Wiring Optimization

[VictorV]

Hi Everyone,
I am trying to plan for a 25kw solar installation. Currently I have a 100a circuit from by house to my workshop (separate building).
There's a 100a panel in the workshop. I am going to be adding a barn and the solar panels will go on the
barn. The inverter(s) will be in the barn as well.

In the diagram below the existing wiring is in red. Typically an inverter would connect to the house panel with its own
breaker as shown in blue. Rather than running an entirely new circuit (in blue) I am wondering if I can just bring the inverter
output to the shop panel (in green) which then eventually goes to the house panel using existing cable (in red). Logistically
this is much easier to accomplish. I need to verify the current rating on the existing circuit. I'm also concerned that, at
208v, 25kw is 120 amps so it may not work at all if the existing circuit only supports 100a.


What do you think? Is this possible?

Victor

 

[snoobler]

Welcome to the forum.

If the 25kW is going to power the house, yes, you have a problem, but if you could utilize it such that you didn't exceed any panel, breaker or wiring limit, I don't see why it wouldn't work.

 

[Hedges]

208V - are you only getting 2 legs out of 3-phase?
Or do you have 120/208Y?
If 3 legs, so your wiring is probably L1, L2, L3, N, G, then 25 kW is 70A on each of 3, 120V phases.

By 25 kW, do you mean 25 kW STC of panels? Or a bit more panels, enough to deliver 25 kW AC?
Either way, instead of all one fixed orientation, you can split your panels between multiple orientations. That would knock peak current down 30%, spread the power out more uniformly across the day.

 

[VictorV]

The 25kw is the rating of the inverters. 25kw is the max our utility allows us to feed back into the grid.
I'm assuming we're 208 3-phase. There are 4 wires between the shop and main house panel.

 

[Hedges]

4 wires plus ground, 3 busbars in the breaker panels? 3 pole 100A breaker in house panel feeding shop?

If so, it can handle 25 kW so long as relatively evenly distributed across the 3 phases. I calculated 70A on each of 3 legs at 120V. The wires can be used for up to 83A continuous while leaving 20% headroom to avoid nuisance trips.

There are rules on PV backfed breaker sizes, to avoid putting too much current through busbars. Main breaker plus backfed breaker not to exceed 120% of busbar rating, assuming backfed breaker is placed at far end of busbar. (If breaker panel has main breaker at center of busbar, then limit is 100% of busbar rating.)

If busbar is rated 200A and main breaker is rated 200A, calculate 200A x 120% - 200A = 40A limit on backfed breaker.
If busbar is rated 225A and main breaker is rated 200A, calculate 225A x 120% - 200A = 70A limit on backfed breaker.
If smaller main breaker than busbar rating, of course larger backfed breaker allowed.
Otherwise, if main breaker is not in main panel, PV can be connected outside the panel by means of additional breakers.


"25kw is the rating of the inverters. 25kw is the max our utility allows us to feed back into the grid."
So this is grid tied. By "inverters", do you mean more than one?
If one or more 3-phase inverters, current will be evenly distributed across the 3 wires.
If multiple single-phase inverters, then if quantity is a multiple of 3 and all same wattage, current will be evenly distributed.
If different wattage or not multiple of 3 inverters, some math is required to evaluate current per phase.

Will you want battery backup to operate during grid failures? There are some ways to do that, with compatible grid-tie PV inverters at the barn and battery inverter/transfer switch at house or shop. Such a system should be planned in advance to get equipment that will work together.

 

[VictorV]

To answer your questions:

"So this is grid tied."
yes

"By "inverters", do you mean more than one?"
I believe a 25kw system will have maybe 3x 7-8 kw inverters (unknown at this time)

"Will you want battery backup to operate during grid failures?"
no

I did a bit more digging to understand what's currently there.

1) 4x 1 awg stranded aluminum wires. 1 common, 1 ground, and 2 phases connecting house panel to shop panel.
2) Wire temperature rating is XHHW-2 (90 deg C) so it should handle 115a for each conductor
3) The voltage across the 2 phases is 240v and 120v between each individual phase and common.
4) 2 busbars in the 100a shop panel.
5) Shop panel main breaker is 100a.

Here's what it looks like from the outside of the shop where the power comes in from the house:



What I'd like to do is add an exterior junction box and wire it like this:


I'd have to re-jigger the existing conduit to go into the junction box rather than directly into the shop.
There would be a short set of wires from the new junction box to the interior shop panel.

Will this configuration meet code? Will there be an issue, for example, if I am running a welder in the shop
on a 50a circuit in the middle of the summer when the solar panels are generating the highest amount of
power? It seems like there would just be less power being fed back to the house and grid.

Victor

 

[Hedges]

It's fine to have grid and solar supply power to a panel, while you draw power from it. Less backfeed as you say.

There are rules on busbar rating and breaker rating. You can't put a very large PV breaker in a 100A panel.
Main breaker plus backfeed breaker can be up to 120% of busbar rating.
You could tap off existing wires from house to 100W panel and add another panel for the PV inverters.
The panel at the house would have to be higher amperage to accept the PV backeed, or else similar tap between separate disconnect and panel breaker.

So you're got 120/240V split phase at the shop, not 120/208Y.
If 100A breaker, 24 kW would be 100% of breaker. I see 115A and 110A for "feeder" in various places of the NEC chart.
80A for about 20 kW would be the most recommended to avoid nuisance trip.

If house has a 225A busbar, 150A main breaker, and 100A backfeed breaker, could be used.
If 200A main, could only have 70A backfeed.
If 200A breaker at meter and wires to a "main lug" panel, then putting a main breaker at the panel would let you tap inbetween.

 

[VictorV]

The house panel is actually 2 panels. Both are 200A. One of them has a 100A breaker going to the
shop. The panels are model HOM4080M200PCVP. I did not see a separate busbar rating so I assume
it's the same as the overall panel rating of 200A.

Sorry if I keep adding more complexity. I was hoping that I could install it myself but as the complexity
grows it may be out of my league. We had a quote to install a 25kw system that included installation
but I'm beginning to wonder if they did not take into account the existing configuration. Here's a more
detailed diagram showing the entire existing system:


The lines in red are existing. I'm wondering if I have to add another main breaker between the meter and the 2 main house panels
and the solar ties into that breaker. Then I can size the new breaker to support the 25kw solar system and the 2x 200A main panels.
That's going to require new conduit directly from the barn to the house which would have to go under the driveway which
I was hoping to avoid.

Victor

 

[Hedges]

Two panels, each 200A busbar. Sure they're not 225A? Common for Square D Q 120/240V split phase panels.
Do you still think you have three-phase service? As I calculated, the 3rd current-carrying conductor allows 50% more total wattage.

If one of those panels gets a 150A breaker (not 200A) and is 225A busbar, the 120% limit allows 270A total. 150A main plus 100A backfeed is OK. Maybe inspector would let you get away with 125A backfeed (275A total), but your existing wire isn't quite enough for 125A.

If you could add a breaker (400A?) right after the meter, then you could have a 3rd panel (or fused disconnect) dedicated to solar.
I think a panel dedicated to PV circuits, with no loads connected, may be allowed backfeed up to busbar rating.

Not really a lot of complexity to install, once you get the design details correct. Issue is what wattage/current can be supported by each component.

You can get more watt-hours with lower peak watts and amps if PV panels have multiple orientations. Power is spread out over more hours.