How do I know if I can run power to a garage from my breaker box?

Question

My breaker box is a 200 amp service. There's 16 openings and only 1 blank space for a new breaker. I have a garage about 100' away from the house that I'd like to run lights and a few plugs for tools from this panel. (The strongest power draw would be a cheap harbor freight air compressor.)

When I add up the amount of amps on the breakers already in the box, its well over 200. I know not everything runs at once, so how do I know exactly how much I can add to the box before I start dimming the lights in the house when I use a drill in the shop, or if the air compressor kicks on, it throws the breaker? Ideally, I'd like to run it off of the house and not install it's own meter and supply since the power company charges commercial rates for the second meter at a single address.

I know they make sub panels but I don't know when those are the only option or how involved a job it is to add one.

I have enough experience to run the wires and do the plugs/switches but this part is new to me.

Answer 1

No need to worry, a 200 amp supply box has more than enough reserve to power your garage. If there ever was an exess draw - anywhere in the system - your circuit breakers would react by tripping.

You could run a sub panel to the garage, in case you ever wanted to use more powerful tools in the garage.

Install a 50 amp duael breaker in the panel, run #6 three-wire line to the garage in a plastic conduit line and connect it to a sub panel circuit box in the garage. Then you can run individual 20 amp circuits from the sub panel to power outlets in the walls and another circiut to your workbench, etc. you could run 3 or 4 20 amp circuits in the garage. This would leave you in good stead for any future needs in the garage.

Answer 2

So you can put one more tandem breaker in your panel, thereby freeing up one additional slot and, with the existing free slot, install a 2-pole breaker to send 240-V power to a sub-panel in the garage.

Does your panel have a diagram showing which parts of the panel will accept tandem breakers and (by implication) which parts exclude tandems? Will you have to rearrange breakers to get the slots in the area which allows tandems? I once tried to install a 2-pole breaker where it wasn't allowed and wondered about this strange interfering tab, but tried to force it in anyway. To my credit I stopped before I actually got out the hammer (just to tap it in, you know).

NEC 408.54 says “A panelboard shall be provided with physical means to prevent the installation of more overcurrent devices than that number for which the panelboard was designed, rated, and listed.”

Your local electrical code inspector will tell you what size breaker you can use to feed the sub-panel. Size the wire generously for lower voltage drop now and so that it could be upgraded later if you or someone wanted to.

EDIT

@Giraffe what are your present high current 240 V circuits used for? It is possible that one or more might be downsized if you have modern energy efficient appliances. This might allow a larger breaker to feed the garage subpanel.

My house has a 50-A breaker for the electric range, but we have replaced the original range with a more efficient one which only requires a 40-A breaker. So I could get 10 A of additional capacity by putting in a new breaker there. I just haven't gotten around to swapping it out.

The other 2-pole breakers are a 30-A for the clothes dryer, 40-A for the a/c condensing unit. We have a new clothes dryer and when I get a new a/c unit soon it may be possible to user smaller breakers there.

In my split bus panel the feeder breaker for the lower section is only 60 A, even though the sticker on the panel says up to 100 A is allowed. (In my panel the feeder breaker is connected to the lower section with heavy wires, but I don't know what size.) I asked the electrical code inspector if I could just assume the wires are sized for 100 A and he said no. He said I must verify that the wires are large enough for a larger feeder breaker. I suppose this means that this wire was, or might have been, installed by the onsite electrician and might have been sized for the original 60 A feeder breaker.

Answer 3

Ideally you should do a few calculations to determine if you can put a new load inside the panel

How many outlets do you have on the 15 A Breaker, 20 A Breaker

How many 30 A Breakers 60 A Breakers do you have

How many double pole breakers do you have, your main breakers are typically 100, 150, 200 for the time being, but all your breakers inside of your panel should add up to the main breaker, with the exception of at least 100 extra Amps depending on your panels load

I = Amps P = Watts V = Volts

( V / P ) = Amps needed on that line ( I / P ) = Volts needed ( I * V ) = Watts

10 outlets on a 15A Breaker probably isn't going to flip the 15A Breaker if you're just plugging in small stuff, but really you should walk around your house and add up everything on each breaker individually and determine if its 80% of the load, Your Main will turn off everything if you go over

Say you have a 200 Main and 120*16

200 * 1920 = 384,000 watts of power

So now you know that you can divide 384,000 into 16 breakers. So if you have 5 - 15A outlets on one 15A Breaker I*V=P What the largest thing youre going to plug in on those 5 outlets. Add all the items you could plug in at once to those 5 outlets and add it into your equation, Then move on to the next circuit. Easy Peazy :)

Then P + P + P = ? Until you get to 80% of 200 Amps

Then you can find the Ohms of each wire by using the R for resistance

( V^2/P ) = Ohms of resistance

The Watt is Power which is equal to the unit of 1 Joule per second, if you wanted to find the Joules you could use

Seconds * Ohms * Amps^2 = seconds * watts = Joules

Resistance says how hard is it to flow

Voltage says how strong is the flow

Amps say how much is flowing

Watts say how much Power is being produced