Trying to figure out what service I need to plan for.
Building will be something less than 800sq ft., a 24 or 26x30, or a 24x40 if my tax refund is good this year.
Utilities guy says I can get a 320amp meter at the house, and have 125, 150 or 200 at the shop, or have a 100amp service run off the existing meter.
I have a large 80gal compressor and two welders (MIG and TIG). I know the compressor draws something like 40 - 50 amps at startup. It would make the lights dim for a second at my old shop (100amp service) but never popped the breaker. Welders, maybe 20 amps each.
FWIW, the most likely cause for those lights dimming every time the compressor kicked on was NOT inadequately-rated service to the building, per se; but rather, you were probably inducing some significant voltage drop due to too-long runs of not-quite-big-enough (but still "legal") wiring.
In any event, those two items (compressor & welders) are very probably two of the three major variables which need to be considered (see below for the third); and you need to presume that from time to time you WILL be welding when the compressor happens to kick on. Also, with that much compressor and two welders, can a plasma cutter be far behind?
Not sure how to plan for lights and outlets. Probably have two breakers for the lights and one for every two outlets. Let me know if this is overkill.
Splitting the lights up into two separate circuits is a good idea, even if it's not strictly "necessary" based just on the load. That way, if/when one trips (or needs to be shut down to work on something), you're not left completely in the dark. When it comes to the switching for those lights, it should be well more than two circuits; but that's a separate discussion, really.
HOWEVER... Putting only two 120V outlets on each branch circuit is DEFINITELY overkill. And here is why:
It will be a one-man shop,
This is the third major variable I spoke of. And given this, the odds on you needing more than two general-purpose 120V outlet circuits to cover the whole garage are somewhere between "slim" & "none". To this, I probably
would add an additional 120V circuit for the workbench area itself (since that area typically becomes a magnet for all manner of plug-in "stuff"), and also dedicated (or at least "semi-dedicated") circuits for any 120V stationary machines with particularly heavy draws (such as table/band/miter saws, heavy-duty bench grinders, etc.). Also, putting the garage door opener(s) on a dedicated circuit is generally a good idea. But in all of these cases, those extra circuits would be there more "on general principles" than because they'd really be NECESSARY in a one-man shop.
Setting aside the "special" circuits for a moment, those two "main" 120V circuits should be run TOGETHER, to double-gang outlet boxes spaced at perhaps 6-8 foot intervals all around the shop, with the bottom of the boxes at least 50 inches or so off the floor. In each of those double gang boxes, put two high-quality duplex outlets, one powered from each of the two circuits I just spoke of. The FIRST duplex in each of these "chains" must be a GFCI type, which (when properly installed) will also provide GFCI protection for all of the outlets "downstream" of the GFCIs.
so the likelihood of having more than one high-current consumer going at once is kinda slim.
See above for two possible exceptions to that; and if you do eventually get a plasma cutter, it goes beyond "possible" -- the compressor and the plasma cutter WILL be running concurrently.
Still, all told, I'd wager heavily that 90-100 Amp service will be quite sufficient for everything we've discussed thus far, unless that compressor is SUCH an inefficient old beast as to be well worth replacing just for this reason. Given that (and pending to some degree on the distance between the house and the shop), running some 2-2-2-4 MHF from your existing main service panel (presumably at the house), out to a moderate size (nominally "100-150A", depending on how many branch-circuit slots you want) sub-panel in the shop, will be your most cost-effective solution. You don't need a separate feed off the meter pan UNLESS there is no workable way to feed from the main service panel. In either case, you WILL need a 90A (not 100A) breaker at the main service panel (or in a separate disconnect box, if you tap directly into the meter) to protect the sub-panel feeder. You will also need a back-fed "Main" breaker rated at
at least 90A (100A or more would be OK) in the sub-panel, to act as the "Local Disconnect".
The one thing which MIGHT cause me to re-think the "90-100 Amp" recommendation is if you were to put a large air conditioning system and/or electric space heat in that shop (gas- or oil-fired heat would be no problem, even if it has a substantial blower fan). But even this, in an ~800 ft.^2 shop, is not likely to make all THAT much difference, as long as said shop is well insulated.
I have never seen a common garage of this nature trip a 60 a breaker on a 2 aluminum wire. Even with regular daily use of 5 hp comp.
I think 60A is more than a little optimistic, in this case. Sure, he MIGHT "get away with it"; but that should not be the criteria for designing a new service installation.
Thanks for the input.
I was a little skeptical of the electrician guy trying to sell me 200amp service.
As well you should be.
Even presuming ALL of the "maybe someday" items (plasma cutter, HVAC, etc.), it's difficult to imagine you needing anywhere near that much juice. Heck, as long as this remains a one-man shop, even 125A would be overkill for what you're currently planning on; and would still be nearly overkill even considering the "maybe somedays".
).
I had all the same sh..stuff in my old 24x24 - less one AC unit - on a 60A. Never tripped it. One day I turned on all 14 T8 fixtures, the DP, lathe, mill, bench grinder (1/2 metric HP), ceiling fan, tripped the compressor and cranked the heater. It was too cold to run the 12K BTU AC unit. I clamped on one of the power legs and that side was drawing 49.1A.
