Feeding a sub-panel

[Flint]

Feeding from the barn to the house won't work due to a variety of factors (primarily, the power company being difficult, but a few others with layout).

So, here's the plan (unless someone tells me I missed something critical):

Existing house panel will be left alone. Above it, I'll use insulation-piercing connectors to lead off a branch which will immediately connect to a 60A fusible disconnect, which will feed the barn.

4/0 wire will run through the conduit to the barn.

At the barn end, the conduit will connect to the REC meter housing. Millbank has a dual-tap kit for this meter housing, so the 4/0 will connect to one set of taps, and the other set of taps will feed the barn sub-panel (through 2ga THHN in conduit).

The inverters will connect to the 4-space 125A main lug panel I already have sitting around - one through a 40A and one through a 50A breaker. The lugs will be connected to the line side of the REC meter (through 2ga THHN in conduit).

In the future, before the 10kW inverter is installed, I will have to install a 100A disconnect by the utility meter and move the 4/0 barn feed to that disconnect, since an external disconnect is apparently required on any system 10kW and over, but not on ones below that. I'd love to do it all now, but it will be a good bit of work to install that external disconnect.

I believe that meets all of the code requirements, right? Thanks in advance for any confirmations (or problems noted).

[Flint]

You may not be able to use the existing wires between barn and house in reverse to feed the house subpanel. It depends on whether the existing feeder has an EGC which is compliant with current code. It used to be allowable to run only three wires (two hots and a grounded conductor) without either a wire or raceway EGC and originate the building EGC from the feeder neutral and a local ground system.

It's an existing conduit (2.5"). It's empty, so I can choose what to run through it.

Southwire's calculator says that 100A through 4/0 for 250 feet will have 2.66% drop. If I went to 300MCM, I could get it down to 1.97%. Not sure if that's worth the extra cost - I'll have to get a quote on 300. Also says that I should use a #6 aluminum ground wire - that seems small, to me?

[inetdog]

I guess another alternative would be to flip things around. Since a service upgrade is necessary, I could have them do new 200A service to the barn, install a panel there, and then use the conduit to feed the house as a sub-panel.

Does that make things simpler in any way (to have the solar be at the main panel)? I could potentially get one of those solar-ready panels?

Having the solar at the main panel gives you at least two advantages:

1. You are at the right place to do a line side tap if your POCO allows it.
2. Without a line side tap, you only have one panel that has to comply with the 120% rule instead of two.

You may not be able to use the existing wires between barn and house in reverse to feed the house subpanel. It depends on whether the existing feeder has an EGC which is compliant with current code. It used to be allowable to run only three wires (two hots and a grounded conductor) without either a wire or raceway EGC and originate the building EGC from the feeder neutral and a local ground system.

[Flint]

I guess another alternative would be to flip things around. Since a service upgrade is necessary, I could have them do new 200A service to the barn, install a panel there, and then use the conduit to feed the house as a sub-panel.

Does that make things simpler in any way (to have the solar be at the main panel)? I could potentially get one of those solar-ready panels?

[Flint]

Sounds like you have it figured out.

Well, I hope I do. That seems to be what should work, from what was described. Just hoping I'm not missing anything.

One last thought: I'm pretty sure you don't need to take a ground to the barn (just add a grounding rod at the new panel) and I think you can reduce the size of the neutral but I'm not familiar with the exact calculation.

I'm pretty sure that the AC panels all need a common ground (bonded to neutral at the meter).

Reducing the neutral is "allowed, but not preferred" according to the inspector. Which I take to mean, "I don't want to see it even if it doesn't violate code, so if I do, I'm going to look for something else I can nail you with."

[tehan]

Sounds like you have it figured out. One last thought: I'm pretty sure you don't need to take a ground to the barn (just add a grounding rod at the new panel) and I think you can reduce the size of the neutral but I'm not familiar with the exact calculation.

[Flint]

The line from the home to the barn should be rated for 100A because that is the maximum combined current from the loads and the generation. I don't think you are going to save any money adding an additional junction box so as to avoid purchasing a 200A panel for the barn; that junction box is going to need to be rated for 200A and so won't be cheap.

Box is cheap - $40-50. Multi-tap splices add some cost, but it will be under $200. If there was a way to do it inside the REC meter box (eg, some sort of two-into-one adapter that could clamp into the meter's lugs, while allowing two 4/0 cables to connect), that would make it even simpler. Not sure if a 200A panel, plus replacing the main breaker with a 100A main, plus a 100A backfed breaker would be any cheaper. Especially since I already have the 100A panel.

As was suggested, you should do a cost/benefit analysis on pulling slightly thicker wire. Your proposed PV system will generate say $3000 of electricity per year, so reducing the voltage drop from 2% to 1% would save you $30 per year or say $300 total. So it might be worth spending about $300 on thicker wire if it meant you could achieve that reduction in voltage drop, but wire in these ranges is expensive so that may be tough.

Southwire's online calculator says I'd need to jump to 600MCM to get under 1%. Jumping over 4/0 starts the price running up, so even going to 300 is going to increase costs, while not necessarily saving much power versus 4/0...
(It's a good thought, though - wasting power for no reason bothers me)

[tehan]

The line from the home to the barn should be rated for 100A because that is the maximum combined current from the loads and the generation. I don't think you are going to save any money adding an additional junction box so as to avoid purchasing a 200A panel for the barn; that junction box is going to need to be rated for 200A and so won't be cheap. As was suggested, you should do a cost/benefit analysis on pulling slightly thicker wire. Your proposed PV system will generate say $3000 of electricity per year, so reducing the voltage drop from 2% to 1% would save you $30 per year or say $300 total. So it might be worth spending about $300 on thicker wire if it meant you could achieve that reduction in voltage drop, but wire in these ranges is expensive so that may be tough.

[Flint]

Conduit is 2.5", so I think I can fit just about anything I'd need to fit.

Actual barn loads will total about 60A, as things currently stand. Having some room for expansion would be nice, but if keeping it down will make things easier, that's an option.

If I'm understanding this correctly, the issue is that with X solar amps and Y grid amps connected to a single panel, the loads could possibly draw X+Y (ie, draw from both at once), which might exceed the safe loading of the panel's bussbars.

So, if the solar connects on the line side of the panel's main breaker, then that whole thing becomes irrelevant, because the loads can only draw what the main breaker allows, right?

Basically, I could run a connection from the solar in the barn to the house breaker, sized for 100A, and connect the barn panel to that wire - the barn loads could never exceed 100A, because they'd trip that panel's main breaker. The solar could never exceed 74A, because that's all the inverters can produce (but we're going to call it 90A, because that's the total of the breaker sizes)(still less than 100A, so it will be safe). The grid can supply 100A through the breaker or fuses at that end.

Now, assuming I'm correct on all that, does it mean the wiring between the house and barn needs to be sized for 100A (the most that either supply could add), or 175A (the most that both, together, could actually add), or 190A (the total of the breakers that can feed power into that wire)?

Sizing for maximum ampacity and using standard aluminum would give me 250MCM if I have to size for the total of the connected breakers. If I can keep it to the actual totals, then I can drop that to 4/0. I can't really go below 4/0 without the voltage drop on the 74A from the inverters becoming significant.

So, summary: replace meter and service with 200A unit. Install two 100A disconnects - one feeds house and one feeds barn. Then wiring gets to barn, it goes into a junction and connects to the barn panel and the REC meter, in parallel. The REC meter is fed from that 125A panel I already have, with one breaker for each inverter (and since there are only four spaces, no one can ever mess that up by adding breakers).

Will that work?

[tehan]

First-rate advice from foo1bar above. You won't find better.

If you need rapid shutdown, I would look seriously at SolarEdge. It's going to be cheaper than a string inverter once you have added in the costs for rapid shutdown, and it's just a better, safer solution. I also really don't like the use of an additional disconnect switch just for rapid shutdown, as some vendors are promoting. To me, and I suspect most firefighters, the obvious way to initiate rapid shutdown is to turn off power at the panel (either at the house or at the barn). I think additional means of operation are just going to cause confusion in the event of an emergency.

[foo1bar]

NH has "group net metering," as they call it. You can even have a dedicated feed-in meter for the array, and other members pulling out power (some folks apparently do that if they have a field that is not near enough to their house for direct wiring).

But it's done in dollars, not kWh.

Ah - well, in that case I can see why you'd want to mostly offset the house usage directly.

It sounds like the best plan is to run a dedicated line from the main meter to the barn, right?

Will a 100A line from the meter to the barn be acceptable to connect both the solar and the barn loads, so long as I connect the solar directly to that line rather than through the panel's bus?

So, can you have a set of wires that are connected to 3 OCPD, one is 100A breaker to the POCO, one is 100A breaker for loads, and one with 90A of breakers for PV.
I'm not sure.
But I'd say the easier solution is a 200A panel at the barn, 100A main breaker (or maybe main lug, but 100A back at main panel - probably I'd go with a breaker at the barn - not sure if it's required or can do a main lug, but it would make it easy to shut off power to the building at the building rather than 250' away at the house), 90A of PV, and various 15/20/30A breakers that you need for loads. 200A panels aren't very expensive ($100-150), and I think most of them have replaceable main breakers that wouldn't cost a lot to change the 200A to a 100A main.

And I would look at sizing of the wire carefully - look at what is the largest alum. conductors you can fit in the conduit you have (and whether you're better off direct-burying a new cable) And what's the cost of those wires and how much voltage drop you will have. (and what that means for your production)

If it were me, I'd put together a drawing of what I'm planning to do, including wiring sizes, what load centers, breakers, etc. And I'd take it to the permit office to see if they see any issues with my plans and what other information they need for a permit for my solar install.


BTW - if you're going to have to replace the main panel, I'd consider it part of the solar installation and if it were me, I'd count it as such on my taxes. (But that's me - I'm not a tax lawyer, nor even a professional tax preparer)

[Flint]

Are you under the 2014 NEC? If not, then rapid shutdown is not even a concept.
You could ask your fire department what they would prefer instead, since the AHJ does not seem to care.
FWIW, one of the nice things about the Midnite Solar Birdhouse is that it is totally in your face and looks like something a fire fighter should pay attention to.

Yup, 2014.

The other nice thing about that system for this project, is that you can install multiple units, as long as you are willing to pay the cost. I figured that one on the house and one on the barn might be a good idea, so it can be triggered from either point.

[inetdog]

They don't seem to know any more than the installers. I'm planning to use a Midnite remote disconnect to meet the rapid shutdown requirements. I asked them where they wanted the box (the meter is actually on the back of the house, so the front actually would seem to be better for accessibility, but might not be noticed by the firefighters if they were heading for the meter). He stared blankly and asked what rapid shutdown was...

Are you under the 2014 NEC? If not, then rapid shutdown is not even a concept.
You could ask your fire department what they would prefer instead, since the AHJ does not seem to care.
FWIW, one of the nice things about the Midnite Solar Birdhouse is that it is totally in your face and looks like something a fire fighter should pay attention to.

[Flint]

Than have a discussion with your AHJ
It may help with what they will accept.

They don't seem to know any more than the installers. I'm planning to use a Midnite remote disconnect to meet the rapid shutdown requirements. I asked them where they wanted the box (the meter is actually on the back of the house, so the front actually would seem to be better for accessibility, but might not be noticed by the firefighters if they were heading for the meter). He stared blankly and asked what rapid shutdown was...

[Naptown]

Than have a discussion with your AHJ
It may help with what they will accept.