Feeding a sub-panel

[Flint]

Here's the situation:

The house has existing 100A service, into an ancient panel. That's due for replacement, but hopefully it can wait until next year.

There's a new (empty) conduit feeding a new barn, where the inverter will be installed. The current plan is to install a breaker in the house panel and run wiring (250 feet) to the barn. There will be a sub-panel at the barn where the inverter will feed in.

Inverter is rated at 32A continuous output, with a 40A max breaker size. There will be a disconnect and REC meter can between the inverter and the panel.

I would like to be able to add an additional 10kW of PV once the barn is complete (42A continuous, 50A breaker/fuse).

I have a 100A load center (Square D QO120M100RB) to mount on the barn (that's a main breaker panel, but the breaker could be removed and replaced with lugs, if necessary - also, I've heard that there may be lugs which can be installed in the input side of the breaker to allow solar to feed there, without actually flowing through the main?). I also have a HOM48L125GRB sitting around that can be used if there's some benefit. I have two 60A disconnects - one fusible and one not.

My thought was to install a 100A breaker into the sub-panel, connected to the REC meter. I could then install the fusible disconnect with 40A fuses and connect the inverter to that. In the future, I could install a second fusible disconnect and also feed into the supply side of the meter.

However, I don't know if that will be compatible with the 120% / 125% rules (we're using NEC2014, here, according to the inspector) for either the sub-panel or for the main panel. Both currently, and also when the second inverter is installed.

I'd prefer to use the stuff I already have, but since I haven't actually installed it or pulled the wires through the conduit yet, I can still change things if it's necessary.

Any input would be appreciated...

[Naptown]

There is no solution I can think of other than a line tap at the house.

[Flint]

There is no solution I can think of other than a line tap at the house.

For the whole thing? Or is there a way to at least get the 7kW system up, with planning for an upgrade in the future to handle the 17kW total? (eg, if upgrading that main panel is necessary, that's already on the planning list - but I would want to make sure that whatever wire I pull through the conduit will be adequate)

And, just to make sure we're talking about the same thing, by a "line tap," you mean connecting a dedicated breaker below the meter, in parallel with the existing house panel? That would be acceptable from my standpoint. Basically, there would just be two main breakers - one that feeds the house, and one that feeds the barn. Would everything else work in that situation? And would the breaker be a 200A or a 100A unit?

A few suppliers make combination meter sockets with four breaker spaces (for example, GE R281CB1). Could I get one of those and stick two 100A breakers in?

[tehan]

Have you thought about adding a second meter at the barn? Saves you the 250' of wire, avoids touching the existing panel, and saves maybe 2% line losses on your PV production. So even if it's expensive maybe worth it.

[Flint]

Have you thought about adding a second meter at the barn? Saves you the 250' of wire, avoids touching the existing panel, and saves maybe 2% line losses on your PV production. So even if it's expensive maybe worth it.

Unfortunately, the local power company has cut their payments for produced power by about half. So, the best way to benefit from the power production is to get it to the house where it will directly offset electricity as its used, rather than just sending power into the grid at half the electric rate. The barn does not have enough load to justify the solar installation - the real goal is to have the power sent to the house, where it matters.

[Naptown]

Flint
What state are you in?

[Flint]

Flint
What state are you in?

New Hampshire.

[solarix]

First rule of backfeeding solar into a load-side tap is that the backfeed current may be no more than 20% of the service's busbar size. Even if you upgrade to a 200A panel that is still too small for your future 17kW. Unless you have barn loads that total less than 100A and ensure that no more than that can be added, the subpanel circuit must be limited to no more than 20A of backfeed which is only a 3.8kW inverter. You could put in a new "solar ready" main panel with a 225A busbar - but that is still a maximum 12.5kW inverter - thus a line-side tap is what you are looking at. Could upgrade to a 400A panel and use the 2nd 200A tap just for solar. Keep your barn loads to less than 100A, backfeed a breaker there and upgrade to a 400A main is what I'd do. The other piece of the equation is where does your utility mandate the AC disconnect to be? Usually they want it at the main panel for easy access although they may allow locating elsewhere as long as there is a permanent map at the main service showing where they can find it.

[inetdog]

Solarix, your simple interpretation of the 120% rule is misleading.
The correct statement is that the size of the main breaker plus the amount of the PV backfeed (determined differently before 2014 NEC) cannot be more than 120% of the bus capacity.
Some 200A panels have a 225A bus, giving you 50A to work with.
Or you can reduce the size of the main, say from 200A to 150A if that will still supply your full load, and thus get an additional 50A of backfeed allowance.
And even the 20% of bus capacity does not apply for center fed main panels or if you cannot locate the PV breaker all the way at the far end of the panel bus from the main breaker.

[foo1bar]

Unfortunately, the local power company has cut their payments for produced power by about half. So, the best way to benefit from the power production is to get it to the house where it will directly offset electricity as its used, rather than just sending power into the grid at half the electric rate. The barn does not have enough load to justify the solar installation - the real goal is to have the power sent to the house, where it matters.

You *may* be able to get the POCO to consider two separate meters/connections to them to be one "virtual meter"
I've read of such things before. I think in that case there were rules like the connection had to be to the same section of the grid (which wouldn't be a problem for you).
I don't know if that was a state-specific law or a particular POCO - or what it was exactly. But I'd read up on it and I'd ask the POCO about it.


If that isn't viable, I think you probably need to change your plans for your subpanel.
I think you are saying you have a subpanel with 100A bus, that you're planning to put in 90A of solar breakers. Which by itself I think would be fine there - but you likely need/want multiple 20A circuits there too for lights, tools, etc. etc.
(Connecting it at the house would be another question you'd have to figure out what you'd do)

For the subpanel at the barn, I'd figure out what all the circuits I could possibly want there. Then look at the price difference between a 125A bus and 200A bus panel. And probably use a 200A bus subpanel with a 100A main breaker because it'll be $40 more for the 200A. Then when I realize a month later I really should have put in a 50A breaker for the welder/EV-charger/whatever I won't have to replace the panel.

[foo1bar]

BTW - multiple subpanels at the barn are a possibility too - but means more wires going to the barn. (more wires = more $ for the wires and question of fitting them in the conduit)

[Naptown]

New Hampshire does allow virtual net metering.
So the barn would be the host and the house would be secondary.

[Mike90250]

Just a heads up, with a large PV install, the inverter may feed enough current, that if it's not consumed locally, you may raise the line voltage enough to trip offline. A panel feeding a panel fed from a tap, may have enough resistance from all the connections and someone skimping on wire size, you get shutdowns. And if there is a long feed from the last transformer on the pole (grid), that resistance all adds up.

[tehan]

Just a heads up, with a large PV install, the inverter may feed enough current, that if it's not consumed locally, you may raise the line voltage enough to trip offline. A panel feeding a panel fed from a tap, may have enough resistance from all the connections and someone skimping on wire size, you get shutdowns. And if there is a long feed from the last transformer on the pole (grid), that resistance all adds up.

On most inverters the trip voltage is configurable to solve this problem. May require a password from the manufacturer. Certainly possible using Enphase or SolarEdge.

[Flint]

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. And, as noted, they've recently cut the rate they will pay for power. Even spending some more money for wiring right now will pay for itself fairly quickly when it gets that power sold at twice the price by offsetting house usage rather than selling it to the grid.

The power company won't discuss disconnects. They don't require them on systems below 10kW, and when I asked them for what they wanted so I could plan for the future, they told me that they can't give information unless they have an application for the system that requires it. Which is ridiculous, but I think they're not pleased with the number of folks who are installing PV, and are trying to make it difficult. I'm assuming that a disconnect located near the meter would be in the plan - if I did a breaker to feed that sub-panel, I was going to run it through an exterior disconnect first, before sending it to the barn (easy enough, as the disconnect would be right on the other side of the wall from the panel).

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?

And, ignoring the future plans, what's the simplest way to get the 7kW system connected up and working?