# 220 to shop

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• posted on April 22, 2006, 11:21 pm
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 shop. 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?
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• posted on April 23, 2006, 12:02 am

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 20a outlets.
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• posted on April 23, 2006, 12:13 am
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
30 amp breaker

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• posted on April 23, 2006, 3:54 am

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?
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• posted on April 23, 2006, 9:25 am

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.
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• posted on April 23, 2006, 11:19 am
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

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• posted on April 23, 2006, 6:10 pm
On Sun, 23 Apr 2006 07:19:46 -0400, "RBM" <rbm2(remove

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

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• posted on April 23, 2006, 11:50 am

shop with an earth ground.
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<%-name%>
• posted on April 23, 2006, 2:45 pm

WRONG.
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Regards,
Doug Miller (alphageek at milmac dot com)
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<%-name%>
• posted on April 26, 2006, 2:09 am
wrote:

try again...shop has been running 220 volt on 3 wire for over 15 years now.
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• posted on April 26, 2006, 3:19 am
On Tue, 25 Apr 2006 22:09:06 -0400, "digitalmaster"

Lots of things that "work" are not legal.
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• posted on April 26, 2006, 10:10 am

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.
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• posted on April 26, 2006, 11:44 am

In service-entrance cable, or overhead triplex, it's ok. In NM cable, it's absolutely not ok.
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Regards,
Doug Miller (alphageek at milmac dot com)
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• posted on April 26, 2006, 11:53 am
wrote:

grounded.
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<%-name%>
• posted on April 26, 2006, 3:07 pm
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.

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• posted on April 26, 2006, 11:43 am

Doesn't matter whether it works or not, it's still a Code violation.
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Regards,
Doug Miller (alphageek at milmac dot com)
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• posted on April 23, 2006, 2:32 pm

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
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• posted on April 23, 2006, 2:43 pm

Excuse me? A 50A breaker on 10ga wire? NOT. 30A is the max.
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Regards,
Doug Miller (alphageek at milmac dot com)
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• posted on April 23, 2006, 5:58 pm

error. Good job.
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• posted on April 23, 2006, 3:48 pm
Doug Miller wrote:

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.