15, for the load, and need to be sized for OCPD protection according to Article 240.

 

[jaggedben]

I have a solar installation that is a bit more complicated.

I have 100A of solar OCPD and I want to tie into a 100A rated sub-panel (B2) in a recreation building. That 100A rated sub-panel is fed from another sub-panel (B1) in the house. It has a 100A feeder breaker with #4 AWG wire. Panel B1 is fed from the main service with #2 and is protected by a 125A rated breaker. The service has a single 240V 400A breaker, single phase.

Here is my proposed design to make this work and my one question I have:

Upgrade the B2 and B1 panel to a 200A for the 120% busbar rule keeping the feeder conductors and feeder breakers the same. Downsize the main breaker to 350A.

My one question is the existing B2 feeder conductors are #4. Am I reading correctly in the 2014 NEC that the feeder conductors can be 83% of the rated ampacity? So in other words #4 AWG is okay with 100A of OCPD?
Is my proposed design within NEC rules on that #4 AWG feeder conductors? There would be 100A of OCPD of solar. 2 inverters, one at 40A the other 60A of OCPD.

Thanks

Seems problematic to me. We just had another thread on this, where I stated my opinion that you'd be ill advised to use the 83% rule without running it by the AHJ. So your feeder to panel B2 is a problem for your inverter output. It's also illegal to be on a 100A breaker, regardless. That could be solved by downsizing the breaker to 80A if the calculated load is less than that. But that doesn't solve your inverter output problem. If the AHJ shoots the whole thing down for either reason you really have no argument.

If you can interconnect to panel B1 instead of B2 you'd be in better shape. Upgrading two subpanels seems like a lot of trouble to go to anyway. Why not go to the service? Are we trying to avoid trenching here?

(You also seemed to say that a 125A breaker is currently protecting a 100A panel (B1). Hopefully that's a typo, but if you upgrade that panel to 200A it solves the issue.)

 

[jaggedben]

Would it make sense to interconnect just the 40A inverter to B2, and downsize the feeder breaker to 80A? And connect the 60A inverter elsewhere? Then no upgrade is even required to panel B2.

 

[electro7]

This is a type of mansion where there is special outdoor tile and would be nearly impossible to get back to the service or sub-panel B1. I think it would require boring under the outdoor tile, if that is even possible. I think it was a typo- B1 is a 125A sub-panel.

I see that 310.15(B)(7)(2) feeders is permissible to be 83% of ampacity of feeder conductors. But I guess the question is whether the conductors feeding sub-panel B2 from sub-panel B1 is considered feeder conductors? If they are not considered feeder conductors then what would they be considered, a branch circuit? The #4 wire feeding panel B2 from B1 that is on a 100A is existing. It is in a guest house type unit and is a separate structure from the house.

I feel like after reading the definition of "feeder" in chapter 1 and reading 310.15(B)(7)(2) it is arguable whether B2 supply conductors are considered feeders. No?

We are looking at a 21kw or more. One inverter has output rating of 32A the other 47.5A hence the 40A and 60A OCPD and 100A of solar breakers.

 

[electro7]

So Jaggedben, do you think I would be ill advised to proceed even if the AHJ says its okay? I am on the fence even if they say it is okay. #4 at 75C is 85A, at 90C 95A. Inverter max output current is 79.5A. Will I ever see the max output on these inverters? And if I do it is still below the 75C ampacity of #4 THHN. I know I am not taking into consideration the 125% of continuous load calc.

Trying to make this work but don't think I want to take the chance of major malfunctions or a fire.

 

[jaggedben]

You seem to be over looking the parts of 310.15(B)(7) where it says "the feeder conductors supply the entire load associated with a one family dwelling...[etc.]". Since the feeders you're talking about don't seem to be such, from your description, you can't use that section.

I think I'd not even propose to an AHJ to export 100A on the 4awg. I'd probably not even where 310.15(B)(7) is legitimately invoked.

If you can't find another way, you might have to downsize the system to have only 80A output. Depending on inverters chosen, you might not have to downsize the DC, or not as significantly. I'm gonna hazard a guess from the 47.5A that you've sold SolarEdge. Well, you can put 10.26kW each on two 7600W inverters, which gets you to 20.52kW, which is awfully close to 21. And then you don't even have to downsize the service breaker.

 

[electro7]

Okay I hear you. I was reading it as those conductors are feeding the entire guest house load (panel B2) and nothing else, a single family dwelling. I am not understanding it right then. So what are feeder conductors? I always understood feeder conductors as being a circuit feeding a sub-panel load. Would that just be considered a branch circuit?

Yes you are correct on the Solaredge. They are wanting the ability to add in the future. And yes I have thought about the 7600 option, it just would be maxed out. It sounds like the homeowner does not want that.

I have thought about their new HD Wave inverters to where they say you can load them up to 155% instead of 135%. That might help having 2 7600's of those.

 

[ggunn]

Okay I hear you. I was reading it as those conductors are feeding the entire guest house load (panel B2) and nothing else, a single family dwelling. I am not understanding it right then. So what are feeder conductors? I always understood feeder conductors as being a circuit feeding a sub-panel load. Would that just be considered a branch circuit?

Generally speaking, feeders feed other OCPDs, branch circuits feed loads.

 

[electro7]

Okay that is what I thought. So for this situation I have the MSP, feeding sub-panel B1, and sub-panel B1 feeding sub-panel B2. Are the conductors feeding sub-panel B2 from sub-panel B1 considered feeder conductors and would the 83% ampacity rule apply to them? 310.15(B)(7)(2) They are #4 THHN protected by a 100A breaker.

Then the next question would be: is it okay to have 100A of OCPD for solar on sub-panel B2 and ultimately backfeeding through the #4 THHN?

 

[Carultch]

Okay that is what I thought. So for this situation I have the MSP, feeding sub-panel B1, and sub-panel B1 feeding sub-panel B2. Are the conductors feeding sub-panel B2 from sub-panel B1 considered feeder conductors and would the 83% ampacity rule apply to them? 310.15(B)(7)(2) They are #4 THHN protected by a 100A breaker.

Then the next question would be: is it okay to have 100A of OCPD for solar on sub-panel B2 and ultimately backfeeding through the #4 THHN?

Circuits from the service point to the service disconnect are service conductors.
Any circuit on the load side of the service disconnect that feeds either a main panel or a subpanel, are feeder conductors.
Feeder conductors can originate from a main service disconnect, a main distribution panel, or from a subpanel.
Circuits that come from a branch breaker in a panel and feed an individual load, a group of receptacles, or a group of lighting fixtures are branch circuit conductors.

The 83% ampacity rule only applies to feeder conductors for feeders rated at the entire load of the dwelling unit, for a 120/240V single phase system. An example is two 200A subpanels fed from a 200A service. Both of them can have conductors that match the service conductor size. Another example is a 200A meter/main as the service disconnect, with a 200A main panel downstream. The conductors from the meter/main to the main panel classify as feeder conductors, but need not be any larger than the service conductors prior to the meter.

An example where this doesn't apply, is if you have two 100A subpanels from a 200A service. They both need 100A worth of wire ampacity for the main supply feeders to the subpanels. Not 83A worth of wire ampacity.

 

[GoldDigger]

If you consider the guest house a dwelling unit (on the same service as another dwelling unit) rather than an auxiliary building, then the section may apply.
Are you OK with the 120% rule all the way upstream to the service?
The dwelling feeder allows you to use conductors of lower ampacity than required by load calculation. IMHO it does not allow you to assign a higher ampacity to those conductors where some other section requires a particular ampacity. But that may be a misinterpretation.

mobile

 

[electro7]

Yeah I am good with the 120% rule all the way to the service.

Panel B2 busbar rating 200A (upgraded) with a 100A feeder OCPD. #4 THHN feeder wire.
Panel B1 busbar rating 200A (upgraded) with a 125A feeder OCPD. #2 THHN feeder wire.
Service 400A busbar, downsizing main to 350A.

My main concern is putting 100A of solar OCPD on B2 that has existing #4 THHN feeder wire (and existing 100A OCPD which the AHJ must have okayed. I don't think they did this without a permit). I keep going back and forth on whether or not I am okay with this even if the AHJ says its okay. Proposed design is 2 inverters with max operating current of 47.5A and 32A interconnected to B2.

Getting either to sub-panel B2 or the service adds a lot of money and work to the job if its even possible.

Thanks for all the feedback as I figure this out within the code.

 

[electro7]

After all of this I am not going to put 100A of solar OCPD on #4 THHN feeder wire.

But just for future reference for me, is a separate building (that is considered a dwelling unit according to code) on the same service as "the main dwelling unit" considered a single family dwelling, according to 310.15(B)(7)(2)? Or is it an "auxilary building"?

I feel like I have seen this a lot where there are #4 THHN feeder conductors protected by 100A OCPD. Never agreed with it in the past but started looking at 310.15(B)(7)(2) again and wondered if in fact this installation is okay since I feel like it is quite common.

It makes me wonder what the reasoning is behind the 83% rule (310.15(B)(7)(2)), and the wire ampacity rating chart 310.15(B)(16). If a single family dwelling unit uses 99 amps at once for 3 hours will #4 THHN be able to handle that? Or is the code saying that will never happen?

Can anybody shed some light on this issue for me?

 

[jaggedben]

It could probably be argued that the separate building is a dwelling unit, if it's really like a guest house. When you called it a 'recreation building' in your original post it didn't seem like that. Let's refer to the actual code language.

For a feeder rated 100 through 400 A, the feeder
conductors supplying the entire load associated with
a one-family dwelling, or the feeder conductors
supplying the entire load associated with an
individual dwelling, unit in a two-family or
multifamily dwelling, shall be permitted to have an
ampacity not less than 83 percent of the feeder
rating.

So if you can reasonably say the separate building is a dwelling then the original installation was and still is okay.

I've backfed all sorts of feeders that were installed to this rule, but never where the backfeed was 100A. That's the tricky question raised by your situation.

 

[Carultch]

It makes me wonder what the reasoning is behind the 83% rule (310.15(B)(7)(2)), and the wire ampacity rating chart 310.15(B)(16). If a single family dwelling unit uses 99 amps at once for 3 hours will #4 THHN be able to handle that? Or is the code saying that will never happen?

Can anybody shed some light on this issue for me?

I wonder that myself. The go-to answer, is the load diversity and statistical chance being minimal that a service sees its nominal load for long enough time to matter.

However, if that is the case, it sounds like the real issue is that the load calculation procedure is too conservative, and really should be corrected to for this, rather than conductors having a special ampacity as service conductors that they don't have in other applications. It's like setting the speed limit 20% higher, because the speedometers are calibrated to display a speed that is 20% above what it really is.

 

[electro7]

I've backfed all sorts of feeders that were installed to this rule, but never where the backfeed was 100A. That's the tricky question raised by your situation.

Yes I understand this. 100A of solar OCPD I agree is pushing the limits with this one.

I believe it would be a dwelling unit according to code- sanitation, living, cooking are all there. The owners are using it more as a recreational building, thats why I called it that.

 

[electro7]

I wonder that myself. The go-to answer, is the load diversity and statistical chance being minimal that a service sees its nominal load for long enough time to matter.

However, if that is the case, it sounds like the real issue is that the load calculation procedure is too conservative, and really should be corrected to for this, rather than conductors having a special ampacity as service conductors that they don't have in other applications. It's like setting the speed limit 20% higher, because the speedometers are calibrated to display a speed that is 20% above what it really is.

I see. I agree that conductors should have a set ampacity rating- same for branch circuits and feeders, for service conductors as feeders, for continuous loads as for non-continuous loads. If a wire is rated for a certain ampacity that should be its rating no matter the application in my opinion.

 

[ggunn]

Yeah I am good with the 120% rule all the way to the service.

Panel B2 busbar rating 200A (upgraded) with a 100A feeder OCPD. #4 THHN feeder wire.
Panel B1 busbar rating 200A (upgraded) with a 125A feeder OCPD. #2 THHN feeder wire.
Service 400A busbar, downsizing main to 350A.

You understand that the 120% rule applies to busbars, right? 125% of your nameplate inverter current rating entering any panel (all panels in series from the inverter to the service) plus the rating of the OCPD protecting that panel cannot exceed 120% of the busbar rating, and the current from the inverter can only enter the panel at the opposite end of the busbar from the end supplied by its protecting OCPD. It has nothing to do with feeder ampacities.