18kpv with critical loads backup panel - does it really need a 70a subpanel breaker?

[FilterGuy]

In the diagram shown, If the critical load panel tried to draw more than 70A from the main panel, the 70A breaker in the main panel would trip. However, the design center of the system shown assumes a smaller total load from the Critical load panel so that would normally not happen unless there was a fault.

The question being asked (as I understand it) is whether the 200A breaker in the existing Critical load panel can be left in place and therefore not have overcurrent protection at that point in the circuit. At first glance, it seems like it would be OK. This circuit has protection from grid current because of the 70 breaker in the main panel. Since the load breaker in the inverter is 200A, it does not offer additional protection. Consequently, there is no over-current protection breaker for current from the inverter. The inverter is self-limiting so if the current got too high it would shut down, but I don't know if the NEC would consider that adequate.

 

[timselectric]

But the 70A breaker in the main panel puts a limit to the amount of current that would ever be allowed through the inverter. The most you would ever see coming out of the inverter is the 70A from the Main panel plus 50A from the inverter. The 200A breaker in the inverter is essentially just a switch.

Correct

 

[timselectric]

The inverter is self-limiting so if the current got too high it would shut down, but I don't know if the NEC would consider that adequate.

It would not.
But as long as everything is rated above the available current, you are fine.
It's similar to a 200A service with a 200A transfer switch. The auxiliary feed can be a 20a generator. The 200A wiring and everything else is more than capable. So no extra protection is required.

 

[nuke]

In the diagram shown, If the critical load panel tried to draw more than 70A from the main panel, the 70A breaker in the main panel would trip. However, the design center of the system shown assumes a smaller total load from the Critical load panel so that would normally not happen unless there was a fault.

The question being asked (as I understand it) is whether the 200A breaker in the existing Critical load panel can be left in place and therefore not have overcurrent protection at that point in the circuit. At first glance, it seems like it would be OK. This circuit has protection from grid current because of the 70 breaker in the main panel. Since the load breaker in the inverter is 200A, it does not offer additional protection. Consequently, there is no over-current protection breaker for current from the inverter. The inverter is self-limiting so if the current got too high it would shut down, but I don't know if the NEC would consider that adequate.

If the grid is down, though, then the subpanel critical load breaker being 70a would be well above the threshold to overcurrent protect the inverter output anyway.

And this is also true in a 'whole home backup' scenario that permits the 200a downstream load.. without grid support the inverter cannot rely on the load breaker to protect itself, that would be insane

 

[FilterGuy]

And this is also true in a 'whole home backup' scenario that permits the 200a downstream load.. without grid support the inverter cannot rely on the load breaker to protect itself, that would be insane

The inverter knows the current it is putting out and will protect itself with or without the load breaker..... but is that sufficient for the NEC rules?

 

[nuke]

The inverter knows the current it is putting out and will protect itself with or without the load breaker..... but is that sufficient for the NEC rules?

If I have a (code compliant) 200a breaker directly feeding the 18kpv to the GRID input and a 200a load center on the LOAD side of the 18kpv, does that violate NEC? The grid could go offline and then the load center could try to draw more than the inverter can produce

 

[ksmithaz1]

If the grid is down, though, then the subpanel critical load breaker being 70a would be well above the threshold to overcurrent protect the inverter output anyway.

And this is also true in a 'whole home backup' scenario that permits the 200a downstream load.. without grid support the inverter cannot rely on the load breaker to protect itself, that would be insane

Just keep in mind, NEC is not about protecting the inverter output, you are protecting the wires and connectors that feed the services. Thus you can come out of your 200A panel, with a 15A breaker connected to #14 wire to a 5-15 socket, and plug a 120v/2A wall wart, which has a 2A fuse that protects the #18 wire from cooking. The breaker does not protect the load, it protects the wiring from the load if it goes south, as Tim states above. If you start backfeeding power it gets wonky because the wiring (the bus bar) was engineered with specific ratings for voltage and current, the latter which could be exceeded with multiple feeds to the bus bar, since the sum of the breakers in the panel generally exceeds the panel maximum. NEC could care less if your inverter falls over as long as all the wires and connectors feeding the loads are protected by appropriately sized breaker for those wires and connectors.

All this is a little esoteric to me. Just isolate your sub-panel to the load lugs on the inverter, thus it only feeds off the inverter, and make sure the wires are suitably sized to the panel breaker. I try to not over-complicate this stuff, If you confuse yourself it will probably confuse the inspector.... I see a box, it has 2AWG that runs as input to this panel with a 100A breaker. Check. If your gonna grid tie, you get to do the 120 math, but I always thought the simplest and safest thing would be to drop your (200A?) primary feed directly to the inverter/grid bus, (via a fusebox or small panel) and run everything from there to your load panels. Then I can backfeed whatever and never overload the buses past the breaker ratings. When i started looking at this weird backfeed stuff with 70A breakers, and "Solar Ready" panels, my first thought was about overloading the bus, then I realized this is useful for a micro-inverter setup that is supplementing the grid where all your solar power just gets dumped into your main panel, and absorbed by whatever loads from there.

When you start wiring up an inverter like the 18kpv/12lv or the big Sol-Ark, you can do it exactly the same way, and just ignore the load lugs, now you don't have any backup. The minute you decide to mix output with some from the inverter and some from the grid and multiple panels it get's complex. Like ICE car, Hybrid car, then EV, if you go ahead and build out your inverter setup to handle all your loads, you re-simplify and use the grid as a backup for your PV setup, at which point you should be able to safely back-feed up to the feed breaker rating if the power company will let you. Your mileage will vary, but to me the real benefit starts when my setup actually can run my whole house.

 

[nuke]

I hear you but I have a meter main combo that doesn't let me interrupt to run to the 18kpv. It has two bolt on jumper bars after the main breaker that theoretically could go to the 18kpv but I'm not sure that modification is allowed. (See attached)

 

[EG4_Jarrett]

There are some wiring diagrams in the manual that do not allow for 200a bypass, and there are some that do.

The one OP is looking at does not. I have one here that will allow for 200a bypass. It requires supply side taps to avoid burning up the bus in the panels.

I have another diagram that may be possible to use with your panel. Diagram 4.4 on page 34. It gets more complex here, but I don't see why you couldn't do this. (not taking cost, space, and labor into account)

Just don't limit yourself to one diagram in the manual. It may not be suitable for your situation. The other options are do-able, and if you have any questions, feel free to DM me and I can explain.