2017 NEC 705.12(A)

[ggunn]

The code says "The sum of the ratings of all overcurrent devices connected to power production sources shall not exceed the rating of the service." I just encountered a service with a 400A panel with a 400 MCB that is supplied by a 150kVA pole mounted transformer bank, and the city is still under the 2017 NEC.

Without looking at the transformer, some might think that it's a 400A service, but (150kVA) / (480V)((sqrt(3)) = 180.4A. I talked to an engineer for the city, and he just told me "it happens". Be careful out there.

 

[tortuga]

That is a very typical install, there is a building I work on that has a 800A 480V service and 75kva bank.

 

[Carultch]

The utility isn't governed by the NEC. It is very common that utility-owned transformers are significantly undersized compared to the size you'd need to use for a customer-owned transformer in an otherwise-identical situation. If you propose a system that exceeds the capacity of the service transformer, the utility would likely require your project to pay for an upgrade to the transformer, or the utility would likely limit you to the existing unit's rating.

 

[jaggedben]

Likely not an NEC problem, but quite likely to be an interconnection application problem if you intended to use the entire service capacity.

 

[electrofelon]

....some might think that it's a 400A service....

It is a 400A service.

 

[ggunn]

It is a 400A service.

Yes, but connecting a 400A PV system to it would be unwise. Even when loads are active there would be a high probability that the system would export more than the full load rating of the transformer.

 

[jim dungar]

Yes, but connecting a 400A PV system to it would be unwise. Even when loads are active there would be a high probability that the system would export more than the full load rating of the transformer.

That would be something the utility would need to consider, it is beyond the scope of the NEC. This is why PV system installations needs to be coordinated with the POCO.

But if you can pull 400A through the utility transformer, why can't you push the same amount back through it?

 

[ggunn]

That would be something the utility would need to consider, it is beyond the scope of the NEC. This is why PV system installations needs to be coordinated with the POCO.

But if you can pull 400A through the utility transformer, why can't you push the same amount back through it?

You can't pull 400A through a 150kVA transformer; the max full current is 180.4A. CPS told me that 115% of full current rating is their maximum load.

 

[tortuga]

One of the first things I do when I look over a new commercial or industrial project is get the utility transformer size and fault current, and in the entire time I have been doing this I have never once seen a transformer match the main breaker size.

 

[ggunn]

One of the first things I do when I look over a new commercial or industrial project is get the utility transformer size and fault current, and in the entire time I have been doing this I have never once seen a transformer match the main breaker size.

Same here. We got burned once by not looking at the transformer rating and noticing that it was the supply for both the businesses we had built PV systems for. We had to eat the cost of upgrading the transformer. Tuition in the School of Experience.

 

[electrofelon]

Same here. We got burned once by not looking at the transformer rating and noticing that it was the supply for both the businesses we had built PV systems for. We had to eat the cost of upgrading the transformer. Tuition in the School of Experience.

I remember that story, but that seems like a pretty unusual case. I mean don't you wait to hear from the utility thru the interconnection agreement process about any equipment upgrade charges before throwing out prices? Or did they originally say they didn't have to do upgrades but then change their mind?

 

[jaggedben]

But if you can pull 400A through the utility transformer, why can't you push the same amount back through it?

To make up somewhat realistic scenario...
If the main breaker is rated 400A, and the transformer is rated 180A, and the load averages, say, 80A, with occasional short duration jumps into the 200-300A range, or even to almost 400A more occasionally, then the transformer will survive. But when a PV system with a 320A output is connected, meaning the site now exports an average of 240A for up to 8 hours a day, the transformer will likely bite the dust pretty fast.

Note that with my (vastly simplistic, but realistic enough) numbers, the daily PV production equals the daily consumption of the loads, so the PV system is not oversized for the load on a kWh basis.

 

[ggunn]

I remember that story, but that seems like a pretty unusual case. I mean don't you wait to hear from the utility thru the interconnection agreement process about any equipment upgrade charges before throwing out prices? Or did they originally say they didn't have to do upgrades but then change their mind?

That's the thing; we had interconnection agreements from the utility. The two systems were separate projects, permitted separately, and no one at the utility noticed that the two services shared a single transformer until we applied for permission to operate. We tried to claim that since the utility had granted the applications that they should be on the hook for the upgrade, but that did not fly. We had two unhappy customers (actually they were part of the same company) who had PV systems installed who could not get their systems turned on, so we were not in a strong bargaining position. We bought the transformer.

It could be argued (and the utility did) that we should have seen the issue in our site assessments. We were not paying attention to utility transformer capacity before then, but we certainly are now.

 

[electrofelon]

That's the thing; we had interconnection agreements from the utility. The two systems were separate projects, permitted separately, and no one at the utility noticed that the two services shared a single transformer until we applied for permission to operate. We tried to claim that since the utility had granted the applications that they should be on the hook for the upgrade, but that did not fly. We had two unhappy customers (actually they were part of the same company) who had PV systems installed who could not get their systems turned on, so we were not in a strong bargaining position. We bought the transformer.

It could be argued (and the utility did) that we should have seen the issue in our site assessments. We were not paying attention to utility transformer capacity before then, but we certainly are now.

I would say that is BS, especially for the utility to say "you should have noticed our transformer......" it's their damn equipment and it's up to them to tell you a price for a project. For them to give you a design and price and then reneg on it later is completely unacceptable. That would almost be worth going to court for just as a matter of principle.

 

[ggunn]

I would say that is BS, especially for the utility to say "you should have noticed our transformer......" it's their damn equipment and it's up to them to tell you a price for a project. For them to give you a design and price and then reneg on it later is completely unacceptable. That would almost be worth going to court for just as a matter of principle.

BS or not, it happened the way I described, though I do not know what you mean by them giving us "a design and price". Like I said, I can see how they missed it; we submitted the designs for permitting separately, and the transformer was plenty big for either system alone. No one, including us, realized that both facilities were on the same transformer until both PV systems were built and ready to interconnect. This was years ago when we were just learning how to do commercial PV; it was an expensive lesson.

 

[wwhitney]

BS or not, it happened the way I described, though I do not know what you mean by them giving us "a design and price". Like I said, I can see how they missed it; we submitted the designs for permitting separately, and the transformer was plenty big for either system alone. No one, including us, realized that both facilities were on the same transformer until both PV systems were built and ready to interconnect.

I think electrofelon's point was that the responsibility for catching the "same transformer" issue lies squarely on the POCO in any fair and rational system.

Suppose two different companies were doing PV installs on the two services and each had separately submitted for interconnection approval. There's no way for any party involved other than the POCO to catch the issue. That's the whole point of submitting to the POCO before the work starts.

The fact that one company did both submissions doesn't really change that. Sure you could have noticed, but the interconnection process has to be designed to put the responsibility on the POCO.

Cheers, Wayne

 

[electrofelon]

BS or not, it happened the way I described, though I do not know what you mean by them giving us "a design and price". Like I said, I can see how they missed it; we submitted the designs for permitting separately, and the transformer was plenty big for either system alone. No one, including us, realized that both facilities were on the same transformer until both PV systems were built and ready to interconnect. This was years ago when we were just learning how to do commercial PV; it was an expensive lesson.

By "BS" I didn't mean I thought you were modifying events of the story, that was directed at the POCO.

I don't see you did anything wrong and I don't see what you could have even done differently. The POCOs transformer is none of your concern. Many times we don't even know where the transformer is or it's in a underground vault filled with water in the middle of a busy street.

I've been in the electrical field for 24 years now, and when dealing with the power company , they always provide a cost for new services, modifications, and DG interconnections. That is what I mean by "design and price.". The design is on their side of the service point, I'm not going to do that for them.

 

[ggunn]

I think electrofelon's point was that the responsibility for catching the "same transformer" issue lies squarely on the POCO in any fair and rational system.

Suppose two different companies were doing PV installs on the two services and each had separately submitted for interconnection approval. There's no way for any party involved other than the POCO to catch the issue. That's the whole point of submitting to the POCO before the work starts.

The fact that one company did both submissions doesn't really change that. Sure you could have noticed, but the interconnection process has to be designed to put the responsibility on the POCO.

Cheers, Wayne

I cannot comment on what I think should have happened, only what did and the lesson we learned from the experience.

 

[electrofelon]

I cannot comment on what I think should have happened, only what did and the lesson we learned from the experience.

What exactly is the lesson and what would you do differently? If that POCO makes changes after the fact then it sounds like you are always at their mercy and anything could change.

 

[ggunn]

What exactly is the lesson and what would you do differently? If that POCO makes changes after the fact then it sounds like you are always at their mercy and anything could change.

The lesson is to thoroughly check out utility transformers when performing a site assessment for a proposed PV system rather than assuming that the service can handle it from only looking at the MDP.

I don't know what you mean by "If that POCO makes changes after the fact..." Everyone involved, including the POCO, missed that both buildings were fed from the same transformer and that the sum of the two separately permitted (on separate meters) PV systems' AC kW ratings exceeded the kVA of the transformer. Nothing was changed after the fact.