Need some electrical advice / suggestions on adding a circuit. Got a
shop close to the house with 100ft of 10-2 wgrnd going from fuse box to
How big a 220 breaker can I use in the main panel in the house? How big
a 220 outlet in the shop? Guess I should put in a disconnect panel in
the shop? and maybe some 120 outlets or should I forget the 120's.
Already some 120's out there on a different 10-2 wgrnd. What about ground?
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
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
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
On Wed, 26 Apr 2006 07:53:06 -0400, "digitalmaster"
The source of the confusion is that you started out by claiming
you were using the bare ground as nuetral, (which is illegal
and dangerous) and now suddenly you have an independant
ground at the service entrance to the detached structure,
and NO nuetral between the two buildings.
to protect it from melting the insulation and catching fire. Voltage
drop is also an important consideration, but that doesn't change the
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.
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.
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