Yes and no, sorta-kinda.

Your outdoor box will become your main panel and the one in the cabin needs to be wired as a subpanel (seperate ground/neutral). It already should be, of course, but check.

Likely the service equipment BEFORE your existing disconnect (ie: meter base, wiring) is rated for 100A. You're talking about 100A load for the cabin, PLUS whatever you might add later. You will thus be exceeding the rating. You can't do that.

I would instead use the 100A breaker in the first two slots as the MAIN, backfeeding it. When you do this you're required to bolt the breaker down, so make sure you have a box where this is possible (you'll need to buy the hardware - usually a bolt and a clampy thingy). Then attach the cabin to the lugs. Now your cabin has a full 100A. You can add other loads later, still automatically not exceed 100A, AND not have the limitation of six - not that I expect you to have more than 5 outbuildings anyway..

"Likely the service equipment BEFORE your existing disconnect (ie: meter base, wiring) is rated for 100A. You're talking about 100A load for the cabin, PLUS whatever you might add later. You will thus be exceeding the rating. You can't do that."

I'm not sure I'm on board with that statement, Mike. What code requirement prevents a 100-amp main breaker servicing, say, a 100-amp feed to a sub-panel (the cabin), a single-pole 30-amp circuit for an RV hookup and, oh, let's say a 20-amp MWBC to a garage?

So could the actual load demand exceed the service capacity under full-use circumstances? Well, sure. But that's true of probably 95% of all 100-amp main panels in houses everywhere. Unless the cabin is already actually demanding current at or near the capacity of the service, it may not be a problem at all. (It'd take a load demand calculation based on many more details and maybe some empirical data to settle that.)

"I would instead use the 100A breaker in the first two slots as the MAIN, backfeeding it. When you do this you're required to bolt the breaker down, so make sure you have a box where this is possible (you'll need to buy the hardware - usually a bolt and a clampy thingy)."

Personally, I'd prefer a "true main panel" in this case. But if you must backfeed a principal service disconnect, then make sure the breaker is rated at 15kVA. (Note that this is an up-leveled requirement exceeding many ordinary 100-amp breaker ratings.)

***

One other consideration that may affect the OP is whether the run from the disconnect to the cabin is three-wire or four-wire. If it is three-wire (two hots plus a combined neutral/ground), then you may run into problems with the 2008 code revisions which deep-sixed three-wire feeds.

This is worth checking out, IMO. How many wires run from the disconnect to the cabin?

[If it's a three-wire feed in an jurisdiction that has adopted the 2008 Code, one might nevertheless argue that the feed is grandfathered when what you're talking about is replacing the disconnect. Still, I'd get the AHJ's (code enforcement official's) take on this before proceeding. Or not. YMMV.]

I'm not sure I'm on board with that statement, Mike. What code requirement prevents a 100-amp main breaker servicing, say, a 100-amp feed to a sub-panel (the cabin), a single-pole 30-amp circuit for an RV hookup and, oh, let's say a 20-amp MWBC to a garage?

I was afraid of that - my statement was a bit confusing. My English, while praised by my schoolteachers, is a nightmare :)

Nothing is wrong with the scenario you've proposed, and all I MEANT to suggest was that he install it just that way.

His original plan, I assumed, was to tie the meter base to the lugs of a lugs-only panel, and insert a 100A breaker to feed the cabin: ie, initial installation only has one breaker, with the bus on the line side. Additional breakers could be added up to the six-throw limitation, with no OCPD upstream of them. I proposed flipping this "upside down", so that additional loads will be added to the load side of the breaker.

I'd recommend the breaker be rated to a number of KAIC, rather than kVA. See? You mis-typed too. I'm not the only one! :D

I am NOT in favor of having a 100A main AND a 100A 'cabin' breaker, as I'm assuming there's another 100A breaker inside the cabin, and having three breakers in series all feeding the same thing feels like a waste of materials.

Maybe I can draw it better than I can say it :)

{{!gwi}}

"I'd recommend the breaker be rated to a number of KAIC, rather than kVA. See? You mis-typed too. I'm not the only one! :D"

Yup, you got me on that brain fart.

"This is what I suggest instead."

Yup. Nice diagram, BTW.

The diagram really helped. Your original is what I had in my head but your suggestion makes sense but how do I appropriately accomplish backfeeding? Do I need a special box and or breaker so the feed off the 100amp breaker feeds the cabin and the box? The box I was looking at has lugs for the load and then 8 places for standard breakers. This was a square D box QO. By the way the box in the cabin does have a 100A breaker and is fed by a 3 wire 2AWG aluminum.

Alright, well the existing feed to the cabin is going to be a problem. Possibly a big enough one to discourage you. You need to replace that with 4-wire. You need to have a seperate ground wire because the cabin panel is now becoming a subpanel. Inside, you will have to remove the bonding screw, install a seperate grounding bar, and make sure all of the ground wires are ground, and neutral are neutral.

If we're still moving forward...

Backfeeding isn't, electrically, any different, it's just a matter of putting the wires in different places. The only difference we care about is a rule. When we backfeed a breaker, we have to ensure that there's no way it can become 'unplugged'.

Without searching too much, I found this (click me) for QO breakers. The picture isn't what I've seen before but it should give you an idea of what we're talking about. The whole point is to make sure there's absolutely no way that breaker can be unplugged without a deliberate effort on the part of someone who, we should hope, knows what they're doing.

PK2MB. That's the part number of what you need. Make sure it'll fit the panel you're looking at.

PK2MB

The present wire from the pole to the cabin is running through 1-1/4 conduit. Do you think the ground could be pulled? Its about 30' from pole to cabin. What size wire would this need to be? What about if I went back to my original plan and upgraded the wire from meter to outdoor panel and go to a 200A outdoor panel to handle the other branches? Thanks again for this info! You have been very helpful!

Assuming what you're working with now are three full-size insulated conductors (i.e., three #2AWG/AL), you'd need to add a #8AWG copper ground or a #6AWG aluminum ground.

For that distance, I'd probably opt for the copper if only because it's physically smaller.

Given that mix of conductors (3 x #2 and 1 x #8), 1 1/4" PVC conduit is the minimum size satisfying fill requirements.

So, yes, it's certainly possible you could add a ground. It won't necessarily be a piece of cake, but it oughta be possible with a good strong pull rope and plenty of lube. You'll probably need to pull the existing wires out and then re-pull them with the ground.

BTW, do you happen to know what version of the Code is in effect in your area? Not all locations have adopted the 2008 code yet and if your area is using an older version, the original three-wire service may still be legal to use in a rework of your main. (If so, better do it fast!)

Second TT's input, except that I'll go so far as to say you DO need to re-pull the existing wires. It's going to be a tight pull, and given the resulting friction, I can envision the new wire "sawing through" the insulation on one or more of the other conductors, even with lube.