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:
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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.
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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.
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?
Best Answer
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?