I have just discovered that our 120v microwave oven is tapped into the 240v stove circuit. The stove circuit is a two wire cable with ground, and the microwave is connected to one hot wire and the neutral is connected to the grounding wire of the stove circuit. I'm not an electrician, but I know intuitively this is not right. What is the major danger? Any ideas for a remedy?
The major danger is the neutral current can show up on the frame of the stove if you have a fault in the grounded conductor.
In fact if this is not type SE cable it was never legal to hook the stove up that way and since 1996 it hasn't been legal for new wiring, no matter what cable you use.
Fire and/or electrocution!
Apparently your house was wired by the village idiot. Get it repaired by a competent electrician.
Sounds like an Edison circuit. Two circuits on three wires. It is possible to split a 240 into two 120's using the common neutral, but I don't know if it is legal to have both voltages. I don't know enough about the code to say if it is compliant or not.
The only way to eliminate it is to run a new wire either from the breaker box or from a junction box that can support the additional load.
The Edison circuit was originally designed by Edison using his DC system, but he lost out to Westinghouse and AC.
*Your best remedy is to install a new 20 amp 120 volt circuit to power the microwave oven.
John Grabowski
There should not be too much danger as long as you pay the Fire Department to park and staff a fire truck in your driveway 24/7. They will likely be able to limit the fire, so it only destroys your kitchen.
For a remedy, call an electrician, to wire it properly!!!
That outlet is probably not even on a correct size breaker or fuse.
Agree with the above. The stove circuit is of the older variety, with one wire sharing the function of neutral and ground. That is assuming that the stove has 120V loads as well as 240V loads, which they typically do. If the stove was just 240V, then the one wire would be just the ground. Assuming it was wired up years ago when it was code compliant, there is no issue there with the stove.
What some yahoo did was tap into it for the microwave and that is not code compliant. The best solution would be to do a new run back to the panel for the microwave outlet. It would also be an opportunity to add other receptacles if you need them.
Contrary to what some are saying, assuming all else was done correctly, there is no fire danger, immediate safety hazard, etc. Like Gfre says, the danger would be if the ground/neutral were to be cut, disconnected at the panel, etc. In that case, the metal case of the microwave would become energized. But even with only the stove, installed to previous code, the same thing would happen with the stove metal itself and that was allowed for 50 years without disasters everywhere. The chance of the one wire being interrupted somehow is very low.
So, it should be corrected, but no need to panic.
My stove is old enough to be 3 wire, and it has an outlet for plugging in a coffee pot, microwave, etc.
Does the OP have a separate 120 outlet daisy chained to the stove outlet? That sounds out of code to me.
From what was described, that's indeed what they have and it is a code violation and should be fixed. The thing I worry about with things like this is if some yahoo did this, what else did they do with that circuit and/or the rest of the house that you don't know about?
Must be running a bit slow today. I take back part of what I said above about the possible disconnection of the neutral/ground being the only safety issue. That is one angle, but there is a big safety issue here, and that is that the microwave outlet is being protected by the breaker for the stove. That's going to typically be a 40A or larger breaker. So, you have a 15 or 20 amp outlet plus the wiring between the outlet and stove, on a 40A or bigger breaker. That is a potential fire hazard.
Thanks, guys, for all the input. There are no 120v devices on the stove (very cheap model). No clocks, outlets or the like. Strictly 240v heating elements. The house was constructed in the late 60's, early 70's (with aluminum wiring) and the kitchen "remodeling" was done in 2010, at which time the jerry rigged wiring was installed. I'm sure no permitting took place.
Glad to learn about the extent and nature of the dangers, though. I will make sure that there is continuity in the ground between the stove and the distribution panel. I'm going to go with having a separate circuit installed as time and circumstances allow.
PS The Anne Arundel County Fire Service was not interested in stationing fire fighters and equipment in the vicinity. The entire community has aluminum wiring and yet there have been no house fires that I am aware of.
Thanks again to all of you for your help.
So you could potentially plug way too many devices into that outlet and never trip anything, that makes sense as the main safety issue.
That 40 A breaker is a double. I'm not sure how they work. Does it take 40 A to trip either side?
Yes.
If, in fact, there are no 120v loads, it is legal today.
The microwave is the thing that pushes it over to the illegal side. If you can drop in a 20a 120v circuit, that is the way to go.
Your point is well taken. You wouldn't believe what we have come across. For instance, the front porch light switch was wired to ground. Turn it on, and the breaker tripped. It has been an expensive nightmare, and it just keeps on.
An electrician that wires a 20A circuit to a 40A breaker should go to jail.
On 03/11/2015 2:43 PM, snipped-for-privacy@spamblocked.com wrote: ...
A "dead short" will certainly trip even the 40A breaker instantly.
The actual fire danger is, imo, quite low w/ the single device; where it would be a serious hazard from that standpoint would be multiple outlets where one could easily put a normal load that greatly exceeds the outlet/local wiring heating limits, but that isn't the case here of the single device.
Is it right? No. Should it be corrected? Yes. But is it an _imminent,standing_ fire or safety hazard? No.
A dead short will trip the existing 40A or whatever breaker for the stove instantly. It's an overload that exceeds the wiring to the outlet, but not the breaker, ie the range of 15A or 20A to 40A, that would be the fire hazard from the wiring overheating.
If you're lucky. only the MW will be destroyed as smoke pours
There really aren't any electrocution issues. No more so than with the stove that also uses a shared ground and neutral that is connected to it's metal case. And that is still code compliant as long as it was wired before the code changed.
Take a 20 A circuit in the kitchen. Each appliance used in the kitchen will normally max out the circuit. Is there a danger with only the microwave plugged in? Not normally.
Plug a microwave and a deep fryer into the same outlet (on a 40A breaker). Fire hazard easily.
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And, of course, while presuming 14ga/15A outlet, any load above that is in violation of Code, the actual breaker thermal trip level is generally about 5-7.5% _above_ the nominal breaker rating at which point the bimetallic strip opens so the actual load might be as much as 43A or so before the breaker itself saw the overload.
But, mitigating that is that the actual current-carrying capacity of wiring or the outlet is quite a lot greater than that for which it is rated for Code application so there's quite a margin of safety before a real fire danger is present.
I don't have the data at hand on what the temperature rise in a 14ga Cu would be in open air at 43A but it'll be warm...will it be fire-setting hot I don't know for certain.
See the previous comment above as well...it certainly should be fixed but I don't believe it's an imminent hazard in the next 10 minutes.
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