Solar Backfeed Question

[charlie b]

I received this from a new member via Private Message. I am posting it on that member's behalf.

In order to gain capacity for a solar backfeed beyond the 120% buss capacity rule, it is possible to reduce the main breaker size, adding the difference between the buss rating and the breaker rating to the backfeed capability. Where can I find the reference to this in the NEC?? I've spent hours trying to find it, but can't seem to locate it!! If you can't help, I guess I'll have to wait till I have postiing permission. But this is an urgent question - I have to demonstrate this is legit to a local AHJ on a pending job.

Thanks
Joe Utasi

 

[ggunn]

I received this from a new member via Private Message. I am posting it on that member's behalf.

The reference is 705.12(D)(2)(b) in the 2014 NEC. Note that it doesn't explicitly say you can downgrade the main breaker, only that the sum of the breakers feeding a bus when the feeds are at opposite ends cannot exceed 120% of the bus rating.

 

[GoldDigger]

Note carefully those two words "bus rating". In some cases, for reasons known only to the manufacturer although we can speculate, the amp rating of the bus will be greater than the rating of the panel and the main breaker that came installed in it.
For example, a 225A bus in a 200A load center. If you can find documentation on the bus rating, you may have more PV headroom than you thought.

 

[mwm1752]

It always kinda made me wonder why they took that section out of the 690 article in 2011 & made it a reference to 705.



"Our job is to solve complicated problems not to complicate solved problems" COA

 

[ggunn]

It always kinda made me wonder why they took that section out of the 690 article in 2011 & made it a reference to 705.

It's something else about which we can only speculate, but it seems to me that it's because 690 is solar only and 705 has to do with all interconnected alternative power sources. The 120% rule applies to all alternative sources, not just solar.

 

[jaggedben]

It always kinda made me wonder why they took that section out of the 690 article in 2011 & made it a reference to 705.

Because the rules in 705 apply across the board to solar inverters, wind inverters, and fuel cell inverters. Repeating them in 692 and 694 would add ten unnecessary pages to the code.

 

[jaggedben]

If you can find documentation on the bus rating, ...

Usually the label is either still there or it isn't...

 

[Carultch]

Note carefully those two words "bus rating". In some cases, for reasons known only to the manufacturer although we can speculate, the amp rating of the bus will be greater than the rating of the panel and the main breaker that came installed in it.
For example, a 225A bus in a 200A load center. If you can find documentation on the bus rating, you may have more PV headroom than you thought.

Busbars are made in fewer available ratings than breakers. So it is very common to have a 300A MCB with a 400A bus, or a 500A MCB with a 600A bus.

It can be very tricky to get a straight answer from looking at the equipment. Often you need to find the manufacturer's job number to then request further information.

 

[Carultch]

The reference is 705.12(D)(2)(b) in the 2014 NEC. Note that it doesn't explicitly say you can downgrade the main breaker, only that the sum of the breakers feeding a bus when the feeds are at opposite ends cannot exceed 120% of the bus rating.

In 2014 NEC, they also modified the language so that rounding errors are not a show stopper.

Instead of using the PV interconnection breaker, you instead use the rated current at interconnection multiplied by 125%. Rather than needing to then round this number up to a standard sized breaker.


Example:
600A bus, 600A main. 96A (AC) of desired solar to interconnect.


For the 96A of solar, you'd need a 125A breaker. However, (125A + 600A)/600A is greater than 120%, and if it is breaker size that drives the rule, this is a show stopper.

By contrast, the new language of the code allows you to not need to round up to 125A for this calculation. 96A * 1.25 = 120A. And (120A + 600A)/600A = 1.2. The NEC allows this calculation for 705.12(D).

 

[mwm1752]

It's something else about which we can only speculate, but it seems to me that it's because 690 is solar only and 705 has to do with all interconnected alternative power sources. The 120% rule applies to all alternative sources, not just solar.

Because the rules in 705 apply across the board to solar inverters, wind inverters, and fuel cell inverters. Repeating them in 692 and 694 would add ten unnecessary pages to the code.

I do agree with both of you -- The code should be convienent to use - not all installers have to use the entire book -- 10 pages may be a strectch but is minimal within 1260 pages (0.79%)when addressing a particular subject. Just an opinion

 

[ggunn]

I do agree with both of you -- The code should be convienent to use - not all installers have to use the entire book -- 10 pages may be a strectch but is minimal within 1260 pages (0.79%)when addressing a particular subject. Just an opinion

Everyone in the trades needs to be familiar with the entire NEC, with all its faults and virtues. If 690 were to contain everything one needs to know to design and install a compliant PV system, with grounding, conductor sizing, conduit fill, etc., it would be huge.

That said, I really wish it would unambiguously address some PV specific issues, such as whether an AC disco on a supply side interconnected UL 1741 compliant PV system is or is not a service entrance and how it should be wired. And no, I do NOT want to start that fight again! :happyno:

 

[Shawn pavich]

In ca we are allowed to derate the main some AHJ want a load calc in order to derate of course you can't go lower then 100 in most cases we derate 200 amps to 175 to get that extra 25 amps so now I can backfeed 65 amps those 175 amp mains are not cheap but they are more cost effective then a service change

 

[Carultch]

In ca we are allowed to derate the main some AHJ want a load calc in order to derate of course you can't go lower then 100 in most cases we derate 200 amps to 175 to get that extra 25 amps so now I can backfeed 65 amps those 175 amp mains are not cheap but they are more cost effective then a service change

Another way you can do it, is with a recording meter to get 15 minute interval power data for an extended time period, that can justify reducing the main breaker for realistic load conditions. Uncommon that the utility would have this information for a residential service, but very likely for a commercial service. Since commercial services are commonly given a demand charge, for having power consumption that spikes well above the base load.

The academic load calculations in the NEC are conservative, and even if they yield a 200A service, in the real world, you might never see anything higher than 100A or 125A. It is also common that the person who built the house originally, rounded up within the standard equipment frame size. So instead of a theoretical 150A main, they might install a 200A main, which allows for future expansion of loads.

A much less ethical way of doing it, is to mark the main breaker as an "existing 175A main" on your permit set, even though it didn't start that way. It sure as heck is going "existing" by the time the inspector gets there. Worst case scenario, you'd nuisance trip the main breaker. Disclaimer: this particular idea is for entertainment purposes only, not serious construction advice.

 

[jaggedben]

Another way you can do it, is with a recording meter to get 15 minute interval power data for an extended time period, that can justify reducing the main breaker for realistic load conditions. Uncommon that the utility would have this information for a residential service,...

Actually PG&E customers can get this data simply by registering their account on the website and using the usage viewing features. I don't think it's readily exportable or anything like that, but it's there...

 

[Carultch]

Actually PG&E customers can get this data simply by registering their account on the website and using the usage viewing features. I don't think it's readily exportable or anything like that, but it's there...

Hmmm... interesting that PG&E customers have access to this data with a simple web login, let alone that PG&E residential meters have the proper hardware to measure and track data this granular. WHAT A CONCEPT!

What is the difference between demand and usage? I understand demand to be the 15-minute average kilowatt rate at which a service consumes power. So if it consumes 10 kW-hr in 15 minutes, that is 40 kW of demand. Demand is the same for a continuous 40 kW steady load, as it is for a load that spikes at 100 kW for less than a second, yet still averages to 40 kw in 15 minutes.

In the big picture of over 8000 hours in a year, this might as well be an instantaneous quantity. And in a practical sense, you need to pick some time period to establish a power rate, and that is every 15 minutes.

 

[jaggedben]

Hmmm... interesting that PG&E customers have access to this data with a simple web login, let alone that PG&E residential meters have the proper hardware to measure and track data this granular. WHAT A CONCEPT!

What is the difference between demand and usage? I understand demand to be the 15-minute average kilowatt rate at which a service consumes power. So if it consumes 10 kW-hr in 15 minutes, that is 40 kW of demand. Demand is the same for a continuous 40 kW steady load, as it is for a load that spikes at 100 kW for less than a second, yet still averages to 40 kw in 15 minutes.

In the big picture of over 8000 hours in a year, this might as well be an instantaneous quantity. And in a practical sense, you need to pick some time period to establish a power rate, and that is every 15 minutes.

Now that I think about it again, the data available to customers online might be hourly instead of every 15 min. Might still be good enough to convince an AHJ that further load analysis is not needed though.

I don't even have our account set up to use it. I mostly know about because the salespeople at work will walk potential customers through it in order to get their usage to come up with PV system sizing. They don't normally look at hourly usage, but it's in there. I remember it mainly because in one case we had a customer who thought his PV system stopped working because his bill had gone up, and we were able to use the hourly data to prove to him that the real problem was, apparently, a relative who had moved in and started using massive amounts of energy in the middle of the night. (We didn't ask about the relative's activities. )

 

[Carultch]

Now that I think about it again, the data available to customers online might be hourly instead of every 15 min. Might still be good enough to convince an AHJ that further load analysis is not needed though.

I don't even have our account set up to use it. I mostly know about because the salespeople at work will walk potential customers through it in order to get their usage to come up with PV system sizing. They don't normally look at hourly usage, but it's in there. I remember it mainly because in one case we had a customer who thought his PV system stopped working because his bill had gone up, and we were able to use the hourly data to prove to him that the real problem was, apparently, a relative who had moved in and started using massive amounts of energy in the middle of the night. (We didn't ask about the relative's activities. )

I remember using my electric meter as a clue to figure out that the well pump was not working.

I noticed the electric bill went up for a couple of months, and checked each breaker to see if turning it off would have a sudden slow-down of the disc. And sure enough, it was the breaker for the well pump, because half the water it was pumping, was leaking back in to the ground.