CT Can and large solar array

[shopkins]

I have a 175 KW solar job to install and the builder took it upon himself to install a 400 amp fused disconnect for the array's AC disconnecting means(100' away) with three 600 MCM & a #2 equipment ground copper conductors to a 400 amp breaker in the SES for the solar back feed. The local utility came out to do a pre-job inspection and noticed they did not pull a neutral to the disconnect and also stated we would need a CT can and a 13 jaw meter socket for this installation. The builder will be responsible for getting the neutral installed from the disconnect to the SES and we will take care of the rest. My question is in regards to which order will the conductors go from the new AC panel board that is fed off the inverters in the basement to backfeed my newly generated power. My first thought is to tie the conduit into the CT Cabinet on the line side and then from the load side to the disconnect and ending on the disconnect load side. The meter just gets #12 wire anyways.

 

[jaggedben]

First of all...

You do not say if the inverter requires is a 3 wire or 4wire inverter, but if is a 3 wire inverter I would ask for a code citation on the neutral. By that I mean isn't required. If a 3 wire inverter then there is absolutely no need to run a neutral 100' to the disconnect at the inverter, since you say the 400amp breaker is at the service disconnecting means. I have seen this debate happen in the field as well as seen it discussed in a number of forums, And while there can be considerable debate over whether a neutral is even required to be brought to the first disconnect, I cannot fathom why it should be required to be brought from the first disconnect to the second, unless the inverter actually requires it.

I also do not understand why you apparently have two OCPDs for the inverter(s). Since you have a 400amp breaker at the SES, the disconnect at the inverter(s) could be an unfused disco.

My question is in regards to which order will the conductors go from the new AC panel board that is fed off the inverters in the basement to backfeed my newly generated power.My first thought is to tie the conduit into the CT Cabinet on the line side and then from the load side to the disconnect and ending on the disconnect load side

I'm really not clear on the question. Is the 400 amp breaker you mentioned attached to this new AC panel board? Is this a supply side connection? How many service disconnect handles are there, and is the PV one of them? Which disconnect are you referring to in the second sentence of the quote?

As for the CT meter, I am not totally clear on whether this is the service meter or a solar production meter, although it sounds like it's the service meter. (which I would then assume will be a net meter?) Assuming it is a service meter, I would think it should go on the line side of the service disconnecting means. Or maybe, just to cover myself, I should say that I don't have any experience installing CT meters, but you should install it as you would on any service and ignore the fact that there is a solar array attached, and in my experience they are typically line side.


Oh, and welcome to the forum.

 

[shopkins]

explanation of facts

explanation of facts

I have a 175 KW solar job to install and the builder took it upon himself to install a 400 amp fused disconnect for the array's AC disconnecting means(100' away) with three 600 MCM & a #2 equipment ground copper conductors to a 400 amp breaker in the SES for the solar back feed. The local utility came out to do a pre-job inspection and noticed they did not pull a neutral to the disconnect and also stated we would need a CT can and a 13 jaw meter socket for this installation. The builder will be responsible for getting the neutral installed from the disconnect to the SES and we will take care of the rest. My question is in regards to which order will the conductors go from the new AC panel board that is fed off the inverters in the basement to backfeed my newly generated power. My first thought is to tie the conduit into the CT Cabinet on the line side and then from the load side to the disconnect and ending on the disconnect load side. The meter just gets #12 wire anyways.

First off, the Utility company is requiring the neutral. They said they need it for metering purposes. It is NOT an NEC requirement, however it will mean that i now have to convert my 3 wire inverters to 4 wire. As far as your question is concerned, the new 480 vac panel board is basically an AC combiner box. My DC circuits will come off the roof to 2 inverters and the output of those (AC feeds) will back feed (2) 125 amp breakers and out of the 250 amp main breaker will run up to the CT Can and disconnect. Does that make sense? I wish I could draw it out for you. My original question is do I take that 250 amp feed to the CT Can first or the 400 amp Disconnect? To answer your other question, yes I now have multiple disconnecting means. One at the SES (400 amp breaker), one at the disconnect outside, and one at the AC Panel Board in the basement of the garage. Which is all good due to the fact that they are all remote from each other.

 

[jaggedben]

First off, the Utility company is requiring the neutral. They said they need it for metering purposes. It is NOT an NEC requirement, however it will mean that i now have to convert my 3 wire inverters to 4 wire.

It sounds like the utility may not have much experience with solar. To my knowledge there are no issues with connecting 3 wire inverters to 4 wire services. (Only way I've personally seen large systems done, actually). I suggest reading this article: http://solarprofessional.com/article/?file=SP5_1_pg26_Hamon
Also, try posting this same question in the forum at solarprofessional.com. The author of the article frequently replies to questions there.

It's up to you, obviously, but if it were me I would try to educate the utility about the (lack of) neutral rather than let them require potentially unnecessary costs for you and your clients on future jobs.

As for the meter, I suppose it may be their prerogative to ask for a neutral at the meter/CT can, but there is no reason you need to bring the neutral anywhere else beyond the service disconnecting means that it not needed for line to neutral loads.

As far as your question is concerned, the new 480 vac panel board is basically an AC combiner box. My DC circuits will come off the roof to 2 inverters and the output of those (AC feeds) will back feed (2) 125 amp breakers and out of the 250 amp main breaker will run up to the CT Can and disconnect. Does that make sense? I wish I could draw it out for you. My original question is do I take that 250 amp feed to the CT Can first or the 400 amp Disconnect?

I still a bit confused about the location of the 400A fused disco and the purpose of the meter.

If it is a service net-meter, I would think it should go out by the SES, especially if there are any other loads on the service besides the solar.

If it is a solar production meter, then it can be between the 250A breaker and the 400amp disco as you've suggested, preferably closer to the 250A breaker. (There can't be any non-solar loads in the AC panel you've mentioned.) However it could also be back towards the SES if that allows you to avoid running the neutral to the solar system location.

To answer your other question, yes I now have multiple disconnecting means. One at the SES (400 amp breaker), one at the disconnect outside, and one at the AC Panel Board in the basement of the garage. Which is all good due to the fact that they are all remote from each other.

You have multiple disconnecting means, but only one service disconnecting means (400A breaker) that you've mentioned. Are there any loads connected to this service that are not the solar?

With regard to the neutral, 250.24(C) requires it to be brought to the SES, but nothing requires it to be brought any further if it is not used.

One other thing: The grounding wire run with those 600 mcm probably needs to be bigger than a #2. Presumably it is a either a GEC or a GES bonding jumper and code requires that to be sized according to 250.66.