I am not an electrician, but I am concerned about the safety of a job that an electrician has proposed for my kitchen. I am installing a 30 inch wall oven where a microwave used to be. The oven power rating is for 240V, 20 amps. Because of the layout of my house, it is very difficult to run a new electrical line from the sub panel to the kitchen. Therefore, the electrician is proposing to use the existing wires from a 120V/20amp wall outlet (that was dedicated for my microwave) to create a 240V junction box by rewiring the neutral line as a second hot line, and using the ground as the neutral line. He told me that the existing wiring is already rated for 20 amps, so this should be no problem. He said he just needs to install a double pole breaker switch in the sub panel where each is rated for 20 amps. I think my main concern is with the size of the wires being used - these are thinner wires from a regular wall outlet. I was under the impression that a job such as this required the much thicker wires that I've seen in other 240V junction boxes.
The size of the wires would be dictated by the current, not the voltage. The same wire size (12 AWG) that is acceptable for 20 A at 120 V would work be acceptable for 20A at 240V.
Remarking the existing neutral and using it as a hot wire is acceptable, as well (if done in a manner consistant with the code). Where the plan sounds questionable is the reusing of an existing ground wire as a neutral. The ground wire is most likely a bare conductor, and cannot be used for anything other than a ground. If reused as a neutral, then there will also be no ground wire to the new oven. The oven MUST be grounded.
It is possible that the new oven does not require a neutral, in which case the installation would be OK as described, except that the existing ground wire would remain as a ground, not a neutral. The manual for the oven will tell what kind of wiring is required.
#38707 - 05/30/0406:22 PMRe: Is my electrician being unsafe?
I looked at the connection again, and noticed that the ground is not insulated, so I can't use it as a neutral.
Here's another scenario my electrician presented:
I'm actually installing 2 ovens, and he said we could connect both of my wall ovens to one junction box instead of using the wiring from the microwave. There is already a juntion box in place for the lower oven. The lower oven is rated 240V 20 amp and the upper is rated 240V 30 amp. Is it safe to connect both ovens to one box and use a 50 amp double pole breaker? The breaker in the sub-panel appears to be rated for 50 amps for this box, and the wiring in the junction box is very thick (It was used on the old oven that I recently removed - it's definitely not the same type of wiring that was in the 120V outlet).
Is this safe? Thanks.
#38709 - 05/30/0407:21 PMRe: Is my electrician being unsafe?
In that situation, the wires feeding the individual ovens (and the ovens themselves) would not have effective overcurrent protection. The wires (and ovens) would be rated for only 20A or 30A, but would be "protected" by a 50A breaker. The wiring could easily become hot enough to start a fire before the 50A breaker tripped out.
The only way I can see to safely reuse the existing 50A range circuit would be to use it to feed a small subpanel, which would house separate 20A and 30A 2-pole breakers to protect the individual ovens. Then you need to have a location suitable for installing the subpanel, as well.
[This message has been edited by NJwirenut (edited 05-30-2004).]
#38710 - 05/30/0407:34 PMRe: Is my electrician being unsafe?
Current code dictates all new installations will be 4 wire for ranges and dryers. Also does the oven run at 20 amps if so it will require a 30 amp feed. Look at amps on the id tag. Breaker will have to be atleast 120% of the given value.
#38711 - 05/30/0407:59 PMRe: Is my electrician being unsafe?
Wouldn't the range tap conductor exception allow using 20A and 30A conductors off the 50A branch circuit: 210.19(A)(3) Exception No. 1: Tap conductors supplying electric ranges,wall-mounted electric ovens, and counter-mounted electric cooking units from a 50-ampere branch circuit shall have an ampacity of not less than 20 and shall be suffýcient for the load to be served. The taps shall not be longer than necessary for servicing the appliance.