I did some work in a building where there were two subpanels run off the same feeder. ie, the main (which was a HUGE RACK - this was a school, not a residence) contained one breaker to protect the wire, which then went to the first panel, which had "double lugs", then another set of wires of the same size went on to the second panel... so both 'subpanels' ran off the same breaker and wires.

I have absolutely NO IDEA if this is legal in your application, since you're talking about seperate buildings, but since there have been no other responses (c'mon people, wake up!) I thought I'd chime in.

Sounds like an awful lot of work to save 25' of wire. I would put another 60 amp sub in from the main panel with 6/3 wire. All inside work and no digging or trenching or outdoor cable.

In most cases the new sub has to be 50 amp or larger..It would be best to check the load amps on the current sub with everything on and running...that would give you a total of what you are currently using for amps from your 60 sub..If you find that you are not using a bunch of power of the sub then i would say yes you could come of that with another 50 amp breaker but i would rather see ya take power from the main panel to the garage. The closer you are to the 200 amp main the cleaner power you will get. By cleaner i mean the less resistance you will get from not feeding through the sub in the shed then from the shed to the garage. In any case check with your local codes first and the AHJD in your area.

"In most cases the new sub has to be 50 amp or larger.."

Really? Is this some new code requirement?

I would think that with the loads the OP is talking about (a few outlets, garage door opener and some lights), as little as a two-pole 20- or 30-amp sub might be entirely adequate, so long as those power tools aren't monsters.

Under what circumstances do you claim a 50-amp sub is the minimum requirement?

With respect to grounding: If you feed from the main panel, all you need to do is run the sub-panel's grounding conductor back to it if the sub is in an attached structure. If the structure is not attached, you are required to drive a second grounding electrode at the site and bond the sub-panel to it in addition to running a grounding conductor back the the main panel.

I'll assume that that has been correctly handled for your shed's sub, and it has its own GE.

Others here will correct me if I'm wrong, but if you feed from the shed's sub, you will need to drive a third grounding electrode for the garage's sub, since you're no longer feeding from a panel in an attached structure.

Personally, I'd pull from the main panel regardless, instead of "daisy-chaining" feeders. More reliable, and just makes more sense.

gblentz: Nice job of raising some of the grounding issues. I'd been mulling those over too and your thinking mirrors mine, but don't take that as authoritative in the least!! Does anyone know for sure how the main-building-to-detached-building-and-back-to-main scenario plays out with respect to earthing?

*****

Back to another subject....

"The closer you are to the 200 amp main the cleaner power you will get."

I must have missed the part about how the OP's shed has a a coal-burning generator.

But since you mention resistance, I'm guessing you mean voltage drop, eh? Well, in this case that's a negligible factor. Let's play with some numbers, ok?

The OP doesn't give the distance from the main panel to the shed, but the worst case scenario would be around 90' from main panel to the shed. That assumes that, under the OP's proposal, the wire to the shed goes the whole 65' to the garage site, then 35' past the garage site to the shed, and then 35' back to the garage, right? (Any other triangle yields a shorter total run.)

OK then, the voltage drop for a run of 90' of #6AWG copper in PVC conduit delivering 55A current at 120/240V is about 1.9% under 60-degree conditions. A "backrun" of 35' using #8AWG copper serving an actual high-water demand load of ~20 amps results in an additional 0.2% drop, for a whopping total of about a 2% drop, which is well within the usual recommended target of 5%. A homerun from the main panel might have a slightly smaller drop, but now we're really splitting hairs.

I grant you that, all other things being equal, short runs have advantages over long runs. I'll also readily concede that the longer the run, the more the resistance. But "clean power" isn't a deciding or important factor here. And (not that it really matters) who said the OP has a 200-amp main panel?

Hey T-tom sorry for the confusion on your part...in my area which is IOWA the use of anything less than 50 for a sub is not allowed..it used to be back in the stone age but we have advanced now. You are able to run a branch circuit like a 14-3wg or 12-3wg but if a panel is to be set then, 50 is the smallest. As for the " closer " to the panel issue its not for the voltage drop itself but the annoying problems you see the further from the main the larger loads get and by larger it could be a shop 240 vac or saw or drill press or vac system but as you may or may not know and I am guessing "not know" The further you get from the main you run the risk of lights dimming when using saws or other loads greater then a "night light" I have seen it many times where this has happened. people put in subs from subs and then at the third sub add a sanyo unit that they put at the bottom of the 24 panel and want to know why all the lights dim every time the thing kicks on...that was my point for saying that..

...in my area which is IOWA the use of anything less than 50 for a sub is not allowed.

Uh huh. That's what I figured. So it's just local lore (or possibly a municipal or state code quirk or something like that) and not a verifiable "general" requirement at all (i.e., NEC or, more to the point, the CEC). Right? Heh, for all I know, the CEC may have a 50-amp minimum.

"As for the " closer " to the panel issue its not for the voltage drop itself but the annoying problems you see the further from the main the larger loads get and by larger it could be a shop 240 vac or saw or drill press or vac system..." etc. etc.

Uh huh. And that has nothing to do with voltage drop, right? The annoying problems are caused by _______? Oh yeah, I forgot; it's unclean power. And there's no point in doing the calculations...it's just better to shoot from the hip because you've seen lights dim in some underwired situation or another. You're right: I didn't know that was the best way to come up with electrical specs. (I do, afterall, live in the stone age.)

I have seen it many times where this has happened. people put in subs from subs and then at the third sub add a sanyo unit that they put at the bottom of the 24 panel and want to know why all the lights dim every time the thing kicks on...that was my point for saying that..

And what, exactly, does this have to do with OP's actual situation? (I must have missed the third subpanel feeding that HVAC equipment in the garage.) Oh yeah, I forgot: you've seen some amazingly underwired and overextended three-deep daisy chains that will barely keep a night light glowing. Well, trust me, so have I.

Ya know, I too would prefer a main-to-sub home run in this case and I'm not adverse to upping capacity to allow for possible future needs. But it still bugs me when folks give DIYers "authoritative advice" that only serves to reduce their valid options and cost them more $$$.

That is all.

adding...

Well, to be fair, there is some advantage in not "daisy-chaining." It's true that motor surges drawn against the shed subpanel would have less of an effect on the garage subpanel. At the moment the pool motor kicks on, that surge would probably be absorbed almost completely by the main panel.

As long as the loads are calculated properly and the feeders are sized accordingly ther is absolutley nothing wrong with feeding a sub-panel from a sub-panel.

>As long as the loads are calculated properly and the feeders are sized accordingly ther is absolutley nothing wrong with feeding a sub-panel from a sub-panel.

Nothing wrong--just a potentially more-involved alternative in this instance if a direct connection to the MP is feasible.

>Does anyone know for sure how the main-building-to-detached-building-and-back-to-main scenario plays out with respect to earthing?

Man, whatever happened to Tom_O? I'd bet he would know. Unfortunately, at least from what I can tell, his last post on this illustrious board was back in the beginning of February. :-(

In any case, he lives on in a concise-but-informative FAQ article, What should I know about installing a subpanel?. And, while it doesn't answer this particular question, the OP (and others) might it find useful.