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FAQ for Wiring Detached Building


New breaker box in a detached garage. 1

230v table saw for use in your detached garage. 3

Run electric to Pole Barn. 4

Rewiring Garage, what size wire to run?. 4

Wiring Garage 60ft from House. 5

Plastic or Metal Conduit?. 5

Wiring a detached garage. 6

Load center for attached garage. 8

Wiring in a pole barn. 9

Electrical service for shed/workshop. 10


New breaker box in a detached garage

Source (


·        QUESTION - How do you run a new breaker box in a detached garage from the existing breaker box in the house?


·        Answer - If your current breaker box is 200 amps then it is fairly simple. Install a 30 amp breaker (house box (a)) to run the garage (b) box outlets. Depending on the distance from box a - b use a suitable gauge wire for voltage drop (10-12) to connect the two boxes. From the 30 amp breaker to the garage box. Always install certified conduit if installing underground. Connect the 30 amp run to the hot post on the new panel then install your breakers accordingly. Never surpass the 30 amp limit on any breaker. Good idea is to have an electrician inspect the work prior to connecting to your house panel. I believe this is not only good for your safety but your insurance.


·        Answer - YOU ABSOLUTELY DO NOT CONNECT #12 WIRES TO A 30 AMP CIRCUIT BREAKER! #12 IS ONLY RATED FOR 20 AMPS! #10 is rated for 30 amps if the distance isn't too far! You need to consult a licensed electrician before doing this job.


·        Answer - One thing that most electricians don't understand, although the NEC is quite clear on this, is that if the sub panel is in a detached building, it gets its own ground. You don't run a ground wire from the main panel to the sub panel. Rather, you run only the neutral and two hots, and bond the neutral to the ground within the sub panel, then ground it to an 8 foot long rod pounded into the ground, or other appropriate ground. 


This is all quite different from the sub panel being in the same building as the main panel. In that case, you run a separate ground wire, and don't bond the neutral and the ground at the sub panel.




230v table saw for use in your detached garage

Source (


·        QUESTION - If you buy a 230v table saw for use in your detached garage what will you need to do to properly wire your shop?


·        Answer - Typically a home is powered from a mains transformer (from the street) which has two powered leads with a common center-tapped ground. The fuse panel in the house typically alternates these two 115v power lines down the fuse box. You need to have a DOUBLE CIRCUIT BREAKER that taps both power lines (230 volts) go to your shop saw.


·        Answer - If your shop is not wired already, I would recommend the following:


Figure out what you intend to power, and figure out the maximum amount of power you need.


Run one circuit from the house to a sub panel in the garage. Our outbuildings are powered off a 40A 220V supply, to give you an idea.


In the garage, install branch circuits for lights, outlets, etc. This will allow for futureproofing as if you want 4 circuits in the garage, you don't need to run 4 circuits from the house. Also you will have 120/240V just like in the house. (Our garage has a 120V circuit for lights, 2 120V outlet circuits, and 1 240V circuit for the air compressor) It is safer as well as one breaker kills the garage completely. Home Depot sells a nice little Square-D sub-panel for $20 that I have used many times in our outbuildings.


While this is simple here, it is actually a big job. Consult an electrician if you are unsure, especially for the high current feed to your sub panel.


Run electric to Pole Barn

(Source Rick Miell –


·        Question - Hello, I have a pole barn that I would like to run electric to. It is around 260 feet from my house. I would like to put a 100 amp service box in the barn. My house has a 200 amp main so I think I'm ok there. My question is, what size cable do I need to use for that distance? Would #1 copper XHHW be ok? Also, how would I connect it to the panel in my house (100 amp breaker)? Thanks.


·        Answer - #1 copper would work good for you. You could probably even go 1 size smaller, if you wanted to.


You would need to terminate in a breaker, in your main panel.


Check out for more helpful info on this type of wiring. Also visit for a good voltage drop calculator. Try to keep your voltage drop between 3 and 5%, never more than 5%.


Rewiring Garage, what size wire to run?

(Source Rick Miell –


·        Question - I'm planning to rewire my garage and need to know what size wire to run. The only circuits will include 3 lights an outside light, 4 outlets, and 2 garage door openers. I’m running this wire to a sub panel in my family room and also need to know what size breaker to use


·        Answer - There are many different things to look at when doing this.


If you run 1 circuit of 20amps (120v) then you could use #12 awg, copper wire and use a 20 amp breaker.


If you run 1 circuit of 60amp (240v) then you could use #6 awg, copper wire and use a 60 amp breaker, but you have to use another panel within the garage. There you will branch out to the lights, outlets using 20amp circuits, (#12 awg, copper).


Check out this site for helpful hints and code references.

Wiring Garage 60ft from House

(Source Rick Miell –


·        Question - I am wiring a garage that is located about 60 feet from my house. I'm planning to wire it through the panel in my house. I've done the calculations and it is big enough (200 amp). I am putting a 100 amp panel with a main breaker in it in the garage. I have 3 questions:

1.    What size wire do I need to run from the house to the garage? I plan to bury it.

2.    Do I need to put in another ground rod for the garage?

3.    When I wire the panel in the garage, do I wire the ground and the neutral together like in the main panel or do I separate them?


·        Answer - To answer your questions, in order:


1.    #2 aluminum or #4 copper


2.    yes


3.    You have two options for this.

a)    If you have not run the 4th wire with your feeder, & you have no other metallic paths common between the house and garage (gas, water, etc...) then you tie them together.


b)                                                   If you have other paths (you have to have the 4th wire), or if you ran the 4th wire, then you separate.


NOTE - The “b” option is the preferred method.


Plastic or Metal Conduit?

(Source Rick Miell –


·        Question - I was also trying to elicit an opinion about whether plastic or metal conduit would be more cost effective and safer (i.e. how does plastic conduit hold up to something hitting it). I'm also getting a headache deciding if to run the wiring through the walls or run conduit on the outside of the interior walls.


I need some objective opinions about which architecture to start with.


·        Follow-up To Question - Is it code to run plastic conduit to circuits inside a detached garage with a 100 amp sub panel and would you recommend plastic over metal?

·        Answer - You can use any of the chapter 3 wiring methods, in a residential garage, unless local codes forbid it. PVC is one of the types, as is metal conduit, nm cable (ROMEX) and others.


Check with your local inspection department to find out if they do allow whatever you choose.


·        Answer - You will need to get local prices for each to compare. I have heard that PVC has increased in price lately, and could be even more than steel. PVC will breaker when hit, though there are certain thicknesses that you can install (schedule 40 is lighter weight that schedule 80). PVC uses glue fittings, unlike metal, which uses either compression or set screw fittings. You are required to pull in a ground wire in PVC, and not in metal, though it is a good idea to do so.


Normally, cable inside of the wall is a cheaper way to go.


Wiring a detached garage

(Source Rick Miell –


·        Question - While replacing a breaker in my detached garage I noticed that there was no equipment grounding wire with my feed wires. I do have a natural gas line to the garage and I have read that I should have an equipment ground with the feeder. The Panel in the garage is a 240 volt 60 amp with # 4 alum 3 wire running 25 feet from home to garage. I am planning on running a # 8 equipment ground wire from home to garage, add a grounding bus bar directly to the panel box in the garage and separate the neutrals and grounds (which they are all connected to one busbar now) in the garage panel, and install a 8' ground rod with #6 bare copper to the ground bus bar in the garage panel. My question the house panel do I need to connect or "tie" in the ground bus bars to the neutral bar or can they remain isolated from each other?


·        Answer - If the house panel is your main panel; that is, there is not a disconnect ahead of it; then do you connect all grounds and all neutrals together? This is the only place where this can be done.


By the way, it sounds like your plan is all correct, according to code.



Load center for attached garage

(Source Rick Miell –


·        Question - I would like to add a 90amp load center in the garage I'm building, I'm thinking a 125 amp panel would be required. The garage is attached to the main house, but the power to the garage needs to go under ground (in conduit) for a short section approx 10' of the 50-60' total run.


1)    Can I use direct burial cable for the whole run (most of the run would be inside the main house) and do I need to use #4 or #6 wire?

2)    I plan on adding a 90 amp breaker in the main house panel and another 90 amp breaker for the main in the new panel in the garage does this sound okay?

3)    How do I go about grounding the new panel?

4)    I'm assuming the new 90 amp breaker in the main house panel would be installed like the other 220v breakers are, is this correct?


·        Answer - You can't use direct burial cable except when directly buried. You could run it in your underground section, and run regular cable in the other locations. Splice in an approved junction box, and protect where necessary. You could use single conductors if you are using conduit.


#6 is not large enough for this load. #4 copper is ok. Use a 4-wire cable (or 4 single conductors), and separate the neutral and grounds in the sub-panel.


You don't need the breaker in the sub panel; you could use the lugs instead.


Visit for helpful hints on wiring.



Wiring in a pole barn

(Source Rick Miell –


·        Question - I am trying to determine what I need to do to bring the wiring in a pole barn up to code. The barn like many has a metal roof that results in indoor rain when the temperature drops outside. The inside of the barn receives no direct sun.


1)    Most of the wiring is already 12-2 UB. I expect I need to replace the non-UB white wire with grey UV resistant UB wire Yes/No.


2)    There was no GFI, I expect I need to add one on all circuits with outlets. Would I also need a GFI on a circuit which only has lighting?


3)    The UB wire is stapled to exposed beams using insulated staples with the junctions inside of galvanized boxes. Do I need to switch to cast al water proof junction boxes and if so, how do I transition from UB 12-2 to the cast al box in a waterproof way or does it even need to be waterproof? I am concerned about condensation forming in boxes and filling them over time.


4)    I pulled the Water Heater as soon as I saw the 30 amp breaker was running on 14-2. Can I use 10-2 wire for hot water heater, pump and base board heater (3 separate runs each of which is less then 16 ft and each on a separate breaker) I assume if I do this I need to color the white wire red. The underground wires going to the pump are colored red, black and green. There seemed to be confusion if I needed 10-3 but nobody seemed to know what to do with the extra wire if I used it.


5)    There are two fans with thermal switches, one at each end of the barn. Wires come from each end to the breaker panel where they were both connected into the same breaker. Is that correct? If not, how should I do it? The barn is 70 ft long and I do not want to create a 120 ft circuit buy doing them in series


·        Answer - To answer your questions, in order:


1)    I would think that most inspectors would classify this area as a damp location. Use of UF type wire would be required.


2)    While not specifically mentioned in the code, I would think you would want to have GFCI protection on any outlet, which would include receptacles or lights.


3)    You should use weather-proof type boxes, but use a regular crimp type box connector when attaching to the box. Locate these connections on the bottom side of the box, so no water can run into the box.


4)    On any load not using a neutral (water heater, baseboard heaters are such loads, while the pump may not be, since it could be using the neutral in its controls), you do not need to run a neutral. 2 wire with ground cable is ok for these types of loads. Verify that there is not a neutral needed for the pump, and wire accordingly.


5)    You can terminate 2 wires on some breakers (Sq. D, GE) but some you can't. Look in the instructions on the inside of the breaker box. If it does not say you can, don't. Just pigtail a short piece of wire, them wire nut the 2 loads to it. Make sure the loads are not going to exceed the rating of the breaker.


Electrical service for shed/workshop

(Source Rick Miell –


·        Question - I have a shed/workshop that I want to bring electricity to.


The main circuit breaker panel for my house has one unused 30 amp circuit breaker in it. The unused 30 amp CB appears to be a double pole version (the unused 30 amp CB has two screw terminals on it to allow two wires to be connected to it).


I would like to use this unused 30 amp CB to feed 120 volt service to the shed/workshop. 10 gage wire from the 30 amp CB to the new panel would be used. The new panel at the workshop would contain one 20 amp CB, and two 15 amp CBs.


Is what I described above allowable by electrical codes?


If what I described is allowable by code, would the load calculation on the existing service (load on my existing main circuit breaker panel) simply be 30 amps or 50 amps (20 amp CB plus two 15 amp CBs)?



·        Answer - You can install a 30 amp circuit from this breaker to a new panel in the shed. Use 4 wires (#10 copper) as the wiring. Use a 60amp rated panel, and install a ground rod at the shed. Keep the neutral(s) and the ground(s) separate from each other.


The load installed will be the actual connected load, such as your total lighting load (i.e.: 3 100watt bulbs = 2.5 amps) along with whatever you are plugging in. Do not add up breaker ratings. You can be sure that the load will not be more than the feeder (30amps) or it will trip the breaker.


If you are going to install a welder, 240v air compressor, or the like, you might want to increase the feeder circuit to 50 or 60 amp breaker & wire.


For more info, visit

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