100amp or 150amp?

[haugy]

Getting the shop wired up. The plan was a 100amp sub panel with #2AL feeder to it.

I started thinking, with me running a 240v A/C unit, a 240v Compressor, and if I happen to be welding with all the lights on.........it's going to be interesting.

Is a 100amp good enough for all that? Or should I go up to 150amp? That's an assload of power I know.

Thoughts?

And yes, I know I'm posting alot. I'm trying to get this wrapped up. Thanks for putting up with my 2 million questions.

 

[dougmac]

Getting the shop wired up. The plan was a 100amp sub panel with #2AL feeder to it.

I started thinking, with me running a 240v A/C unit, a 240v Compressor, and if I happen to be welding with all the lights on.........it's going to be interesting.

Is a 100amp good enough for all that? Or should I go up to 150amp? That's an assload of power I know.

Thoughts?

And yes, I know I'm posting alot. I'm trying to get this wrapped up. Thanks for putting up with my 2 million questions.

For what it is worth........... I am running everything that you are except the A/C unit and am doing just fine with a 60 amp sub panel.

 

[Stuart in MN]

Without knowing the current load of your compressor, welder, air conditioner, lights or all the other stuff in the shop there's no way to tell.

 

[jvitez]

One thing that confused me until I had it clearly explained is that it's not really the amperage that counts, its the wattage first.

A 100 amp panel at 240V= 24,000 VA or watts (power factor 1.0 for residential use). Add up the wattage of each item you could possible use at the same time, whether 240V or 120V, ie all lights, electric heaters, welder, running air compressor, etc. Are you over 24 KW? If not, you're fine.

For a shop/garage you need to use the actual wattage ratings, not percentage calculations like in a residence, which assumes you won't have everything on at the same time. You add up the gross wattage of everything, then divide by 240 to see if it's below 100 amps. 24 KW running at the same time is a lot of power.

 

[dougmac]

For a shop/garage you need to use the actual wattage ratings, not percentage calculations like in a residence, which assumes you won't have everything on at the same time.

Hummm ... I would have thought that you would not have to calculate 100% for a shop considering that you would never use all of your equipment at one time?

 

[Falcon67]

For what it is worth........... I am running everything that you are except the A/C unit and am doing just fine with a 60 amp sub panel.

I'm running all that plus AC or the G73 heater on a 60A feed, no issues. I use a 125A panel in there to get plenty of breaker space.

 

[lametec]

Guesstimated running loads:

15A for the comp (3.5HP), 10A for the a/c (20,000 btu), 50A for the welder (maxed out stick welder!), under 20A if you're talking sub-200A welder.

I'd say you have more than enough with 100A, but then again, depending on how much more the 150A service will cost, you might want to go that way anyway.

 

[jvitez]

Hummm ... I would have thought that you would not have to calculate 100% for a shop considering that you would never use all of your equipment at one time?

Partially correct. Maybe I wasn't clear: you need to add up the gross wattage of everything YOU might use at the same time. Of course AC wouldn't be used at the same time as an electric heater. If it's a one man shop, you won't be using a spray booth at the same time as a welder. So look at everything you have or might want. Figure out how you work, and what things you could see being used by you at the same time, then add up that particular combination of wattages.

If you're really close to being overloaded, could you reduce the load by how you work? Maybe just flip off the switch to a monster air compressor when you're welding, just in case it decides to recharge exactly when you're running a bead, that sort of thing. If you absolutely hate the idea of doing that, then you must add the compressor load to the welder load for your calculations. That's what I mean by calculating how you yourself plan to use your shop. Otherwise, POP, lights out!

This is why the specific percentage calculation of NEC/CEC load calcs used for residential construction won't work for workshops/garages. Be specific, be accurate. Life is great if you do.

 

[mrb]

cant supply #2 AL with 100 amp breaker, 90 is ok.

 

[haugy]

With a 5hp 80 gal compressor, 12 high bay 6-bulb lights, a plasma cutter, and an A/C unit, I think I'll be pushing it. And I can see me using all of these at once. That's what has me wondering.

I talked to the guy, and he said it was only another $300 for the 150amp. So I think I'll go ahead and up it. I'd rather have too much, than have something go out right as I'm trying to do a precision weld on one of my hot rods and the lights go out.

 

[Aceman]

Since the other thread mentioned you're feeding this from your house panel, I'll just mention a couple things:

1. Do a load calc on your home first. And your shop. Maybe someone can find an online worksheet for you. This will tell you what you really need for power in your shop and if your house has enough available for everything. You may not really need a 150 amp panel in your shop.
2. 2 pole 150 amp breakers aren't common. You'll need to make sure you can even get one for your existing house panel as well as price it out. Big breakers=big bucks. You're load calc will tell you if you even need one this large, the biggest 2 pole residential breaker I usually come across is a 125.

 

[dougmac]

Maybe I wasn't clear: you need to add up the gross wattage of everything YOU might use at the same time.

Ok.... that makes more sense to me......

 

[hillbilly1]

I think you will be fine with 100 amp, breakers are are designed not to trip instantly when you hit there rating, there is a cushion built in that allows temporary overloads. If your doing precision welding, it's not likely you will have the welder turned all the way up, even if you do, you are limited by the duty cycle of the welder. You risk overheating the welder from continous welding anyway and will have to allow cool down time between welds. Unless your A/C unit is really big, you shouldn't have a problem.