Help with inverter installation

[sfav8r]

We are installing a 54Kw inverter that is for the sole purpose of eliminating short term peak demand. The system monitors the current flow at the main panel and when it senses a surge, it begins supplying power from the inverters until the surge is gone. This is very short term (minutes). The system is fed from a 100a breaker at the main panel. We install a 3 phase disconnect, then a new sub panel with three 30a 2-pole breakers (one for each of the inverters 3 18kw inverters for a total of 54kw). We have done a number of these installations in various jurisdictions and have never had a problem. We are currently in a jurisdiction that is requiring some changes that don't make sense to me. Essentially they want the wire, disconnects and panels rated for 200a, not 100a. Their position is that the panels is fed with 100a from the main panel breaker and 100a from the batteries (inverter) so it needs to be rated at 200a. We, as well as the manufacturer have explained that it is only one direction or the other, that the current cannot be flowing in to charge the batteries and out to feed the utility at the same time. They insist that that fact is irrelevant and want everything rated at 200a. they are citing 705.12 as a reference. It should be noted that this equipment CANNOT back feed to the utility. It only REDUCES the load temporarily. I cannot follow their logic at all. Even if we were to conceed that the new 3 phase panel would be rated at 200a (which it is anyway) who would the wire feeding the panel need to be rated at 200a if it is proteted with a 100 breaker. It simply doesn't make sense unless I'm really not understanding what the review engineer is saying.

This is a big issue because we have several more installs to do in this jurisdiction and some of the wire runs are quite long. The impact on installation cost would be significant.

Any help is appreciated.

And Merry Christmas to everyone!

 

[GoldDigger]

What they appear to be doing is applying a rule that the sum of all current sources on the bus cannot exceed 100% of the bus rated current.
This does NOT make any sense when the load is limited to less than the bus rating and the individual feeds are on opposite ends of the bus with the loads in the middle.
Now for PV (and by extension for other grid interactive inverters, which is a separate code article from the DC side of the inverter), the compromise is what is known in the PV trade as the 120% rule. That states that you may take the bus rating (which may be 225A for a nominal 200A panel; check it out!) and multiply that by 1.2.
Then as long as the utility feed and the inverter feed are at opposite ends of the panel bus, the sum of the main breaker and the backfeed breaker from the inverter can be up to that number.
So if you have a 200A panel with a 225A bus, your working number is 225 x 1.2 = 270. If you use a 200A main breaker, you can then add up to 70A of backfeed at the opposite end of the bus.


Their concern really does make sense, in that if there is a problem with the loads, the load on the panel bus can increase until it reaches the sum of the two breakers before even coming close to a breaker trip. If the main and backfeed were located together at one end of the bus, the total (abnormal) current would be flowing in the portion of the bus from the sources to the first load. Maybe even farther.
If you put the two sources at opposite ends, the current on the bus can never exceed the greater of the two OCPD values.
By this reasoning, you could have both main and backfeed equal to the bus rating, but the CMP was not comfortable with that an compromised with the 120% rule instead.

Now if you have a 200A panel with a 200A bus, you are limited to only 40A of backfeed. So by all means check your panel.

Unfortunately in the 2011 NEC the 120% rule is in 705.12(D), and 705.2 seems to exclude battery driven systems from the definition of hybrid systems and maybe from the scope of 705 entirely.
But you may be able to introduce this to persuade the AHJ that if you satisfy the 120% rule you are OK.
There is no way that you will be able to talk them into accepting more than that though, IMHO.

Now the special case of a panelboard which has only a main breaker, a backfeed breaker and a feeder breaker to the assorted loads in another panel may offer you an escape because you know exactly what the load limit on the panel is. You might then also have to add a placard stating that no additional load breakers may be installed.
With multiple load breakers you have more of a problem, since the calculated load can be lower then the sum of the breaker values, but that means that the sum of the breaker values can easily be greater than the bus rating. Normally the main breaker will then be the enforcer of the bus limit. But you lose that when you add backfeed.

 

[sfav8r]

What they appear to be doing is applying a rule that the sum of all current sources on the bus cannot exceed 100% of the bus rated current.
This does NOT make any sense when the load is limited to less than the bus rating and the individual feeds are on opposite ends of the bus with the loads in the middle.
Now for PV (and by extension for other grid interactive inverters, which is a separate code article from the DC side of the inverter), the compromise is what is known in the PV trade as the 120% rule. That states that you may take the bus rating (which may be 225A for a nominal 200A panel; check it out!) and multiply that by 1.2.
Then as long as the utility feed and the inverter feed are at opposite ends of the panel bus, the sum of the main breaker and the backfeed breaker from the inverter can be up to that number.
So if you have a 200A panel with a 225A bus, your working number is 225 x 1.2 = 270. If you use a 200A main breaker, you can then add up to 70A of backfeed at the opposite end of the bus.


Their concern really does make sense, in that if there is a problem with the loads, the load on the panel bus can increase until it reaches the sum of the two breakers before even coming close to a breaker trip. If the main and backfeed were located together at one end of the bus, the total (abnormal) current would be flowing in the portion of the bus from the sources to the first load. Maybe even farther.
If you put the two sources at opposite ends, the current on the bus can never exceed the greater of the two OCPD values.
By this reasoning, you could have both main and backfeed equal to the bus rating, but the CMP was not comfortable with that an compromised with the 120% rule instead.

Now if you have a 200A panel with a 200A bus, you are limited to only 40A of backfeed. So by all means check your panel.

Unfortunately in the 2011 NEC the 120% rule is in 705.12(D), and 705.2 seems to exclude battery driven systems from the definition of hybrid systems and maybe from the scope of 705 entirely.
But you may be able to introduce this to persuade the AHJ that if you satisfy the 120% rule you are OK.
There is no way that you will be able to talk them into accepting more than that though, IMHO.

Now the special case of a panelboard which has only a main breaker, a backfeed breaker and a feeder breaker to the assorted loads in another panel may offer you an escape because you know exactly what the load limit on the panel is. You might then also have to add a placard stating that no additional load breakers may be installed.
With multiple load breakers you have more of a problem, since the calculated load can be lower then the sum of the breaker values, but that means that the sum of the breaker values can easily be greater than the bus rating. Normally the main breaker will then be the enforcer of the bus limit. But you lose that when you add backfeed.

GoldDigger, thanks for your detailed reply. I see what you are saying about if there are problems with the loads. I have two clarifications. There are NO loads on the panel. The only purpose of the panel is to feed the battery system. I suppose from the AHJs perspective, loads could be added later. However our big issue with their decision is not the panel rating (which is already a 225a rated panel). They want 200a conductors from the source to our panel and from our panel to the batteries. That's the expensive part and I just don't follow the logic on how this could possible be appropriate since the breaker at the source is 100a and the combined breakers from the battery source is 90 amps.

 

[GoldDigger]

Yes, if that is what the inspector is asking for it makes no sense at all.
If a conductor is protected at both ends at 100A or less, there is no reason I can think of to require the conductors to be more than 100A.
I did not get that from your original post.