I would like to feed a sub panel with a 43 amps of pv. The sub panel has a 200 amp busbar and is protected by a 100a breaker from the main panel. Am I correct when I say that (in theory) I can feed a maximum of (1.20 x 200) - 100 = 140 amps into this sub panel? I think I'm missing something here and would like some clarification/direction.
Sub Panel allowable ampacity
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jaggedben
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As far as the rules of the code...
Yes, you are correct. (I added the red part to be super clear.)
However, as a practical matter, if you backfeed more then 100A then your 100A breaker for the sub will trip. So in reality you are limited to only 100A.
ALSO you must consider the rating of the main panel as well. In your case the configuration in the main panel must also be suitable for backfeeding 43A.
So if the main panel is 200 amp bus and 200 amp main breaker, then I'm out of luck because I'm > 40 amps?
shortcircuit2
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That is correct.
Can you give me a code reference please. I suspect I am looking at the wrong thing. I'm looking at 705.12(D)(7) .."In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors. Am I not interpreting this correctly?
While technically, per Code, that is correct. But I have always considered this requirement for upstream bussing and feeders beyond the subpanel feeder OCPD an error in thinking.
When a compliant main panel is commissioned, it includes the calculated load of the subpanel. When PV current is backfed to the main, none of subpanel loads are using power from the main. As such, the total current on the main panel bus is reduce by the amount not used by the subpanel and increased by the backfed amount. In a compliant system, under normal operating conditions, the current on the main panel bus will never exceed the bus rating.
PS: I'm doubting the amount is 43A. The value applied to this determination is the OCPD rating, which 43A is unlikely, being a non-standard OCPD rating.
shortcircuit2
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Depends entirely on what you call normal.
The situation which you are not accounting for is when the normal loads in the subpanel are small or are turned off. In that case the entire PV system output becomes a backfeed into the next upstream panel and it therefore has to be applied to the 120% rule at that panel, where there are loads and two separate feeds.
I agree that it makes no sense at all to apply the 120% rule to the feeder conductors if they run only to that one subpanel, but we are currently stuck with that by the strict Code wording, although not all AJHs will enforce that.
shortcircuit2
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- South of Bawstin
You are correct if the loads are small compared to subpanel feeder circuit rating. If this were an actual consideration under Code, there would likely have to some measure to base "leniency" on the calculated load of the subpanel vs. feeder circuit ampacity and PV backfeed capacity.
But for the sake of discussion, let's assume the feeder circuit and OCPD rating is just large enough for the calculated load of the subpanel...
jaggedben
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You are looking at the right section. The subpanel is connected in series with the main. Thus the first overcurrent device connected to the inverter is used for calculations on both the sub and the main, as well the conductors in between them.
As Smart$ pointed out, 43A cannot be your breaker size. If that's your max inverter output then you need a 60A breaker. In that case, your sub is fine, your feeders for your sub must be rated 133A, and your main panel is a problem. Your options include upgrading the main, downsizing the main breaker in the main to 175A, finding another way to connect (e.g. supply side connection), or installing a smaller solar system.
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I will be happy to assume that, but I will also note that the real load may be a lot less than the calculated load, while the 43A will continue to be 43A or will get higher because of use of a larger nearest OCPD.
Perhaps, but the total subpanel supply OCPD's cannot exceed 120% of its rating, and the service-supplied OCPD must be sized not less than the calculated load (with continuous factored at 125% typically). The main panel must also be sized to handle this load at a minimum. The PV system cannot backfeed any more current upstream than the feeder circuit rating for downstream loads. And in order to backfeed any amount of current, the entire calculated load is not being supplied by the service to the subpanel.
Let's say for example it's a 100A subpanel with 50A calculated load, 50A feeder OCPD, and 50A PV breaker. The max backfeed current to the main panel would be 50A, and whether it supplies other loads in the main panel or backfed to the service, the total current handled by the main bussing will not exceed its rating because no current is being supplied to the subpanel by the service. (Let's not get into half-voltage inverters for now
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This time you are, I think, overlooking the fact that the 120% rule is only needed if you assume that either by poor design or by a fault the loads on the panel can draw more current then the main breaker alone could supply
Nothing you have described applies to that particular case.
It is common for the total of load breakers to exceed both the main breaker rating and the bus rating, so you cannot rely on those branch and feeder breakers for protection.
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Nothing you have described applies to that particular case.
It is common for the total of load breakers to exceed both the main breaker rating and the bus rating, so you cannot rely on those branch and feeder breakers for protection.
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I am not overlooking any facts. I can see we're getting nowhere fast...
Thank you for your input. Just for clarification, the 43A is the continuous output rated current of the inverters. My understanding (although I haven't confirmed it yet) is that the 2014 code, which has already been adopted here in Mass, utilizes this value now in calculations instead of the OCPD rating. I've still got some homework to do.
shortcircuit2
Senior Member
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- South of Bawstin
It is actually 125% of the output current. So...43 x 125% = 53.75
Sorry, think I used the wrong verbage. The 43A (which should actually be 44A) has already been adjusted for continuous output. I'm installing 39 Enphase M215's (34.9A x 1.25 = 43.7A)
Huh....that would be new! I like using just 125% instead of the breaker rating. It is a bit more leeway.
Could you copy & paste the actual wording from the NEC?
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