So this is a question that's been asked a lot, but reading individualized responses is creating confusion for me, not answers. I have a detached garage with existing power, and I have added a few items to the circuit. Because I don't know what the original design of the circuit was, I'd like to remove the additions and add my own dedicated circuits to operate what I need.

I will leave the 1/2 hp garage door opener, the single bare bulb light fixture (although I could remove this with no stress on me) and the two duplex receptacles on their original circuit.

I have only three empty slots in my main breaker box and will run 12/3 wire about 200 feet to my garage (unless suggested to do otherwise.)

What I'd like to power is:

  • 6 four foot florescent light fixtures, two T8 bulbs each will be switching to LEDs as budget allows, on a switch near the entry door.

  • 8 LED recessed lighting fixtures in the eaves of the garage, with one motion detecting flood light out front, dusk to dawn switched in the house and the garage.

  • 8 receptacles that will be responsible for powering: 3 to 4 power tool rechargeable battery chargers that may or may not be used at the same time. a 7" power saw, a 12" miter saw, a 10" table saw and eventually a light to medium duty MIG welder (110 v) and a 2.5 hp air compressor with intermittent auto start, a bench grinder, a shop vac, and assorted hand tools, grinders/drills/soldering irons/routers etc.

The power tools (with the exception of the chargers) won't be used at the same time, I only have 2 hands. But I can envision using the big saw and having the compressor kick on. I obviously don't want to suddenly be working in the dark, or to burn my garage down.

What should I run from the main breaker box to the garage? Should I install a sub panel? What size breakers for each circuit? All three circuits now, or two with future expansion? How would you design the circuits to accomplish what I want to do?

I understand, ahead of time, that the absolute best way to do this would be to hire a reputable electrician, but I am a big time DIYer, and would like to learn as I go. I also understand that I am only asking for advice from experienced folks, and am still responsible for making my own decisions, and facing any potential consequences....

To be more specific, I have a GE 200 amp main box on the outside of my house, say street west side. The detached garage (attached by a breezeway roof overhang) is in the back yard east side. I stepped it off and being pretty liberal came up with a 167 foot run. The plan is (until advised otherwise) to run a conduit from the main box up to the attic, through the attic and the breezeway connector to the garage. My local code prohibits aluminum wire, so suddenly my small project just became a major expenditure and overhaul. The three empty slots in the main are randomly placed, I would have to relocate one double pole one position upwards to gain two adjacent slots for an additional double pole. I researched and simply muddied my own waters. Went looking for answers, and came back with more questions.
house sketch

As you can see from this very, very rough sketch, burying the feeder wire is not an option. The solid black line between the two buildings represents the breezeway which connects the two, and through which the existing wire is run, down from the attic to the first story ceiling and through the connecting run. I priced some 2/2/2/4 for the feeder and holy COW, I think I need a copper mine of my own.

Anyway, those are the distance specs, with the same power tool toys as described above. Still--any suggestions?

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    Welcome to SE. A sub-panel is probably what you'll want so you have more control over individual branch circuits and a higher current capacity. As it is, though, your question is too broad. Do some reading on the subject and refine your post to a single, specific question. – isherwood Feb 14 '16 at 20:04
  • @Tester101 has a "standard garage subpanel" answer which I can't seem to find at the moment. It covers the bases pretty well. At this point the price of copper .vs. aluminum may make aluminum feeders more sensible. I would not bother to leave the old circuits as is; run a subpanel and be done. – Ecnerwal Feb 15 '16 at 1:24
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    It would be very surprising if your local code prohibits aluminum conductors of all sizes. Most will prohibit 10 and 12ga as formerly used for 20 and 15 amp circuits with resultant problems. Large feeder conductors are a completely different animal. – Ecnerwal Feb 19 '16 at 4:17

As Isherwood and Ecnerwal advised, it would be in your best interest to set a sub-panel in the garage.

TL:DR Set a 60 amp sub-panel and feed it with #6 NM wire from the main panel. Separate all your neutrals and grounds at the sub-panel and buy the ground bar kit for the ground wires to attach to in the sub-panel. DO NOT install any bonding screws between the box and neutral bar. (If your welder draws more than 20 amps you should probably upsize this to 100 amp panel fed with #3 wire if you plan to have the compressor and welder on at the same time.)

Longer version: The size of the panel depends on what you plan to run now and what you may plan to run in the future. The 2.5 hp compressor is the largest load you listed and will draw a little over 24 amps at according to Table 430.248 of the National Electrical Code. Let's say 28 amps at 120 volts. (Is there any way to re-wire the compressor for 240 volts? This would halve the current and reduce your voltage drop.)

The proper way to size this is 125% of the largest motor's listed current from the Table 248 in the NEC plus 100% of all the other motors table listed currents. Then, take all your other devices and add the current draw on each one at 100%. Take the total and divide it by 2 since you will be running a 240 volt circuit and all the loads are 120 volts and the current is split between the two legs. Now you have your current draw for all loads in the garage and this is the current draw for the feeder to the sub-panel.

For the sake of argument let's say you come up with 40 amps (this may be high or low for what you have listed but you didn't have a current draw for the wire welder so I will press on). You could then run a #8 circuit if the feed was shorter but to compensate for distance and voltage drop you upsize one wire size to #6 wire. Since you can only get sub-panels in certain sizes you will most likely have to buy a 60 amp sub panel without a main breaker in it. (Since the garage is attached through the breezeway you don't have to drive a separate ground rod. So don't worry about that.) If you go with a 100 amp sub because of the draw of the welder then You may end up with the #2 wire to compensate for voltage drop.

Now, unless your local building code requires it (and Chicago is the only place I know that requires Conduit in residential) you don't have to use conduit. You can use standard NM copper cable to make this run and it will be way more affordable than running conduit and pulling wire.

Power factor is not considered when sizing circuits in residential occupancies. End of story.

The number of circuits is a consideration for convenience but is not taken into account when calculating the size of the sub-panel and its corresponding feeders and breaker.

So, there you have it. Simple eh?

If you have to run a wire 200' you'll need to consider the voltage drop involved here plus the devices you plan on powering. A 12/2 cable on a short run is good for supposedly 20 amps. At 100" it's hard to get my 1.5 hp compressor to start so it won't be long before you're replacing that 12/2 for something that works.

For comparison I just ran a new line to my new garage 135' from an FPE Panel using 60 amp commercial breakers that take double the room but ran a 6/3 cable and have no plans on running anything that heavy in it except maybe a 1.5 hp vacuum to clean my boat. Distance is the one thing that will burn out electric motors because the voltage drop. That 12/2 won't start your compressor nor will it run your 12" miter saw or MIG welder and if your table saw starts the motor won't last long either. You had better be looking at at least the 6/3 if you expect to play with any of your electrical toys.

I'm unsure if you can get the Cutler Hammer 125 amp sub panel but unlike the smaller FPE that only holds 8 circuits the Cutler Hammer can be turned from 8 to 16 circuits by choosing the narrow breakers that come in 230/240 volt with a 120 volt on each side or two 120 volts instead of a single 120 volt. It gives you lots of room for breaker protection for the toys you have there to play with

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    Richard, you need to visit the help center and pay particular attention to the "be polite" part that's spelled out there. Your answers (including, but not limited to this one) display a tendency that does not fit the civility standard diy.stackexchange.com/help/how-to-answer – Ecnerwal Feb 15 '16 at 1:21
  • You are right Ecnerwal and I'm trying to put the nonsense of others who tend to copy and paste answers they look up on the internet just to try and make themselves look better than everyone else, including extremely qualified professionals that end up leaving because of it. I do thank you though. This answer above is meant to save time and money for the user and wasn't meant to be disrespectful, only the truth. Undersizing wires cause fires. I just saw something I missed though ;) – Richard Feb 16 '16 at 16:57
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    We have plenty of qualified folks who manage to express themselves politely - and in paragraphs, even. – Ecnerwal Feb 16 '16 at 21:52
  • Richard, for what it's worth, yours was exactly the straightforward, no candy mouthed sugar coated answer I was looking for. I personally didn't see anything impolite about it, and appreciate your candor. I can run the wire. I can make the connections, and I can throw the switch. But what wire? What connections, and what switch? I'll be adding to the overall comment and question, and look forward to your answer as I get more specific. Incidentally, I (as my handle infers) am a trauma nurse at a very busy ER. That's my area. If I tell you to "hold pressure there" you had better listen. :) – TraumaRN Feb 17 '16 at 4:43
  • Thank you TramaRN, I don't bother mincing words when someone has something very wrong in mind and throwing a straigh forward answer out, is what I do. Not sure why Toys had to be edited to devices since mine are all toys when you like to play. The main thing is to always go bigger than needed rather than smaller. ;) lol Since I don't do much in Trauma but if you say pressure is needed, you'll get pressure there :) – Richard Feb 17 '16 at 15:32

Power factor is very important when considering cable runs.. (My own situation demands I be very careful with this as I only have a light duty cable to my shed).

If the power factor of your saw is 30%, and it might need to be measured, then that means that if it's rated at 1 kW, the saw will draw approximately 3 times that power in terms of current! I appreciate this can be confusing for those not electrically minded.

So, the power factor of all devices needs to be known, and if poor, should be corrected with the appropriately sized capacitor on each device. Then you have to consider how many devices will be run simultaneously. This includes heating/ air conditioner. Plus, startup current needs to be considered, air compressor, fridge, air conditioner, etc. The compressor most likely will be the most 'vicious', on start up. Even a good sparky can get all these things wrong!

If I were you I'd run a conduit through your breezeway to a subpanel in any method that keeps the length to a minimum. Then run two #4 black and one white plus a bare ground. For your major toys that are all 110 volts, skip the 14/2 and use 12/2 wires to an outlet close to where you want each of your major toys and use a separate breaker for each. Try to find a subpanel that has at least 16 circuits. Now when wiring the two drawing the largest amps that chance starting at the same time, make sure you get each on a separate 120 volt line feeding the sub panel. So when you see the two black #4 cables entering the subpanel,one will feed off of each, ensuring if they start at the same time nether will be trying to rob amperage from the other when they start at the same time. The LRA is always,or lock rotor amperage, starting amps could be double the running amps. My heat pump draws 60 LRA with just under 20 amps after it starts

The floresants, three per circuit off one switch but try to use low energy draw balasts or not over 15 amps per 3. The other recepticles you can run 12/2 to feed 4 only recepticles of a long amp breakers and use a 60 amp breaker in your main panel. It's unlikely it will ever kick but the #4's will ensure there will be no voltage or amperage issues and I highly doubt you will need all the subpanel circuits. By using 12/2 wires to your circuits you can use 15 amp breakers for anything up to about 18 amps use 20 amp breakers but if your compressor draws more than 20 amps use a 10/2 with a 30 amp breaker and match the plug from the compressor to the recepticle.

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