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Our Whirlpool GJC3034R Cooktop needs to be replaced so while doing the research I realized that it is rated 40A (here is the excerpt from the manual):

A 4-wire or 3-wire, single phase, 240 volt, 60 Hz., AC only electrical supply is required on a separate, 40-amp circuit, fused on both sides of the line.

but when I checked the panel I noticed it was installed on double 30A breaker and was operational for last 8 years like that.

I'm about to purchase a replacement but that one is also rated as 40A.

Anyway, is it okay to install a 40A appliance on a double 30A breaker and am I correct in thinking that the worst thing that can happen is a tripped breaker if we try using all four elements simultaneously?

EDIT

As suggested by someone in the comments section I checked the element wattage of the Maytag MEC7430BB which I intend to purchase and it was 8600 Watts combined. Converted to amps that is 35.83 so again, as long as we don't use all 4 elements simultaneously the strength of the current will be below 30A and all should be fine, other than the possibility of a tripped breaker,... or it also depends on the gauge of the wire?

  • What gauge is the wire in the circuit? – ThreePhaseEel Oct 29 '17 at 19:10
  • @ThreePhaseEel Any way to establish that without removing the panel? – Dean Kuga Oct 29 '17 at 19:11
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    If the wire is NM and is visible in the basement you might find printing on it that tell you what gauge it is. – Tyson Oct 29 '17 at 20:08
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    You can also look at the specs for each burner and add them up. I bet at max draw combined they don’t add up that high, sadly one must do calculations like that to get past the marketing hype. (Also the reason they are asking the wire size is to determine if 30amp is the max for the wire, or if you can upsize the breaker because the wire is bigger) – Tyson Oct 29 '17 at 20:14
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    The breaker size is determined by the wire gauge. If it 10 gauge that 30 amp breaker will need to stay, if the wire is larger (smaller gauge number) then the breaker size can be increased. – Tyson Oct 29 '17 at 21:10
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Let's start with the easy answer. If your installation manual is calling for a 40A circuit for your cooktop and you should install it per their directions. This is because the manufacturers engineers have determined that the recommended circuit is necessary for the cooktop to operate properly. Failure to do so could void the manufacturer's warranty if there is a problem with the cooktop.

Now the more confusing part. NEC Article 220.55 covers calculations for residential cooking appliances and it does allow derating of these circuits. For example if you have a range rated at 12K you can install a circuit rated for 8k. You can run a single circuit for more than one cooking appliance and tap the circuit for the reduced load of each appliance. I have to say that it is one of the most confusing and complex calculations mentioned in the NEC.

How can you be sure that what you have will work with your new cooktop?

You are on the right track by reading your manual and trying to determine the correct KVA to your cooktop, but what you really need to find is the Nameplate rating of this unit. It usually is not in your Installation Manual and it may be hard to find on the unit itself but it will be there somewhere. Another place to find it is to go online to the manufacturer and see if there is a specification sheet for the unit. On the nameplate you will find it's operating voltage and amperage or the required KVA. Then you will have to take that rating and calculate the demand from NEC 220.55. Personally I think that it needs to be supervised by a professional Master Electrician or Engineer or a very competent Journeyman.

Hope this helps, good luck and stay safe.

  • I'm having a hard time understanding NEC 220.55. I have a cooktop with a nameplate KVA of 7.5 and it's being run on a double 30A breaker with 10 gauge wire. Is that in spec? – Brad Nov 30 '17 at 4:48
  • If I am understanding what you are saying, you have a range top that is being fed by a separate feeder from the panel. In simple terms, since you only have the cooktop on this feeder and the name plate is 7.5 KVA per NEC you are not allowed any diversity. So you must connect it with a 40A breaker and use a #8 conductors. – Retired Master Electrician Nov 30 '17 at 14:34
  • What I am trying to say is that if you had a single feeder connecting something like your cooktop and a built in oven. Then you could use NEC 220.55 and using its diversity. You could run the feeder and tap the conductors per its direction. So if you are getting into a remodel situation rather than have two full circuits for your kitchen. It might be more economical to see if you could run one circuit and cover two appliances. – Retired Master Electrician Nov 30 '17 at 14:53
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You're supposed to have it on a 40A circuit with 8 AWG copper wire.

Using a 35.8A appliance on a 30A circuit is exceeding the limit, but the limit is a generic one meant for bundles of wires in insulated walls to be safe. In open air, 10 AWG wire can handle in excess of 40A.

But following rules to the letter can actually make life more dangerous, and there are no shortages of such examples in America. Consider the likely hood of that wire causing a fire. It's very low. Consider the likely hood of leaving pizza boxes on the stove and having your dog scratch the controls to the stove causing it to turn on and ignite the pizza boxes. That's much more likely. If you have the choice between upgrading the wiring and installing a fire sprinkler in your kitchen for the same cost, the fire sprinkler will be orders of magnitude better at preventing a fire.

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    Actually, the sprinkler won't prevent a fire. What it will do (which is VERY useful) is keep a small fire from becoming a BIG fire. – manassehkatz Feb 15 '18 at 2:55

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