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Old 03-18-2011, 08:29 AM   #1
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240/120 question


If I have 2x20A breakers in my house sub panel, run to my garage underground 210' with 10-3 UF to a panel, could one 20A breaker be 120V and one 20A breaker be 240V? What are my options? Thanks.

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Old 03-18-2011, 08:41 AM   #2
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240/120 question


No; you could have 2 120 v lines or one 240 v line (assuming you replaced the 2 x 20 amp breakers with a double pole breaker or had them tied together if possible for that breaker model).

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Old 03-18-2011, 10:42 AM   #3
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240/120 question


Thanks Bob. OK then...and I apologize for my ignorance...if I did the double pole 2x20 @ 240V at the house, would the 2 x 20A breakers at the shop be sending out 240V or 120V? In my pea brain I was thinking since I would have 2 conductors with the 10-3, and some 240V capability would be nice to have at the shop, I could have 2x20A breakers with 120V capability and 1x20A breaker with 240V capability (not cut on at the same time). I pretty sure that is not how it works, but I was thinking of the water hose analogy, 240 hose is split with 2 valves, one takes 240, the other is split into 2x120's...not cut on at the same time. School me.
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Old 03-18-2011, 10:54 AM   #4
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240/120 question


Joe,

if you want to have both 120VAC and 240VAC in the shop, you can.. here is what you want to do....

1) replace the 2 individual 20A breakers with a dual pole breaker. with a 10-3 you can go to a double 30A. (when installing breaker in your panel, make sure that it has the terminals on opposite phases)
2) then in the shop, terminate into a small pony panel. (8 slot or something).
3) now you can feed 120vac circuits and have 240vac circuit(s).

you will be limited to a 30a draw on each leg, so try to balance it out.

good luck

rod

Last edited by rditz; 03-18-2011 at 10:57 AM.
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Old 03-18-2011, 11:13 AM   #5
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240/120 question


You are running 210 feet so you should leave the breaker at 20 amps to go with your #10 wire.
You are using the 2x20 to describe your breaker. Does this mean you have two separate 20 amp breaker or one breaker with the handles tied together?
Your cable has a black, red, white, and ground - correct?
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Old 03-18-2011, 12:28 PM   #6
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240/120 question


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Originally Posted by a7ecorsair View Post
You are running 210 feet so you should leave the breaker at 20 amps to go with your #10 wire.
?
just to explain why corsair and rditz come up with different recommendations on the breaker size.


legally, you could use a 30 amp breaker with the #10 wire but due to the distance of 200+ feet, you will be faced with voltage drop.

Actually, at that distance, you shouldn't feed more than 7 amps at 120 volts or 10 amps at 240 volts as anything greater will cause more voltage drop than is recommended.
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Old 03-18-2011, 02:08 PM   #7
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240/120 question


This is another rehash of previous discussions by this poster who has been told repeatedly that he can not use a 10/3 cable to get both 120 and 240 unless he installs a sub-panel.

10-3 UF-B for 1 outlet use 200' from house.

He must think if he asks often enough some one will give an answer he wants to hear. (And it might happen).
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Old 03-18-2011, 03:03 PM   #8
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240/120 question


Dear Handy,
I do not mind getting an answer that differs from yours. Check the post you reference. I asked about the difference in 120/240. Now I was asking if I could have both. I appreciate positive input. Thanks to all of you as I learn something new every day. This is a learning site for learners and teachers. If you know it all, you are here to teach or argue.
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Old 03-18-2011, 03:11 PM   #9
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240/120 question


nap,

would appreciate your further explanation, I am not an electrician, but I do understand the concept of voltage loss due to length of cable run, but don't know the complete reason..

if there is a voltage drop, then the combination of VxA should equal lower W. and seeing as though the wire is capable of 30A at 240V, what is the risk?? (assuming that there is a risk)

thanks

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Old 03-18-2011, 03:31 PM   #10
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240/120 question


Quote:
Originally Posted by rditz View Post
nap,

would appreciate your further explanation, I am not an electrician, but I do understand the concept of voltage loss due to length of cable run, but don't know the complete reason..

if there is a voltage drop, then the combination of VxA should equal lower W. and seeing as though the wire is capable of 30A at 240V, what is the risk?? (assuming that there is a risk)

thanks

rod
Voltage drop is a result of the resistance of the wire itself. What happens at the end then is light bulbs might not be a bright which may not be a problem but electric motors will have to run at a lower voltage which could damage the motor. The conductors feeding the building will also run warmer which is not good.
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Old 03-18-2011, 03:54 PM   #11
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240/120 question


so basically, the wire is acting as a filament which is causing the wire to be a "load".

what I don't understand then is why can't we still use the rated breaker??
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Old 03-18-2011, 04:20 PM   #12
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240/120 question


If you have #10 supporting a 120 volt circuit and you want to limit the voltage drop to 2% here's some data:
5 amp and 225 feet of wire
20 amps and 55 feet of wire
30 amps and 35 feet of wire

I don't have a table for 5% voltage drop but you can see the effect of the wire's resistance.
So, in the above example, if I wanted to make sure my voltage drop didn't exceed 2% for my shop that has 55 feet of wire from the breaker to the load, I would have to put a 20 amp breaker in my panel.

Last edited by a7ecorsair; 03-18-2011 at 04:22 PM. Reason: spelling
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Old 03-18-2011, 04:37 PM   #13
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240/120 question


Quote:
Originally Posted by rditz View Post

what I don't understand then is why can't we still use the rated breaker??
You can use the larger breaker, at least by electrical code. It's your equipment to damage.

I have been made aware there is a code that does require voltage drop to be considered though. If whoever alerted me to that reads this, please list it again. I have forgotten where it was and would like to see if it might be applicable to this situation.
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Old 03-18-2011, 04:55 PM   #14
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240/120 question


appreciate you patience... so what do you do to improve the distance??? xfmr??

I am familiar with radio communications and when we want to minimize loss due to cable length, we use a better grade of cable (thicker core).

rod
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Old 03-18-2011, 05:27 PM   #15
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240/120 question


Quote:
Originally Posted by rditz View Post
appreciate you patience... so what do you do to improve the distance??? xfmr??

I am familiar with radio communications and when we want to minimize loss due to cable length, we use a better grade of cable (thicker core).

rod
Increase the wire size.
Here is the same data but with 12 gauge wire.
5 amps 140 feet where it was 225 feet with #10
20 amps 35 feet where it was 55 feet with #10

Copper wire is the most economical "good" conductor but there are other metals that have lower resistance per foot but the cost, and therefore the demand, makes them impractical for residential wiring.

Aluminum is used but mainly for large service conductors. Typically you need one size larger in aluminum than copper for the same current carrying capacity.

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