RustyShackleford

Summary of this whole escapade: hearth.com/talk/threads/electrical-horror-show.191254

Broke into another 3-gang switch box and one of the cables has the neutral clipped off right where it comes out of the sheath. Just venting I guess ...

More horror: a 3-way controlled light that's wrong - both switches must be in the correct position for light to glow. I literally can't figure out what they did. It appears maybe they're trying to switch neutral in one of the 3-way switches. The other feeds hot to one of two wires, which is normal, but the cable has no neutral.

The bigger question: the electric wiring in this house is a nightmare, as you can tell from the various things I've brought up here. At what point do I draw the line ?

I was just gonna use a GFCI outlet temporarily for the test you suggested: run wires from breaker to "line" and then connect the branch circuit conductors to "load".

I think I'm going to leave well enough alone there. Except I did offer to replace the Federal Pacific panel - considerably more ambitious than anything I've done before except maybe the grid-tied PV installation I did last year - but I feel pretty confident.

Looked in the panel, and the single-pole breakers alternate black & red wires. So I reckon the whole house was done with MWBCs. I can't tell for sure how they're paired up, because the NM sheaths don't extend into the box - everything comes in from the crawlspace via a 2" PVC nipple, so I guess they just put the conductors thru it (haven't looked in the crawl yet). Which makes me wonder why some of the boxes I've looked in just have power coming in via a white&black cable (incl. the light that lives on 2 circuits). I guess it's ok to come off an MWBC with a 2-wire w/ one of the hots though.

On your GFCI test, I guess the GFCI needs to be at the breaker (so the breaker connected to the "line" terminals, and the branch circuit conductors connected to the "load" terminals). Not just hung off the circuit somewhere; if you do that, and the load is just a light bulb or something, of course the neutral and hot currents will be balanced and the thing won't trip.

Good point, re the smart switches. Right now I've kinda got the opposite problem. Two circuits (home runs, it appears) that go to one light and join up. And the extra conductor that I won't be using in the 3-wire that goes between the two switches (one of which has an outlet brought off it).

Maybe we shiuyuld move this to discussion, but I'm not sure how.

Found another amusing thing: They tapped off a 3-way switch circuit to add an outlet - so the outlet is driven by a traveler and neutral. So if the switch in the other room is in the wrong position, then the outlet is dead. And ... wait for it ... the outlet is for the propane tankless water heater. Guess I'll remove the switch in the other room, and send hot&neutral to the water heater room (and discard the third conductor); so the water heater will always be energized, and the light in that room can only be switched there.

What's an example of why it'd be advantageous to have two circuits in the same box ? A switch box (perhaps 4-gang) from which lots of loads are driven ?

Ok, well I can't find ANYTHING else on the "ring", so I guess i'll just disconnect at this light.

Found a MWBC to the kitchen, tied to two independent single-pole breakers. At least they're neighbors, so hopefully opposite phase ...

But yeah, as far as MWBC, I should first make sure the two breakers are driving separate NM cables (as opposed to two conductors from a 3-wire cable).

In case you guys aren't auto-notified, I tore into the thing and edited OP.

A wood stove with electric heat strips ? Wow. Yeah, I wonder why equipment manufacturers can't bother to put a simple and cheap piece of circuitry in, to avoid stuff like this (2 circuits). My range runs the oven off 240vac, but because the internal logic (gas controls etc) run off 120vac, it requires a neutral wire; pretty annoying.

That's a cool idea about temporarily using a GFCI

If it were up to me, I'd outlaw MWBC. Just to save 25% on copper costs ?

I actually didn't mean to use conduit bodies, but that's a good idea, come to think of it.

Maybe we better move to the chat-room, lest we get slapped down again.

I'm not actually talking about trying to get the conduit into the corner, but rather it'd sort of cut off the diagonal, so stick into the room a little bit. Kind of a 'U' with a squared-off bottom and then turned upside down (so it goes into knockouts in the top of the two. panels.

How is #6 NM-B acceptable ? It's good for 55 amps it appears. And you're sayng I can't round up if the calculation gave more than 55 amps. I believe we can get down to 55 amps though. The 300 sq-ft study is not meant to be habitable; it just happens to be where the modem/router/phone is. We can pretend I'll run an extension cord to them from living room. That'll save 300*3/240 amps right there, no ? To me, of course, the real concern is, will the breaker feeding the subpanel (from the main panel) trip in ordinary grid-on times.

So I gotta use #6 THWN even if we could massage the calculations down to 54amps. I believe that requires 3/4" conduit (EMT of Sch40 PVC), EGC counting as a "conductor" for conduit-fill calc, I believe. I think I could make that work. Do I read correctly that if we can't get the numbers down to 55 amps, and I have to use NM-B, that it'd have to be #4 ?

@Harper-ReinstateMonica that would only help with the calc for backup mode, I believe, and I'm fine with the 27 amps that 3PE calculated.

I'm confused what is required to operate that load-shedding relay. I can't just have a switch that I flip, whenever I switch the interlocked breaker-pair in subpanel ? If not, I'm not clear what you recommend by "auxliiary contact on the generator breaker". What about simply operating the relay (with 120vac coil) off the wires coming in from the generator inlet ? Maybe that's what you said. I guess if you had a running generator hooked up while in grid-on mode, it could disable the ovens, but who cares (unless AHJ) does ?

I don't understand your worry about overloading main panel busbar. The subpanel is still one of the loads protected by the 200amps main breakers. I understand about the 120% rule with my PV inverter: I'm ok there, because panel rating is 200amps, 120% of that is 240 amps, so I have 40 amps for the solar, and my inverter has max output 16 amps, so I'll be using a 20amp breaker for it. And I'll put the solar breaker at the bottom of the main panel (opposite end from main breaker).

To be clear, your calculations say the feed from main panel to subpanel should be rated at 60amps (so 60amp breaker in main panel, as well as 60amp breaker in the subpanel (the one in the interlocked pair that is connected to main panel), and 6awg THWN (even though I believe technically 8awg would do, something about being able to up-rate that 55 amps to 60amps). And then 30amps for the breaker (in the subpanel interlocked pair) that's connected to the generator inlet.

I believe I can get conduit to work. Won't be able to get it completely buried in wall cavity, but who cares. EMT can allow much tighter bends than PVC, with those "pull elbows". I just hate Romex in those higher awg's.

As far as shedding the range, I'm still astonished it's acceptable for me to do that (interrupt one wire with the relay). Anyhow, I'm going to have to add a few boxes in crawlspace and attic, to extend critical-loads home-runs that won't reach the new subpanel, and the range is one of those, so the contactor can go there. Just need a SPST relay; these are nice ones like I used for my DIY water-heater timer: automationdirect.com/adc/shopping/catalog/relays_-z-_timers/‌​… (click "power relays, open style").

Thank YOU @ThreePhaseEel for this comprehensive response and patiently answering my questions.

Also, imagine AHJ will want to see plywood wall panels above and below the main panel; although apparently it wasn't an issue when they signed off on the house in 1988.

Here's the site. I would remove temporarily remove that piece of plywood so as to use a recessed-type main-lug load center for the subpanel. Central-vac in right-foreground sits only 26" out from the wall where the subpanel will be; dunno if that's ok, or I need to shift subpanel to the left a little way (moving the invisible-fence controller and its outlet). The vacuum exhaust pipe will be a minor bother in routing the conduit.

So my primary concern is that, in grid-on mode, that 60amp breaker doesn't trip if we're baking in both ovens, the minisplit is on, and the well pump starts up. And it looks like we're good there, as far as I can tell.

It's good to have those calculations on what the grid-on and backup feed to the subpanel should be. But based on my experience with the AHJ here, I'm not sure he'll go that deep, probably just looking to make sure stuff is done in a workmanlike manner and that the conductors are sized properly.

Following the link on those contactors I provided on Q&A, they say it is "UL Recognized E191059". Does that work ? It'd please me to learn the water-heater timer I built is "on the reservation". I put it inside a NEMA-1-rated box in my crawlspace, 12vdc coil controlled by this nifty widget: amazon.com/gp/product/B008999RYY/…

It's 200 amp panel (I assume that means the busbars, it is Siemens G2040MB1200CU). Main breakers are 200 amps (pair of 100s). GTI breaker will be 20 amps, at bottom (opposite end from mains) of busbars. I think I'm good to go there.

Ok, I've finally got it - thanks. But, "reasonable" probably isn't good enough, right ? Reasonable, to me, is telling DW "please don't use the oven" (maybe my generator will burst into flames if it's overloaded, but I guess that 50amp breaker I have connected to the generator inlet, would trip first). I like the idea of using relays between 40amp breaker and range, but surely there's all kinds of regulatory issues there, no ?

Yep, just figured that out. Sigh. But let's say you decide on 80 amps for connection to main panel. I don't necessarily need 80amp connection to generator. But I DO need some plausible/reasonable/UL-listed/whatever way of reducing the backup load requirement to that 50 amps. Do I have it right (finally) ?

Just install a little switch on the L2 wire on the oven ? Kidding. Though I've fantasized over doing the same thing on the wire to the defrost element on the fridge.

I don't understand. Siemens makes double-pole branch-circuit breaker up to 100amps (or more), there's whatever gauge THWN, and ? Maybe getting a generator input jack of high-enough amperage ?

See another image I'll add to OP; it doesn't really make sense to me though.

I'm still confused: If use the same size wire from subpanel to generator inlet, as from main panel to subpanel, and make all 3 breakers the same size (the two interlocked ones in the subpanel and the one in the main panel to whch the subpanel is connected), I'm good as far as NEC, am I not ?

I added wiring diagram for range.

Are you saying I have two single-pole relays/contactors between the 40amp breaker and the range, and control those according to if I'm in grid-up or grid-down mode ? Surprised NEC allows that. Although I did somethijng quite similar with my water heater, being dissatisfied with available water-heater timers.

But I WILL have the range's breaker turned on, during power outages.

Coincidentally, I just changed the contactor on my heatpump compressor earlier today.

Am I wrong though, that I can simply run the same size wire to the generator inlet and I'm ok with NEC ? As far as overloading whatever I have connected to the generator inlet, we'll simply know "don't turn the oven on". Is that unreasonable ?

I can easily figure out which leg powers the controls. Tell me more about using a contactor though.