• Have you tried out dark mode?! Scroll to the bottom of any page to find a sun or moon icon to turn dark mode on or off!

diy solar

diy solar

Help with neutral wiring w/critical loads and bypass

bitbanger

New Member
Joined
Jun 19, 2023
Messages
6
Location
US
I'm hoping someone can help save me the embarrassment of a failed inspection (or worse). I just can't figure out a neutral arrangement that makes sense for this setup. I'd like to use a 100A subpanel with interlock kit to act as an inverter bypass for the critical loads. However this seems to be complicating the neutral wiring.

NEC 300.3(B) from the top of my head, requires ungrounded conductors (neutrals) to occupy the same conduit or raceway as the ungrounded conductors. This seems to conflict with how I see critical load panels sometimes wired - just bringing over the hots from the main panel.

Additionally there's an issue with parallel neutral paths in numerous places.

Lastly if the SolArk 12k does an automatic N-G bond under grid-down, that means a double bond with the main panel via the critical load bypass feeder, resulting in objectionable currents.

Is there any way to make the below NEC compliant and functionally correct? I was hoping to avoid the cost of a 3pole 50-60A transfer switch (in place of the 100A panel interlock kit), but I think that would solve some problems.

This is a limited-to-home setup; no intention for supplier agreement/sell-back. Not shown below are CTs and conduit ground bonding.

Thanks for any thoughts folks.

1705544155922.png
 
I don't think I'd care about a loop in neutral and ground, so long as each path was high enough ampacity to work if other went open-circuit.

The two (?) black hots and one red hot cross over without neutral or ground. I'd want to route the through same conduit as where neutral and ground tie between those panels.
 
I don't think I'd care about a loop in neutral and ground, so long as each path was high enough ampacity to work if other went open-circuit.

The two (?) black hots and one red hot cross over without neutral or ground. I'd want to route the through same conduit as where neutral and ground tie between those panels.
I suppose the issue with neutral loops is there's no guaranteed return path. So if the inverter was for example backfeeding via GRID, and the critical loads also wanted to return via that neutral connection (due to lower impedance), then it would potentially exceed the sizing of the GRID 50A feeder neutral.

I'd have to check sizing but I'm not sure if it's feasible to get the inverter bypass and all branch circuits in one conduit. I guess if it's kept short as a nipple then no derating needed.

Preciate the thoughts.
 
Have to figure out whether those add or subtract. I'm thinking subtract but I'm not 100% clear where loads and backfeed are. Also consider 120% rule for busbar, and where connections are made.

Yeah, no derating under 2'. Doesn't say anything about a length of wire outside the nipple as heatsink, but my thinking is there should be.

Easier to overstuff if conduit is a straight shot. Curves cramp my style.

I'm planning to look for 2.5" EMT for one elbow, and will have to holesaw the opening. Just a 45 degree into a pull box.
 
Thanks - yep if I put GRID connection at bottom of main bus, and derate to 175A (by swapping out 200A MCB) I'm under bus limits w/9kW max backfeed.
 
Thanks - yep if I put GRID connection at bottom of main bus, and derate to 175A (by swapping out 200A MCB) I'm under bus limits w/9kW max backfeed.

bitbanger,

How did the wiring turn out? I am doing the same thing on my NHX-12k inverter. I am taking a sub panel that I installed to dump all of my critical loads into and making it a "convertible panel" with an interlock kit so I can easily switch back and forth from an inverter critical loads panel back to a sub panel. My thought is that in the case of inverter maintenance or issues that I can flip a switch and convert the crit panel back to a sub panel by the flip of a breaker via interlock. BUT I am concerned about the neutral and ground wiring creating some sort of loop that may damage the inverter or cause shocks or something along those lines.

See my post here - https://diysolarforum.com/threads/new-nhx-12-killowatt.91611/page-3#post-1299794

I cannot get a good answer and I am in the process of firing it up for the coming winter storm tomorrow. So hopefully you have gained some knowledge about this wiring scheme that confirm that it's either a good or bad idea to wire it this way.

My work around for now is to disengage the "sub panel" neutral and ground from the main box and shutting off the breaker on the hots, which is basically disconnecting the main panel altogether. If I needed to do maintenance on the inverter down the road I can simply shut down the inverter and rewire the "sub panel" wiring back into the sub panel and manually convert it back to sub panel status.

I hope that you did well on your setup! This is my first inverter install and I am a little pit panicked about blowing it up internally. I have pics of everything on the link I sent.

Thanks for any insight BB.

Happy New Year!

John
 

diy solar

diy solar
Back
Top