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Old 02-22-2011, 10:16 AM   #1
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120/240 Question


Could someone explain the difference/benifits of buying 10-3 rather than 10-2 for a run from the house to a tractor shed. The way I understand it is the 2 conductors available on 10-3 would allow me 2x20A circuits instead of 1x20A for the 10-2. A air condition fellow told me the 10- 3 would allow 240V instead of 120V and this was better for the ballast on FL lights. He said the 120V causes them to fail early. How does this 240/120 thing work? What would make 10-3 240V rather than just 2x120V? Help me.

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Old 02-22-2011, 11:16 AM   #2
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It has to do with what is called a multi wire branch circuit - MWBC.
A typical branch circuit consists of one cable with a black, white, and ground. The black is connected to a single pole breaker and the white to the neutral bar and the ground to the ground bar. You get 120 volt service.
In a MWBC a three wire cable is used and there are two hots, one neutral, and one ground. It is connected to a 2 pole break which connects to opposite legs of the incoming power. You have 240 across the two hots and 120 from either hot to the neutral. If this were connected to a 2 pole 20 amp breaker, you could draw roughly 20 amps on each side or 20 amps across the two hots or a combination of the above.
You would have 4800 watts available on a 20 amp MWBC
You would have 2400 watts available on a 20 amp single pole breaker.

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Old 02-22-2011, 11:18 AM   #3
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You can get 230 volts from 10/2 or 10/3. With 10/2, you can only have either (1) 230 volt circuit or one 120 volt circuit. With 10/3 you can have (2) 120 volt circuits (MWBC) or (1) 230 volt circuit.

Input voltage has no affect on ballast life provided you use the correct ballasts.

Search at the top of this page for "wire shed" wire garage". Key words like that. This is by far the most common asked question on the forum.
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Old 02-22-2011, 12:52 PM   #4
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Thanks...kind of the same concept as 12v vs 24v parallel or in series. I did not see the connection and the AC man's comment on ballast life confused me even more. Thanks for unassing this for me.
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Old 02-22-2011, 01:21 PM   #5
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[quote=Joe Willie;595719]...kind of the same concept as 12v vs 24v parallel or in series.

uhh, not really...

here is how it works...

if you purely want a 240vac circuit and nothing else, then you can run a 10/2 wire. This gives you 2 conductors to carry your power and the bare copper wire.

if you want a combination of 240vac and 120vac power, then you would run a 10/3 wire. this again, gives you 2 conductors to carry power, 1 conductor as a neutral and the bare ground conductor. This would run to a sub-panel for distribution.

how this works is that AC is alternating current and changes it's phase as it flows. in the panel there are two power bus bars. each power bus bar is on an opposite phase as the other one. so if you measure voltage across them you will get 240vac. if you measure between each of them to neutral you will get 120vac. think of the neutral as the common/reference point.

using your 12vdc batteries illustrative purposes... if you put them in series (batt 1 - positive terminal connected to negative termina of batt2), you get 24 vdc between negative terminal on batt1 to positive terminal on batt2. but if you measure from negative on batt1 to the positive of batt1 you get 12vdc. the centre is effectively the neutral.

if you are going to possibly require multiple 120vac and or 240vac in the shed, you will want to run 10/3. if you will only have 240vac (compressor or welder) then you can run 10/2. in my opinion, just run the 10/3 and put in a little panel, your options are much broader now.

rod
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Old 02-22-2011, 02:28 PM   #6
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120/240 Question


Quote:
Originally Posted by Joe Willie View Post
Thanks...kind of the same concept as 12v vs 24v parallel or in series. I did not see the connection and the AC man's comment on ballast life confused me even more. Thanks for unassing this for me.
It is the AC man that needs unassing.
Ballast are made to run on whatever voltage is called for.
You can buy ballast that run on 120v, 240v, 277 volt, and higher voltage.

Using one voltage over another has no impact on the life of the ballast.

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