Max Solar Backfeed on a 200A Residential Service

[timfekete]

What is the maximum amount of solar amperage backfeed allowed on a 200 amp residential service? I've heard conflicting interpretations of the code. Just curious what others are doing. Thanks.

 

[GoldDigger]

There are three different rules that apply to solar backfeed, and you only need to comply with one. Usually the most liberal is the one known as the 120% rule. You take the amapacity of the bus itself inside the panel, (which may be 200A or 225A and it is difficult to determine which is the case for your panel), multiply by 1.2, and subtract the main breaker amperage. For a 200A bus and keeping original 200A main breaker, you get 240-200 = 40A. For a 225A bus, keeping original main breaker, you get 250 - 200 = 50A.
You can increase either number if the calculated load is such that you can substitute a smaller main breaker.

 

[Zee]

200A facst are all good.

In a 225A bus panel:
225A bus ---> 1.2 x 225 = 270 (not 250)

270-200=70

70A breaker for solar (not 50A)
if the main breaker is 200A

 

[timfekete]

I appreciate the responses but I should have been more clear. We are not doing a breaker backfeed. We are doing a line side tap so bus rating doesn't come into play. I am looking for the maximum amount of power I can backfeed a 200 amp residential service utilizing a line side tap configuration. Thanks again.

 

[GoldDigger]

Using a line side tap, the only NEC limit is the nominal size of the service, in this case 200A. But the POCO may have their own limits based on both the service size and the voltage stability of your specific transformer secondary and its primary feed.

 

[electrofelon]

Using a line side tap, the only NEC limit is the nominal size of the service, in this case 200A. But the POCO may have their own limits based on both the service size and the voltage stability of your specific transformer secondary and its primary feed.

Is there any ambiguity here though? I would have to double check the language as I don't remember the exact wording offhand, but doesn't it say "rating of the service"? Is that the ampacity of the service conductors, and what about the 83% sizing?

 

[jaggedben]

I concur that per the NEC the max is 200A, if that is the rating of the service.

Is there any ambiguity here though? I would have to double check the language as I don't remember the exact wording offhand, but doesn't it say "rating of the service"? Is that the ampacity of the service conductors, and what about the 83% sizing?

This is indeed ambiguous in my opinion. Put it this way: maybe the OP needs to settle the question of what the service is really rated. 200A would be a huge system for residential though. Hopefully this is not the controversy.

 

[Hv&Lv]

The 200 amp service starts at the meterbase or main breaker.IMO.
Anything on the line side of the meter you have to get POCO permissions and should be part of your power purchase agreement if there is one..
Back feeding on the line side is also limited by the transformer and the utility wire size. Kva is kva, regardless of which way it’s feeding.

 

[jaggedben]

The 200 amp service starts at the meterbase or main breaker.IMO.
Anything on the line side of the meter you have to get POCO permissions and should be part of your power purchase agreement if there is one..
Back feeding on the line side is also limited by the transformer and the utility wire size. Kva is kva, regardless of which way it’s feeding.

Per the NEC, the point of demarcation is the service point, not the meter or the disconnecting means (unless they happen to be the same).

Around here that would typically that the conductors between the weatherhead and the meter/main need to meet the AHJ's idea of 200A.

 

[Hv&Lv]

Per the NEC, the point of demarcation is the service point, not the meter or the disconnecting means (unless they happen to be the same).

Around here that would typically that the conductors between the weatherhead and the meter/main need to meet the AHJ's idea of 200A.

You don’t have underground there? Some, if not most places for UG services have a demarcation point at the meter.
Also, a line side connection could still be at the top of the weather head.
We have lots of them on our system that are connected on line side.

 

[electrofelon]

You don’t have underground there? Some, if not most places for UG services have a demarcation point at the meter.
Also, a line side connection could still be at the top of the weather head.
We have lots of them on our system that are connected on line side.

We don't have much URD in central ny, so service point is at wx head for overhead, and for underground......I'm actually not quite sure: We run 10 feet of conduit up the pole and leave enough wire to get to the tranny. POCO puts u guard above the conduit. I'm tempted to say service point is where the conduit ends and u guard starts, but I'll bet $1 if the conductors fail under the u guard, they will say they are my conductors.

 

[GoldDigger]

Just as a side note, many POCOs will have a limit on system power (not necessarily backfeed) which forms the dividing line between a residential net metering tariff and a wholesale producer tariff.
This is often a mixture of state law and corporate policy.

 

[electrofelon]

I concur that per the NEC the max is 200A, if that is the rating of the service.



This is indeed ambiguous in my opinion. Put it this way: maybe the OP needs to settle the question of what the service is really rated. 200A would be a huge system for residential though. Hopefully this is not the controversy.

So Ben what in your opinion would be the max backfeed for a 200 amp residential service that has 4/0 AL service conductors? Considering the continuous load adder I doubt there would be a problem going 160A inverter output=200A calculated, but should we keep it at 144A inverter output/180A calculated?

 

[jaggedben]

You don’t have underground there? Some, if not most places for UG services have a demarcation point at the meter.
Also, a line side connection could still be at the top of the weather head.
We have lots of them on our system that are connected on line side.

Of course we have UG here. (I wasn't trying to be exhaustive.) In neighborhoods which were originally built with UG the service point tends to be at the meter/main meter terminals. In places that were converted at some point from OH, there tends to be a transition box that is the service point. Seen a couple weird ones recently where the old service riser still goes up to the roof and then was extended back down to the ground to meet the new transition box.

 

[jaggedben]

So Ben what in your opinion would be the max backfeed for a 200 amp residential service that has 4/0 AL service conductors? Considering the continuous load adder I doubt there would be a problem going 160A inverter output=200A calculated, but should we keep it at 144A inverter output/180A calculated?

I don't have a strong opinion, and I've never had to deal with the question in the field. I've just noticed that the code has never explicitly addressed it. I suppose my initial take would be 'if it's good enough for 200A in, it's good enough for 200A out.' The counter-argument might be 'the allowance for dwellings in 310 basically assumes the current is not continuous, but inverter output would be continuous.'

FWIW, 2020 705.11 changed it from 'rating of service' to 'ampacity of service conductors'. But this just raises a new question of whether the 'ampacity' must be considered before the allowance for dwellings is included, or may be considered after.

 

[shortcircuit2]

I would agree the 144 complies with the Code for 4/0 aluminum

Then hope that meter base and meter doesn't melt under the continuous load...especially if it is in direct sunlight.

 

[Hv&Lv]

I don't have a strong opinion, and I've never had to deal with the question in the field. I've just noticed that the code has never explicitly addressed it. I suppose my initial take would be 'if it's good enough for 200A in, it's good enough for 200A out.' The counter-argument might be 'the allowance for dwellings in 310 basically assumes the current is not continuous, but inverter output would be continuous.'

FWIW, 2020 705.11 changed it from 'rating of service' to 'ampacity of service conductors'. But this just raises a new question of whether the 'ampacity' must be considered before the allowance for dwellings is included, or may be considered after.

My argument would be you better forget the NEC and ask the POCO.

 

[electrofelon]

What is the maximum amount of solar amperage backfeed allowed on a 200 amp residential service? I've heard conflicting interpretations of the code. Just curious what others are doing.

I would agree the 144 complies with the Code for 4/0 aluminum

Then hope that meter base and meter doesn't melt under the continuous load...especially if it is in direct sunlight.

A little off topic, but also kinda on topic: One "trick" I do with almost every resi ground mount I do is replace the meter socket with a class 320 and run another set of service conductors out to the array "structure" using 230.40 exception #3, and connect load side to that service panel (kinda indistinguishable from being considered a supply side connection depending on a few things). The 320 socket isnt really there for capacity (but is nice to use crimped lugs and its heavier duty) but just is a convenient way to split/tap the conductors, and it seems like almost always the existing meter socket is due for replacement anyway. ITs costs a little more that making a tap in a box or wireway, but is all around cleaner looking. Of course you are still limited to that 144/180/200 amps if you arent replacing what is on the line side of the meter socket.

My argument would be you better forget the NEC and ask the POCO.

OF course. I would think it would all get flushed out during the interconnect agreement process, but I guess some who dont know what they are doing would just build it then call the POCO to come hook it up

 

[jaggedben]

My argument would be you better forget the NEC and ask the POCO.

Not sure if you're being facetious or just not following the thread, but...

First, the POCO has no say about NEC governed service conductors, which was the subject of the post you quoted, if you read back.

Second, what kind of say the POCO has depends on state utility regulations. In California I can build anything I please up to 30kW, and as long as it's on a transformer shared by more than one customer, all the POCO can do is tell me to wait a year for them to upgrade the transformer. (Well, that's a big annoyance, but better than being told to pay something...) Of course, your state will vary, and 200A single phase is well over 30kW.

 

[Hv&Lv]

I would think it would all get flushed out during the interconnect agreement process, but I guess some who dont know what they are doing would just build it then call the POCO to come hook it up

One interconnect requirement is testing to make absolutely sure the solar generator doesn’t backfeed onto the lines and potentially get someone killed.

I would hate to know I connected an installation illegally and without the POCO knowledge, and without any regard for the lives of the guys working on the lines.

one can bet lawsuits will be many for the homeowner, electrician, and or solar company...