Calculating backfeed for Solar PV

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rynd2it

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Hi,

We have always used a simple formula for calculating the backfeed of a Solar PV array and the allowable backfeed of the main electrical panel:

Max Continuous AC Current of the inverter(s) * 1.25 = backfeed
Panel size * .20 = max backfeed allowed.

I have always understood that this was to comply with the NEC but a homeowner challenged it when we told him he need an upgrade to the main panel. I cannot confirm if it is code or not.

Help please


David
 
Hi Don,

Thanks -but unless I'm misreading it that simply specifies the load cannot exceed 120% of the bus rating. If the bus rating is 200 amps that tells me I can have a backfeed of not more than 240 amps - which cannot be right surely?

And I still need some confirmation of the 125% of the backfeed amps from the inverter.

David
 
Hi Don

Thanks for the response but that section seems to say that the load cannot exceed 120% of the bus size, which on a 200 amp panel is 240 amps. That does not seem right somehow unless I'm misreading it.

Also, where does the inverter MCC * 1.25 come from?

I appreciate the help, reading these codes is not my strong point

David
 
rynd2it said:
Hi Don,

Thanks -but unless I'm misreading it that simply specifies the load cannot exceed 120% of the bus rating. If the bus rating is 200 amps that tells me I can have a backfeed of not more than 240 amps - which cannot be right surely?

And I still need some confirmation of the 125% of the backfeed amps from the inverter.

David
Click to expand...

I think your over looking that the Buss Can not be "fed" from more then 120% of its rating that is both utility and separately derived system, if you have a 200 amp main that means you could only back feed 40 amps onto the buss. If you have a larger PV system you need to reduce the size of the main breaker so you are not over 120% combined.
 
acrwc10 said:
I think your over looking that the Buss Can not be "fed" from more then 120% of its rating that is both utility and separately derived system, if you have a 200 amp main that means you could only back feed 40 amps onto the buss. If you have a larger PV system you need to reduce the size of the main breaker so you are not over 120% combined.
Click to expand...

Nailed it.:cool:
 
Thanks guys - that makes that much clearer.

But, where does the 25% factor for calculating maximum backfeed come from? Our formula adds all the inverters together then multiplies the Max Cont Current by 1.25 to arrive at the backfeed total. I can't find this anywhere

Appreciate some more insight

David
 
I think your over looking that the Buss Can not be "fed" from more then 120% of its rating that is both utility and separately derived system, if you have a 200 amp main that means you could only back feed 40 amps onto the buss. If you have a larger PV system you need to reduce the size of the main breaker so you are not over 120% combined.
Click to expand...

I don't think that is entirely true. If you have a 225A panel with a 200A main, you should be able to install up to a 45A breaker for the PV. You are assuming that the panel is rated for 200A and that may not be the case.

I know this is an old post, but, I just saw it and wanted to put my 2 cents in.
 
jotw said:
I don't think that is entirely true. If you have a 225A panel with a 200A main, you should be able to install up to a 45A breaker for the PV. You are assuming that the panel is rated for 200A and that may not be the case.
Click to expand...

The person asking the question stated it is a 200 amp panel in post 3.

A 225 is typically a 3 phase panel. :)
 
Oh, right. All 225A panels I've ever seen have been single phase. Anyway, what difference does it make whether it's a single or three phase panel? Just curious.
 
If a panel is rated at 125 amps and the breaker is 100 amp, It seems to me that you could add 25 amps backfeed plus another 20 amps based on the 120% rule. This would make 45 amps solar into a 100 amp(125 amp rated) panel.
For a 200 (225 rated) panel it would be 65 amps.
 
rynd2it, check out 690.64 (B) (2). It clearly states sum of the over current devices supplying power. Iwire and mlnk have it right.
 
rynd2it said:
125 amp service panel can have maximum 25 amp backfeed according the NEC code. No more.
Click to expand...
The calc is "fed" by no more than 120 % of the connected input to the bus. So a 100 amp panel x 120% = 120 amps max combined backfeed and Service. If you want more than 20 amp backfeed either tap ahead of the main or put in a larger service. I would not expect to lower a main lower than 100.
 
Back feed calculations

Back feed calculations

All UL listed PV grid connected inverters will have installation instruction and perhaps a label that specifies the maximum fuse/CB to be used for the AC connection.

A lower ampacity device can be used if it meets the 125% rule for inverter output current based on PV array rated watts, inverter efficiency, and the low limit for line voltage.
 
backfeed

backfeed

If an OCPD is rated 100% continuous use for an ac circuit it can indeed be used to protect the conductors of the inverter output, which are sized at 125% of the inverters rated output. That is inverter output... input is figured at the lowest voltage expected x inverter efficiency x max isc on the DC side ONLY when it is coming from a battery bank, any pv source circuit or output circuit current for sizing conductors and OCPD is figured at strings in parallel x1.25 x 1.25.
 
Too many numbers in my head....

Too many numbers in my head....

reread the last post and I lost track of myself.....
this is right :)
If an OCPD is rated 100% continuous use for an ac circuit it can indeed be used to protect the conductors of the inverter output, which are sized at 125% of the inverters rated output. That is inverter output... input current is the rated power / lowest input voltage / inverter efficiency at max power on the DC side ONLY when it is coming from a battery bank, any pv source circuit or output circuit current for sizing conductors and OCPD is figured at max ISC of strings in parallel x1.25 x 1.25.(or 1.56)
 
I think nearly all residential main CBs are not rated for 100% continuous duty. But I think you can still figure 120% times the CB rating plus add the excess buss bar rating. For a 100 amp panel, 20 amps plus 25 amps = 45 amps maximum solar back feed. If you think otherwise tell me about it.
 
mlnk said:
I think nearly all residential main CBs are not rated for 100% continuous duty. But I think you can still figure 120% times the CB rating plus add the excess buss bar rating. For a 100 amp panel, 20 amps plus 25 amps = 45 amps maximum solar back feed. If you think otherwise tell me about it.
Click to expand...


Nope, max back feed is 120 of the bus rating for Resi.
If its a 100amp main then it's 20 amps ,, that's it.
You can feed ahead of the main and be higher.
 
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