220 to shop

Easy part is that you can't put 120v on a 10/2 240v line; no neutral. Assuming it is copper, you can use a 50a breaker; but then you will need a subpanel to run 20a circuits for your tools. Unless you plan on running several large 240v tools at the same time, just put a 20a breaker in and run

Easy part is that you can't put 120v on a 10/2 240v line; no neutral. Assuming it is copper, you can use a 50a breaker

I'm sorry, you are certainly correct! 100' of #10 does not exceed the voltage drop requirement for 50a, but #10 can't can't handle 50a!

That brings up an interesting question... If there is little voltage drop, then there is little power being lost as heat. If there is little heat, why can't it handle greater current? Isn't the capacity of wire related to heat?

The capacity is related to the heat of the wire and insulation heat rating. As # 10 copper wire has a resistance of about 1 ohms per 100 feet and there are several wires in the cable and not if free air where the heat buildup can be dissapated very well , it will not take too much power to build up a lot of heat. Do the calculations and see how much heat you get at the various currents. It is power is equal to the current squared times the resistance. Remenber this is for just one wire in a bundle and you have several wires.

Also, the NEC recognizes the ampacity of AWG 14,12, 10 gauge to be higher, but still limits those conductors to 15,20, and 30 amps regardless of insulation type

you use the bare wire as the neutral..but you must have another panel in the shop with an earth ground.

The size of the breaker (Ampacity) is matched to the size of the wire to protect it from melting the insulation and catching fire. Voltage drop is also an important consideration, but that doesn't change the first statement.

A 50A circuit might have a 1A load connected to it. Great. There will be very little voltage drop because the current is so low.

Now if the circuit is fully loaded (unusually) to the full 50 Amps, the voltage drop becomes a big deal and the wire may get warm to hot, yet still be within its rated value. The power lost in voltage drop is I^2 x R (I=current R=Resistance of the wire) The squaring factor is the main consideration. Even with an arbitrary low value for R, say 0.25 ohms, this yields P = 50 x 50 x 0.25 = 625 watts distributed evenly through the wire! That is a lot of lost power!

If the wires are confined to cable or conduit, they are going to get hotter. If they are outside in free air, they can handle more current for a given size wire (the code allows for derating).

To get a perspective of scale on this, some overloaded or fully loaded transmission circuits may have 1000 Amps or more running through individual conductors. If the line is at max. capacity, the conductors may operate in the temperature range of 100 C (212 F), the boiling point of water. Thus they are "hot" in more than just the sense of being "energized" with high voltage.

Beachcomber

Excuse me? A 50A breaker on 10ga wire? NOT. 30A is the max.

WRONG.

Incorrect, a 10ga wire can indeed be protected by a 50A circuit breaker under limited circumstances. Under typical residential conditions 30A is the max though. See NEC 240.4(D-G) and referenced articles.

The bigger issue is the fact that the 10-2 w/grd does not provide the required separate neutral and ground conductors to feed a sub panel. Without the separate neutral and ground conductors you are limited to using this as a single branch circuit, either a 30A 120V circuit (hot, neutral and ground), or a 30A 240V circuit (hot, hot and ground). You can not feed a subpoena or use it as a 120/240V circuit.

Pete C.

Yup, you caught me in an error; 11 hours after I acknowledged it was an error. Good job.

Just to follow your lead... but some people might think of just table

For those disputing this, read 2005 NEC 240.4(d)

tom

And which of those apply to this situation?

None.

Of course he can use it to feed a subpanel. What he *can't* do is use it to feed a subpanel that supplies both 120V and 240V loads.

Consider: if the subpanel supplies only 240V loads, there's no need for a neutral, and 10/2 WG is fine. Also, if the subpanel supplies only 120V loads, there's no need for two hots, but of course only every other breaker stab in the panel will have power.

Well he could run the 240 and use a transformer for 120

try again...shop has been running 220 volt on 3 wire for over 15 years now.

Lots of things that "work" are not legal.

Don't understand why 3 wire to a detached structure is not legal in your area. Mine was inspected and approved. The detached structure must have its own ground rod.

Doesn't matter whether it works or not, it's still a Code violation.