subpanel in out building

There are two considerations in selecting wire size. The first is heating. 10 AWG wire with the types of insulation normally used in homes is capabable of carrying 30 amperes without getting hot enough to create a fire hazard. That's why 30 amp breakers are used, so that any load that exceeds what the wire can safely carry will trip the breaker before the wire gets hot enough to be a problem.

The second consideration is voltage drop. All conductors have some resistance. The longer the wire run, the more electrical resistance is encountered, and the greater the voltage drop for a given current. Remember, 120 volt tools don't like to run on 100 volts. I don't have my references at home, so can't give you an exact number, but if I were going more than a few 10's of feet I would use the next larger size wire just to make sure I didn't cause enough voltage drop to be a problem.

Standard residential wiring is a split phase 120/240, 4 wire configuration. It is single phase, 240 volts between the two hot wires, with the neutral (and ground) in the center, which "splits" the 240 volt service to give 120 volts between the neutral and each phase. With your proposed configuration (2 hots, 1 neutral, one ground), and ignoring voltage drop issues, you could have two 30 amp 120 volt circuits, or one 30 amp 240 volt circuit, or some combination of 120 & 240 as long as the total current in each hot wire did not exceed 30 amps (for example, two 15 amp 120 volt circuits, one on each phase, and one 15 amp 240 volt circuit). You cannot connect the two hot leads together to double the current capacity, as this would short across the 240 volt input.

The neutral never carries more current than ONE of the hot wires; when both hots are providing 30 amps, the currents in the neutral cancel, which is why the neutral is the same size as the hot wires. A ground wire can be smaller, since it only carries current in a fault condition, and then only has to carry it long enough to trip the breaker.

By the way, actual loads are limited to 80% of the rated capacity, which means that in reality you could only draw 25 amps on each 120 volt circuit.

By convention, power panels are rated for the allowable current in EACH hot conductor. A 60 amp panel is rated for 60 amps at 240 volts, which provides two 60 amp 120 volt feeds.

If you really only want 30 amp service, that's fine, but using a 60 amp panel is overkill.

I would STRONGLY suggest you go to your local library & find a book on basic wiring. They should also have a copy of the national electrical code. If you can't understand the code, you have no business messing around with something as dangerous as your household electrical service.

Also, get a permit. Inspectors are generally a great resource, although I have to admit I've run across a few who had their own interpretations of some of the code sections. Inspections can be a pain, but it'll keep you from burning down the house or electrocuting yourself.

By the way, just so you don't accuse me of blowing smoke, I'm a practicing electrical engineer with 30 years experience, and in a past life I designed power distribution systems for LARGE government communications facilities.

Keith

You _may_ be able to feed hot, hot, & neutral to your building, and provide an earth ground there.

But the other suggested way with 4 wires & not grounding the neutral & ground witres together is the better way to go to account for code most everywhere.

--->Paul

10-3 provides the requisite 4 conductors, but does not have enough ampacity to service a 60 amp sub-panel. The max overcurrent protection for
10 AWG is 30 amps, regardless of wire type. The circuit breaker or fuse which protects this subpanel feeder should be located back at the main panel.

You do not have to provide a 60 amp feeder to a 60 amp rated subpanel. But the ampacity of the feeder should be equal to or greater than the composite "calculated load" of the circuits fed by the subpanel. To my mind, 10-3 is only enough for a few lighting and convenience receptacle circuits in the remote building, and nothing more.

You also need to check the wire size rating of the lugs in your subpanel. It is common to have a minimum wire size specified, and your 10 AWG may be too small for mechanical reasons. Watch the total distance and voltage drop as well.

If you have only 120 volt loads, a 60 amp panel can supply 120 amps of 120 volt power, assuming the total load is perfectly balanced between each hot leg and neutral.

10/3 with ground wire is capable of supplying 30 amps across its two "hot" legs. Again, if the loads are pefectly balanced, you can theoretically supply 60 amps worth of 120 volt loads with 10/3 wire.

You CANNOT get the full capacity out of a "60 amp subpanel" with 10/3 wire. I am well aware that it has TWO hot wires (and a neutral, and a ground). It still has half the capacity needed. Why do you think they put such large lugs on the panels, if 10 AWG would be big enough?

10/3 wire MAY provide adequate serviceability for a remote building provided the total load is limited. Be sure to protect the branch circuit with a 2-pole 30 amp breaker, and you probably won't burn down your house (or the remote building). If the building has only lighting and 120 volt convenience outlets, this may work fine for you.

If you want to put welders or air compressors or water heaters or any other 240 volt loads out there, the 10/3 wire is woefully inadequate, and you will be tripping the branch circuit breaker all the time.

You will have it burnt to the ground before he gets there.

If you already have all the answers, why do you ask the question?