Load calculations/main OCPD/Solar 120%/subpanel

[ESolar]

Two questions regarding panel sizing with solar and breaker placement. 200A service and panel. Solar feed required derating to 175A.

1. When doing load calcaultions for additional appliances, EV charger etc., is one limited to 175A, or can one still use the 200A limit of the panel and feeder. And if you're limited to 175A, is that answer due to safety concerns? I thought, maybe naively, that the solar output (if any) would offset the demand, which is why the solar OCPD is usually placed at the far end of the bus bar and the bus bar is protected by the main OCPD.

2. Suppose the solar is on a subpanel along with an EV charger. At night it is an EV load, and during the day it is a solar feed. Where is the optimal/proper location for the subpanel feeder/OCPD in the main? (note that, although not important for this question, in the subpanel the solar OCPD is at the far end)

 

[ESolar]

One additional piece of info that has nothing to do with code. The panels are half east facing and half west facing. The calculated maximum 125% amperage is a little over 40, hence the derating. Therefore, the array will NEVER hit the 40 amps associated with the 20% rule to protect the bus bars.

 

[ggunn]

One additional piece of info that has nothing to do with code. The panels are half east facing and half west facing. The calculated maximum 125% amperage is a little over 40, hence the derating. Therefore, the array will NEVER hit the 40 amps associated with the 20% rule to protect the bus bars.

Unfortunately, the NEC does not allow us to consider the anticipated situational inverter output when we are sizing conductors, determining OCPD ratings, and complying with Article 705 rules. Even if we load an inverter only to a DC:AC ratio of 0.5, we still must design everything to the maximum rated output of the inverter(s).

 

[ESolar]

Unfortunately, the NEC does not allow us to consider the anticipated situational inverter output when we are sizing conductors, determining OCPD ratings, and complying with Article 705 rules. Even if we load an inverter only to a DC:AC ratio of 0.5, we still must design everything to the maximum rated output of the inverter(s).

Right. I am aware. My reason for raising that issue was that if the answer to question 1 was that "it is not covered by code" - in that case safety would be the only a concern. Sizing the loads based on a 200 amp capacity is nota safety concern in this case.

 

[tallgirl]

In reality, an array can produce over what you calculate as the maximum output based on orientation and STC nameplate. What the inverter shouldn’t do is produce more than it’s rated for as it’s maximum output. 50% is probably low enough that all the odd little things that will increase array output momentarily won’t reach the inverter’s maximum output, but 80% isn’t.

 

[jaggedben]

Two questions regarding panel sizing with solar and breaker placement. 200A service and panel. Solar feed required derating to 175A.

1. When doing load calcaultions for additional appliances, EV charger etc., is one limited to 175A, or can one still use the 200A limit of the panel and feeder. And if you're limited to 175A, is that answer due to safety concerns? I thought, maybe naively, that the solar output (if any) would offset the demand, which is why the solar OCPD is usually placed at the far end of the bus bar and the bus bar is protected by the main OCPD.

You're limited to 175A. The idea behind load calculations is to make sure the service or feeder rating is sized big enough that the breaker protecting it won't trip. The ancillary saftey issue, from a very general standpoint, is to make sure people don't jerry rig inadequate electrical installations to try to get more juice out of them than the wires are safe for. That's how fires start. A case in point here: You know what's going to happen if the homeowner really needs 200A and not 175? Their 175A breaker will trip, and then they'll have you or someone else back out to put the 200A back in, and then the installation will violate code.

2. Suppose the solar is on a subpanel along with an EV charger. At night it is an EV load, and during the day it is a solar feed. Where is the optimal/proper location for the subpanel feeder/OCPD in the main? (note that, although not important for this question, in the subpanel the solar OCPD is at the far end)

I don't think it makes a meaningful difference.

 

[jaggedben]

One additional piece of info that has nothing to do with code. The panels are half east facing and half west facing. The calculated maximum 125% amperage is a little over 40, hence the derating. Therefore, the array will NEVER hit the 40 amps associated with the 20% rule to protect the bus bars.

If it's a string type inverter (including Solaredge), maybe just spec a smaller inverter.

For Enphase microinverters, you can now install consumption CTs that support Power Control Systems. The gateway will tell the microinverters to throttle output so that it never goes continuously over the code allowed max. In your east west configuration, the PCS will probably never have to kick in anyway, but you can pass inspection. This works for other inverters, too, if you don't like the first idea. Only thing left to do is explain it to persnickety customers.

 

[ESolar]

If it's a string type inverter (including Solaredge), maybe just spec a smaller inverter.

For Enphase microinverters, you can now install consumption CTs that support Power Control Systems. The gateway will tell the microinverters to throttle output so that it never goes continuously over the code allowed max. In your east west configuration, the PCS will probably never have to kick in anyway, but you can pass inspection. This works for other inverters, too, if you don't like the first idea. Only thing left to do is explain it to persnickety customers.

For which version of enphase microinverter are those available. The inverters on the install are a mix of 6 and 7.

 

[jaggedben]

For which version of enphase microinverter are those available. The inverters on the install are a mix of 6 and 7.

7 and 8 for sure. Doubtful about 6, you could ask them.

 

[ESolar]

You're limited to 175A. The idea behind load calculations is to make sure the service or feeder rating is sized big enough that the breaker protecting it won't trip. The ancillary saftey issue, from a very general standpoint, is to make sure people don't jerry rig inadequate electrical installations to try to get more juice out of them than the wires are safe for. That's how fires start. A case in point here: You know what's going to happen if the homeowner really needs 200A and not 175? Their 175A breaker will trip, and then they'll have you or someone else back out to put the 200A back in, and then the installation will violate code.


I don't think it makes a meaningful difference.

I am still not convinced that one is restricted to a 175A load for the case presented. Instead, the 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 derated (from 200A) 175A OCPD conservatively protects the 200A feeder wire. It was derated to protect the bus bar, not the feeder, in the presence of a solar feed. That derating calculation specifies that the solar feed is limited to (120% of Bus Bar Rating A) - (Main OCPD A). The calculated derating is to protect under conditions when the load (sans solar) on the panel may be more than 120% of the busbar rating (200A) and not trip the OCPD (because solar reduced what the OCPD sees). The derating to 175A was not designed to limit the load beyong that limited by the 200A panel and feeder. The code says nothing about reevaluating and reducing your load (which might exceed 175A) when adding solar - it instead specifies limits on the breaker (175A) to protect the busbar.

I think we need a NEC expert to answer this because limiting to 175A seems contrary to the math and safety concerns.

 

[ESolar]

Also: Granted, if the load exceeds 175A when there is no solar being produced, the breaker may trip - but it was derated to protect the busbar when the load exceeds 240A with solar pumping out more than 40A.

 

[jaggedben]

Your last post gets it right. The one before that is confused.

You are correct that the load is not the reason you are downsizing. But you must still verify that the calculated load doesn't exceed 175A because that is what is now available to serve the loads. If you could claim that your interconnected source is as reliably available as the grid to serve a portion of the load, then that might change the argument. That might apply to an industrial site with cogeneration. But for the time being it doesn't apply to typical residential solar systems.

Btw, the feeder cannot see current in two directions at the same time, so it is moot here.

 

[tallgirl]

Your last post gets it right. The one before that is confused.

You are correct that the load is not the reason you are downsizing. But you must still verify that the calculated load doesn't exceed 175A because that is what is now available to serve the loads. If you could claim that your interconnected source is as reliably available as the grid to serve a portion of the load, then that might change the argument. That might apply to an industrial site with cogeneration. But for the time being it doesn't apply to typical residential solar systems.

Btw, the feeder cannot see current in two directions at the same time, so it is moot here.

Likewise, the bus bar isn’t carrying current from the solar in one direction, and the grid in the other. In the hypothetical situation where the solar is producing the limit, whatever current is used by the 2nd to bottom breaker comes from the solar, up to what it’s producing, and so on up the bus bar towards the main OCPD.

 

[ESolar]

Likewise, the bus bar isn’t carrying current from the solar in one direction, and the grid in the other. In the hypothetical situation where the solar is producing the limit, whatever current is used by the 2nd to bottom breaker comes from the solar, up to what it’s producing, and so on up the bus bar towards the main OCPD.

At this point, after the education, I agree with everything said.
It is nevertheless interesting to consider how conservative the code is for this problem. In this case, the code protects against a load that is > 240A that may not be seen by the main OCPD due to the solar feed. The derating to 175A is too achieve that end. Therefore, the code is expecting that the load may be much (65A) greater than 175A, and yet one still has to limit the load to 175A (a 65A cushion). Take the extreme: Imagine that I have two 100A (200A) solar feeds on that panel - I derate to 40A. Now I am limited to a 40A load. Clearly the busbars can take 200A more in this case - now I have a 200A cushion on my load. The only solutions to thisproblem seem to be a line side connection or a new panel with a much higher rating on the busbars.

 

[wwhitney]

Take the extreme: Imagine that I have two 100A (200A) solar feeds on that panel - I derate to 40A. Now I am limited to a 40A load. Clearly the busbars can take 200A more in this case - now I have a 200A cushion on my load. The only solutions to thisproblem seem to be a line side connection or a new panel with a much higher rating on the busbars.

If you have 200A of solar (160A of continuous inverter output) on a 200A service, then the 120% rule is the wrong way to do the interconnect. You need to either do a line-side connection, or do a feeder tap on a 200A feeder.

Cheers, Wayne

 

[tortuga]

Take the extreme: Imagine that I have two 100A (200A) solar feeds on that pane

The last time I had a residential project with over 100A of solar inverter output I used a 400A commercial panelboard, with commercial panels you can order all kinds of handy options, I got a 400A frame breaker with a 200A trip unit, so the service main was still 200A. Sure commercial panels cost allot more but they sure simplify things in the end, like having the proper size lugs for all your grounding wires etc..

 

[tallgirl]

At this point, after the education, I agree with everything said.
It is nevertheless interesting to consider how conservative the code is for this problem. In this case, the code protects against a load that is > 240A that may not be seen by the main OCPD due to the solar feed. The derating to 175A is too achieve that end. Therefore, the code is expecting that the load may be much (65A) greater than 175A, and yet one still has to limit the load to 175A (a 65A cushion). Take the extreme: Imagine that I have two 100A (200A) solar feeds on that panel - I derate to 40A. Now I am limited to a 40A load. Clearly the busbars can take 200A more in this case - now I have a 200A cushion on my load. The only solutions to thisproblem seem to be a line side connection or a new panel with a much higher rating on the busbars.

Don't get too carried away. I was explaining the direction of current flow, not whether or not you could have more than the rated total current flowing through those bars, just because some of it's only on one half, and some of it is only on the other half.

Heat will be generated by whatever current is flowing, regardless of direction. Thermally, the answer is 200 amps -- regardless of direction -- for continuous total loads and total production.

 

[ggunn]

One thing that is worth knowing is that Eaton Cutler-Hammer 150A - 225A panels all have the same 225A busbars.

 

[jaggedben]

One thing that is worth knowing is that Eaton Cutler-Hammer 150A - 225A panels all have the same 225A busbars.

This is the CH line, right?

 

[ggunn]

This is the CH line, right?

Yes.