Rewiring 120v Branch to 240v?


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Old 08-08-14, 09:23 AM
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Rewiring 120v Branch to 240v?

Hi,

Assuming a branch circuit that consists of a single 120V outlet w/ ground, is it acceptable (code-wise) to rewire the branch to 240V by using the neutral as hot (black tape on both ends to label it) and replacing the breaker with a 240V breaker with the same amperage rating?

Thanks!
 
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Old 08-08-14, 09:58 AM
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Welcome to the forums. It will depend on the amperage rating of the wire versus your proposed load and whether or not the item you will be installing requires a neutral. What will be plugged in there?
 
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Old 08-08-14, 11:12 AM
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Hi Chandler -

Thanks for your reply. It is a commercial microwave oven. Its power requirement is 3100W 208/240V (12A @ 240). It has a NEMA 6-20 plug on it.

While we're at it, there is something I don't quite get... What makes the difference between 240 and 208 volts? I know that the difference between the A and B legs of my home breaker box is 240. Is 208 a three phase thing? In any case the microwave is spec'd for either so I guess it doesn't matter?

Sounds like at the very least I probably need to make sure the existing circuit has wiring that will support 20A, no? Because at 208V (if that is what is available) the thing will draw 14.9 Amps. Or perhaps I should just stop speculating :-)

Thanks!
 

Last edited by Matt Redmond; 08-08-14 at 11:27 AM.
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Old 08-08-14, 11:51 AM
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208 volts is most commonly found on a commercial 3 phase supply. 240 is standard residential single phase voltage. The current circuit needs to be a minimum of #12 wire and the breaker should be 20 amps. A 20 amp breaker is good for 16 amps continuous* so you are with in the safe range for a 20 amp breaker.

*The microwave isn't a continuous load. Just using as rule of thumb.
 
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Old 08-08-14, 03:36 PM
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Matt..... just wanted to add one thing. Be sure the circuit you are converting only consists of a single receptacle or you could be in for a shock.... literally.
 
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Old 08-08-14, 03:51 PM
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Agreed.

There is nothing quite as annoying, or as satisfying, as mapping out ALL of the circuits on a house.

After finally doing the leg work, I cannot over ephasize how much it can help to have a full,
wire by wire diagram of the layout of a house.

Knowing that any particular room is on a specific circuit, helps immensely when working out loads,
wiring layouts, etc.
 
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Old 08-09-14, 05:05 AM
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For your (208/240 volt) microwave oven it does not matter whether the two conductors gave you 208, 220, or 240 volts and it does not matter whether the utility feed (service) was fed by a 3 phase system.

(For those eavesdropping) Generally appliances rated for just 240 volts may not be run on 208 volts or vice versa. There is a not too complex set of rules that can be used to obtain an educated guess as to which may.

The original 120 volt circuit can be rewired to provide 240 volts at the same amperage it was rated for originally. (Restated for emphasis) Be sure there are no other outlets (receptacles, lights, etc.) on that circuit that are not also proactively converted to 240 volts.
 

Last edited by AllanJ; 08-09-14 at 05:22 AM.
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Old 08-09-14, 09:32 AM
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Originally Posted by Matt Redmond
What makes the difference between 240 and 208 volts?
I know that the difference between the A and B legs of my home breaker box is 240.
Is 208 a three phase thing?
Double checking terms now....

Most US power is split phase- you get 240v between hot leads, 120v between hot and neutral.
Some US power is 3 phase, you get 208v between any 2 of the 3 hot lines.



Originally Posted by Matt Redmond
In any case the microwave is spec'd for either so I guess it doesn't matter?
Yep.
You do need to make sure the wiring is rated for 20 amps; and you'll have to swap out a single breaker for a double wide breaker.
 
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Old 08-09-14, 11:08 AM
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Some US power is 3 phase, you get 208v between any 2 of the 3 hot lines.
That would be 208/120V 3-phase 4-wire. You could also have 480/277V 3-phase 4-wire, 240/120V 3-phase 4-wire, 240V 3-wire or 480v 3-wire. Ummm....what did I miss?
 
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Old 08-09-14, 11:56 AM
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Originally Posted by CasualJoe
Ummm....what did I miss?
Not much-

Unless it's a European appliance with two flat prongs and a ground which resembles a
NEMA 6-30 at first glance, but the UK 240v plug requires a different sort of 240v power.
 
 

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