Derating main OCPD with solar

[ESolar]

Am I correct?: One concern with adding solar is that the backfed solar source can allow the loads on the bus bar to be greater than the OCPD without tripping because they are offset by the solar. As a result, the busbar rating may be exceeded by the loads causing expensive and dangerous catastrophic failure. The derating of the main OCPD, if necessary, specifies that the solar feed is limited to (120% of Bus Bar Rating A) - (Main OCPD A). The derating is to protect under conditions when the load may be more than 120% of the busbar rating and not trip the OCPD (because solar reduced what the OCPD sees). The derating is not designed to limit the load beyong that limited by the panel and feeder. The code says nothing about reevaluating and reducing your load (which might exceed derated OCPD) when adding solar - it instead specifies limits on the OCPD to protect against a load significantly exceeding the busbar rating without tripping.

 

[wwhitney]

The code says nothing about reevaluating and reducing your load (which might exceed derated OCPD) when adding solar

No, but it's an obvious logical consequence. The breaker size is always required to be at least as large as the calculated load. So when you put in a new breaker, you need to perform a load calculation to confirm the new size is still at least the calculated load.

Cheers, Wayne

 

[ggunn]

Am I correct?: One concern with adding solar is that the backfed solar source can allow the loads on the bus bar to be greater than the OCPD without tripping because they are offset by the solar. As a result, the busbar rating may be exceeded by the loads causing expensive and dangerous catastrophic failure. The derating of the main OCPD, if necessary, specifies that the solar feed is limited to (120% of Bus Bar Rating A) - (Main OCPD A). The derating is to protect under conditions when the load may be more than 120% of the busbar rating and not trip the OCPD (because solar reduced what the OCPD sees). The derating is not designed to limit the load beyong that limited by the panel and feeder. The code says nothing about reevaluating and reducing your load (which might exceed derated OCPD) when adding solar - it instead specifies limits on the OCPD to protect against a load significantly exceeding the busbar rating without tripping.

Well, if you apply Kirchoff's current law to the busbar, even when the OCPD rating is the same as that of the busbar and the solar breaker on the other end is the same, even though there is potentially 2X the amount of total available current than the busbar rating, no point on the busbar can get more than it is rated for. A "finger" of the busbar could if a breaker failed during a fault, but not the busbar itself.

 

[ESolar]

No, but it's an obvious logical consequence. The breaker size is always required to be at least as large as the calculated load. So when you put in a new breaker, you need to perform a load calculation to confirm the new size is still at least the calculated load.

Cheers, Wayne

The point of the derating (e.g., often by 50 amps on a 200A panel) is to protect the busbars when the load is GREATER than their rating and offset by solar. In other words, the busbars and feeders are still proteced under the old load calculation for 200A - that was the point of the derating. I can see that the derating might cause tripping with the old load. But it would not appear to be a safety issue.

 

[wwhitney]

The point of the derating . . .

Making a change to the system to comply with one NEC rule doesn't change the fact that you have to comply with all the other NEC rules.

Cheers, Wayne

 

[wwhitney]

Well, if you apply Kirchoff's current law to the busbar . . .

That's correct, but the 120% rule is based on heat dissipation, not current in a cross section of the busbar. It was arbitrarily decided that ~4% additional worst-case heating would be OK without any additional testing.

Cheers, Wayne

 

[ggunn]

That's correct, but the 120% rule is based on heat dissipation, not current in a cross section of the busbar. It was arbitrarily decided that ~4% additional worst-case heating would be OK without any additional testing.

Cheers, Wayne

Of course, but the operative word is "arbitrarily"..

 

[ESolar]

Making a change to the system to comply with one NEC rule doesn't change the fact that you have to comply with all the other NEC rules.

Cheers, Wayne

I agree - rules are rules. But the result is likely to be extremely conservative. For example, take an extreme: A 200A panel, 200A solar feed, and a 40A derated main breaker. Now the load, with all of that potential solar offset is limited to only 40A even though you're producing 200A of solar.

 

[ggunn]

I agree - rules are rules. But the result is likely to be extremely conservative. For example, take an extreme: A 200A panel, 200A solar feed, and a 40A derated main breaker. Now the load, with all of that potential solar offset is limited to only 40A even though you're producing 200A of solar.

I'm not sure what you are saying. Derating a MCB to accommodate more solar backfeed is done to comply with 705.12(B)(3), which says nothing about loads.

 

[winnie]

As I understand the example, if you have a 200A panel with a 200A solar feed, you would need to reduce the main breaker to 40A to comply with 705.12(B)(3).

But as a consequence of having a 40A main breaker you would be stuck having a calculated load of less than 40A.

IMHO this is a crazy result that comes from a crazy misapplication of 705.12(B)(3). This bit of code is a permission to allow a solar feed breaker on the main panel, but there are other ways to tie in the solar. In this misapplied example, you have a 200A PV breaker, but only an expected 40A of load and a 40A path to the grid. In other words this example installation simply wouldn't work.

In the real world, most '200A' panels actually have 225A or even 250A bus bars. And a 40KW PV install is unlikely at a home with a 200A panel...and if you are actually installing a 40KW PV system you can probably afford to upgrade the panel appropriately.

Jon

 

[ggunn]

As I understand the example, if you have a 200A panel with a 200A solar feed, you would need to reduce the main breaker to 40A to comply with 705.12(B)(3).

But as a consequence of having a 40A main breaker you would be stuck having a calculated load of less than 40A.

IMHO this is a crazy result that comes from a crazy misapplication of 705.12(B)(3). This bit of code is a permission to allow a solar feed breaker on the main panel, but there are other ways to tie in the solar. In this misapplied example, you have a 200A PV breaker, but only an expected 40A of load and a 40A path to the grid. In other words this example installation simply wouldn't work.

In the real world, most '200A' panels actually have 225A or even 250A bus bars. And a 40KW PV install is unlikely at a home with a 200A panel...and if you are actually installing a 40KW PV system you can probably afford to upgrade the panel appropriately.

Jon

You realize that when I brought up the scenario of a 200A panel (MCB and busbar) with a 200A solar backfed breaker I was only talking about an extreme example to illustrate Kirchoff's current law, right? Of course no one would ever try to do that.

 

[winnie]

You realize that when I brought up the scenario of a 200A panel (MCB and busbar) with a 200A solar backfed breaker I was only talking about an extreme example to illustrate Kirchoff's current law, right? Of course no one would ever try to do that.

I didn't comment separately on your analysis, but I like it.

I was commenting on ESolar's example which was also a 200A solar feed forcing a reduction of the main.

Jon