How do you calculate how many amps you need for a new shop?

  • Advertise with us

« back to Focus on the Workspace forum

Forum topic by Rob posted 02-06-2014 05:34 AM 21980 views 0 times favorited 46 replies Add to Favorites Watch
View Rob's profile


704 posts in 3274 days

02-06-2014 05:34 AM

I’ve finally decided that I’m wasting too much time dinking around with my minimal set of power tools, and if I can scrape together enough money I’m going to clear out my single-car garage this spring and convert about half of it into a small woodshop. (Unfortunately I still need to share space with the mower, grill, bicycles, etc.) The biggest problem is, it has minimal wiring…an outlet in the ceiling for the garage door opener, and may one or two double outlets on the one wall shared with the house. I know that’s not going to be enough to run my tools, because I already trip the breaker every time I try to use two tools at once.

At first it seemed obvious how to calculate the number of amps I’ll need: just add up the amps from all the tools, and give that number to the electrician. But I’ll never use all the tools simultaneously, so that seems excessive.

If I have a list of tools that I have or plan on getting, how do I calculate how many amps I need? Do I just add up the amps on all the tools that will be running at the same time, and tack on an extra 20A or so for breathing room? Or do I have to also figure out where each tool will be, which outlets will be on separate circuits, etc.?

For example, the largest assortment of tools and other electric devices I could see myself running at one time would be the air compressor, the dust collector, a shop vac, the lights, cordless drill chargers, computer, garage door, an A/C or heater, and either two random orbit sanders or a saw (table, miter, circular, or jigsaw). Suppose all that adds up to 100A.

1. Does that mean I just need to tell the electrician I need 100-120A in the garage? Or do I add up all my tools, put each pair of outlets on a separate circuit, multiply the number outlets by 15-20A, and give him that number so I never have to worry about tripping the breaker? How much breathing room do I need?

2. Should the 110V circuits be 15A or 20A? (I don’t think any of my tools have a 20A plug, but I think the dust collector is technically rated at 20A.)

3. Since most tools are rated at 12-15A, does it make sense to just put each pair of outlets on a dedicated circuit?

-- Ask an expert or be the expert -

46 replies so far

View Loren's profile


10477 posts in 3851 days

#1 posted 02-06-2014 05:45 AM

Get a shed first and put all the stuff in it.

Considering your space, unless you’re planning to buy a 5-function
combination machine, you’re going to have to make some
real compromises with machinery for furniture making.

It’s not the panel that will limit you, it’s the space itself.

That said you want 20 amp circuits and no, you don’t need
dedicated circuits but you may want one to run the dust
collector on itself. It’s the simultaneous running of
machines on one circuit that blows breakers, not the
number of outlets on that single circuit.

View OhioMike's profile


79 posts in 2366 days

#2 posted 02-06-2014 06:28 AM

Normally I would say that a small one-person shop (like mine) can function just fine with a 60 amp (240volt) subpanel. However, since you also want to power your HVAC system from the same subpanel, it might need to be enlarged a bit, depending on the HVAC system requirements.

Your electrician should do these calculations for you.

Remember, a 60A, 240V subpanel can provide up to 120 amps of standard 120 volt circuits.

As for standard receptacles, I would put them all on 20 amp circuits and I would put the lights on a 20 amp circuit too if your local code allows. (Some local codes require lighting circuits to be limited to 15 amps)

Again, your electrician will know the local requirements.

For reference, my small one-person shop has:

Three 20 amp general purpose receptacle circuits. (My 4.5 gallon air compressor shares one of these circuits)
One general purpose 240 volt 20 amp circuit (table saw, planer and jointer)
One dedicated 240 volt 20 amp circuit for the dust collector
One 20 amp circuit for the lighting.


View Fred Hargis's profile

Fred Hargis

5177 posts in 2697 days

#3 posted 02-06-2014 12:16 PM

Here’s the thing: once you’ve Decide on a subpanel (and you should do that, if you haven’t) the extra cost of 100 over a 60 is fairly minimal. I truly believe that a 60 amp sub would serve you well, but I also believe a 100 amp will give you more slots (more circuits) and plenty of power if you decide to, say, buy a small welder; or some other high current draw tool. Make all your circuits (120V and 240V) 20 amp, and you still have room for any larger ones you might need in the future.

-- Our village hasn't lost it's idiot, he was elected to congress.

View jmos's profile


902 posts in 2573 days

#4 posted 02-06-2014 04:48 PM

I’m not clear, are you looking at a sub-panel? Being in the garage, I’m guessing your house electrical panel in there. Are you just coming off the main panel, or looking at a sub?

Anyway, I would think you have a lighting circuit already, even if you expand the number of fixtures, you should be fine. I’ve got 7 2-tube florescent fixtures and a shop air filter on my 15amp lighting circuit, plus a few other regular compact florescent fixtures. Never had any problems.

Assuming everything you have is 110v;
I would plan on one new circuit for the dust collector, sized for what it needs, 15 or 20amp
another dedicated circuit for the HVAC
maybe one for the air compressor if that is something you use a lot
two 20 amp circuits for tools

You just won’t be running that much at one time if it’s just you. Now, if you plan on having someone else in the shop with you also running equipment, you may want to reconsider.

-- John

View CharlesA's profile


3351 posts in 2001 days

#5 posted 02-06-2014 05:14 PM

How many amps do you need for a garage? Depends on how many guitars you got . . .

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

View bigblockyeti's profile


5286 posts in 1924 days

#6 posted 02-06-2014 05:41 PM

That’s a tricky one. For example some tools like a dust collector, air cleaner and any fan you might be using will pull close to the nameplate amperage with little deviation except for start up. Tools like the tablesaw will rarely come close to pulling the amperage listed while running, but way more during start up. If you add the amperage of all the tools you have, as well as those you’ll likely add in the future, that would provide you with plenty of juice to run everything with lots of capacity to spare! I have a 60 amp service going to my shop and theoretically I could pull nearly 300 amps with the larger tools (the air compressor could pull 36 amps alone). I’ve never had a problem, but I have to remember that starting any tool while the dust collector and one other tool is already running could trip the 60 amp breaker in the main panel feeding the sub panel.

-- "Lack of effort will result in failure with amazing predictability" - Me

View Rob's profile


704 posts in 3274 days

#7 posted 02-06-2014 08:10 PM

Thanks for all the advice, guys!

I’m planning to add a subpanel because I figured it might not cost that much more than running wires all the way to the garage from the main panel in the basement on the other side of the house. I guess that’s a question for the electrician.

There is some small welding equipment I’ll have to factor that in, but I don’t see myself ever using it at the same time as the woodworking equipment. I remember hearing somewhere that intense heat and wood don’t get along….

I didn’t think about the conversion from 240V to 120V in the panel; that means I’ll be able to get by with a lot less than I was thinking before. I’m planning to put in at least a couple 240V circuits for a couple stationary machines and/or welding equipment. I’m leaning toward a 240V table saw but I guess it’ll depend on what I can scrounge up for funds.

I’ll have to look into whether there’s a good place to put a small shed in the back yard for the mower, etc. The real issue is that we have a 4×8 trailer that I probably have to stash in the garage during winter, and it takes up about 1/4 of the garage.

If anyone else has anything to add, keep the advice coming.

-- Ask an expert or be the expert -

View jmos's profile


902 posts in 2573 days

#8 posted 02-06-2014 09:41 PM

If you are doing a sub panel, I agree you should cost out 60, 80, and 100 amp; probably won’t be much of a difference. I had to do a sub-panel for my shop and ended up at 80 amp at the same cost as 60amp. You won’t want to have to do this twice.

If you’re thinking many dedicated 240v circuits, count them up; since they take 2 slots per, you want to make sure your box is large enough.

-- John

View Dan Lyke's profile

Dan Lyke

1520 posts in 4329 days

#9 posted 02-06-2014 10:08 PM

You only need enough to run your largest tool and dust collector simultaneously. That probably means 40A for tools (2 20A circuits), and a lighting circuit (which may exist off your other panels). But I agree with everyone here: There’s no reason to not just put in a 100A panel and be done with it, even if you only have the electrician run four 20A circuits.

Heck, both my wife and I work in our shop, but even simultaneously it’s rare that we fire up the bigger tools when the other person is in there, if only because when we’re in there we’re being kinda social, so it’s band saw and mini-mill at the same time, not router table + dust collector and track saw + other dust collector. I’ve got 100A total to the shop, we’ve probably used 25A peak, that’d be lights + big router in table + shop vac as dust collector.

Re ” ...the conversion from 240V to 120V in the panel…”, if they’re in the same building, you’ll run 4 conductors from your main panel: 2 hot, one for each leg of, 1 neutral, 1 ground. 120 is just one of the hot legs and the neutral. 220 is both hots (and usually you run the neutral and ground too).

And a warning: I planned out the electrical for my shop to put a socket assembly on each leg of a shared neutral circuit in each box. So at each outlet I have 4 sockets, 2 on one circuit, 2 on the other. I thought I’d be able to run fewer wires, save a little money. However… all of your sockets will need to be GFCI protected, and the shared neutral means I had to run GFCI breakers, rather than using a GFCI socket and hanging the rest of the circuit off the socket. Dang, those GFCI breakers are pricey.

-- Dan Lyke, Petaluma California,

View oldnovice's profile


7334 posts in 3571 days

#10 posted 02-06-2014 11:20 PM

A lot depends on how many tools will be operating simultaneously. You can assume that the dust collection system will be running for whatever other power may be used and you HVAC system.

Dust Collection + power tool(s) + HVAC = power required, solve one formula for 110 and another for 220

I do not have enough power in my garage but I work around it.
  1. 220…...30…......1
  2. 120…...20…......2
  3. 120…...15…......4
    As you can see, very under powered!

-- "I never met a board I didn't like!"

View RogerM's profile


799 posts in 2603 days

#11 posted 02-07-2014 05:22 PM

Rob – Panels do not cost a lot of money but it does cost a lot of money to upgrade one. A 200 amp panel will probably more than meet all of your needs and there will be room for more breakers and circuits should need them in the future. I have a 200 amp breaker in my shop which works fine and gives me a lot of confidence that should I need additional circuits in the future it will be a simple matter of adding a breaker and stringing some wire. Don’t skimp on this, the bigger capacity breaker panels gives one a lot of peace-of-mind.

-- Roger M, Aiken, SC

View oldnovice's profile


7334 posts in 3571 days

#12 posted 02-07-2014 05:39 PM

+1 RogerM

-- "I never met a board I didn't like!"

View Richard's profile


1922 posts in 2894 days

#13 posted 02-07-2014 07:35 PM

start at 200 amps and work up from there. :)

View Rob's profile


704 posts in 3274 days

#14 posted 02-07-2014 09:59 PM

Aww, man, just when I thought I was starting to get a handle on this and thought 60A might be enough and 100A would leave me room to expand, 3 of you guys come and say 200A. It’s still at least a couple months down the road, but I’ll keep watching for more recommendations here, then I’ll get a bunch of different quotes and will report back.

-- Ask an expert or be the expert -

View oxyoke's profile


57 posts in 2557 days

#15 posted 02-07-2014 10:20 PM

60amps will work fine 100amp will give plenty of space to expand your not running a production shop.Half of a 1 stall garage,

-- Bill Byron Center MI

showing 1 through 15 of 46 replies

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics