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.

  • 3
    the Sub panel is your best option. Running underground wires - run them in a conduit designed for underground use. Make sure you mark out where that underground wire is running - just in case you sell the house. How much Amperage your current service is already using will determine how much you can use for the Sub Panel. You can't just pick how much you want. Also you will need to check local codes to see if you require a permit and or inspections to do this.
    – Ken
    May 6, 2017 at 20:41
  • How many square feet is your house, and what do your other existing loads look like? May 6, 2017 at 21:51
  • 1
    With only one slot left in your main panel you would not be able to have 240-V power in the garage, unless you rearrange your panel to free up at least one more slot. If you only want 120 V power, then I think you would not need a true subpanel just a wire from the main panel to the garage to a box with a cutoff switch. But since the trouble of trenching 100' is the same for 120 V and 240 V you would be advised to rearrange your panel to free up another slot. Depending on the model of your panel you may be able to free up one or more additional slots with so called tandem breakers. May 6, 2017 at 23:09
  • m.startribune.com/… According to this website, I can use 4 double breakers on my box because my model number is TC1620, meaning I have 16 maximum spaces and 4 maximum doubles. Since I already have 3 doubles, I could add 1 tandem into my box, giving me a total of 16 and 20. That would give the 240 of love to have out there, right? Once the cables make it to the garage, I can run it into a sub panel and then divide as need be, correct? How do I know how many amps I can run on that line?
    – Giraffe
    May 7, 2017 at 2:03
  • 1
    @JimStewart you can have a 120V subpanel. There are several ways to do that. May 7, 2017 at 3:04

3 Answers 3


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.

  • The breaker box only has an opening for a single breaker so the double breaker idea won't work. THREEPHASEEEL, As for my other loads, there a double 30, double 25, double 50, then a 20, 20, 15, 15, 20, 20,20,20,and 15 all in singles. By my math, that's a total of 270 amps but obviously it's not all being drawn at once and those are the max loads. KEN, I've checked with local coses and they do allow me yo run power from the house box like this, as long as it's within safe limits. The power company is actually the one that suggested I try to come off of the house supply.
    – Giraffe
    May 6, 2017 at 22:12
  • What type of heat gas? With the loads described I would add the 4th double breaker that will free up a 2nd slot to run a double to your shop. From the breakers listed even if everything was full load for the breakers listed at the same time it would be right at 200 in truth it would probably be below 150 or less how often do you turn all the stove elements and oven elements on at the same time at full power and leave them on for 3 hours every light in the home on and fully load all the outlet branch circuits. Based on the above information I would there is enough power to run a 60amp 240 feed.
    – Ed Beal
    Dec 28, 2017 at 14:55

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.


@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.

  • The actual panel has doubles on 1/3, 2/4, and 14/16. How do I know if the double breaker can be 30 amps each or only 20 each?
    – Giraffe
    May 7, 2017 at 14:24
  • You almost certainly will not be limited to 30 A. I'll bet the electrical inspector will let you put in at least 60 A and maybe more (100 A?). BTW when describing double pole breakers one does not specify the current limit as being "each". That is understood. (And one never adds the current in the two sides to get a total current.) The inspector will tell you what he will allow. Some of the experienced people on this site can tell you what to expect. May 7, 2017 at 15:35
  • @Giraffe your photobucket link above gives me an error. May 7, 2017 at 18:52
  • Sorry about my incorrect term usage. I'm more of a sawdust maker(woodworker) or grease monkey.
    – Giraffe
    May 11, 2017 at 15:40

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

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