Backfeeding one subpanel via the main from another subpanel

This isn't common in my area. I'm surprised this passes code. Did you identify this "normal" breaker as being the feed so that someone working on the panel knows how to de-energize it? I don't have a current NEC, but I'd like to see the section reference that allows this practice. I'm also surprised that all circuit breakers would function properly in this configuration. I think most are still thermally activated so they likely don't care which direction the current is flowing, but I could imagine a design that was current direction sensitive and might not work properly if backfed.

Matt

That was the only requirement; a sticker identifying the disconnect breaker (IIRC, the sticker said "OFF" ;-).

BTW, the reason it was done this way is I wanted to use a 100A box for a 40A service to a subpanel in my garage. 40A boxes didn't have enough slots and 100A service was way more (and expensive) than I needed.

I believe they're generally thermal and magnetic. How would the breaker know which direction the current was flowing? It's AC.

The panel has to be approved for that application. (It ain't much: the extra main breaker should require a screwdriver to remove it. Just a reminder to the electrician that it isn't a just another 240 circuit.)

vinging a second 2 pole slot be used as an

Well, "they" make panels that just don't have a main breaker. With the prevision for making a particular two pole breaker difficult to remove, these can be used as service panels or branch panels or whatever.

You can't buy a 40 or 50A main breaker for a larger panel?

If the breaker is thermal only, it won't know which direction. If it is magnetic (has a coil) or a capacitor, the phase difference will indicate direction. I don't know if this matters as I'm not familiar with the innards of a modern circuit breaker, but I certainly wouldn't automatically assume that polarity is immaterial.

Matt

Not at least at the time (mid '80s). A 100A panel required a

No, they're *both* magnetic and thermal (thermal for long time- constant events and magnetic for gross overloads). Either way, the breaker has no idea where the source and where the load is. Only the current matters, since the breaker doesn't even know about ground (ground isn't connected to the breaker at all). The breaker *cannot* know which end is which.

That's good to know. It wasn't rated for any such thing, but passed the inspection with flying colors. This was in the mod '80s so perhaps the code anticipates such things better now?

...good to know. Thanks!