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I think the wiring to my cook top and oven might be backwards. My oven requires 40A and my cooktop requires 30A. The oven wire coming to the junction box is 12 ga. but the wire from box to breakers is 8 ga.

The wire to the cooktop is only 10 gauge. I am purchasing new items, and find my new cooktop definitely needs 40A, and most ovens I am looking at are 30A or even 20A.

What can I do to fix this?

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I'm not sure what your question is, you seem to be stating facts. What is it you need to know? –  GdD Oct 30 '12 at 10:05
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Can you perhaps post a clear picture of the junction box? Additionally, are those conductors copper or aluminium? –  Tim Post Oct 30 '12 at 14:32
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3 Answers

10 Gauge (AWG) is only good for 30 amps. You will need to have a 40 amp circuit installed for the new cook top. Additionally, you will want to ensure that the oven is connected to the feed from the panel using the same gauge as landed in the panel, which is #8 AWG. #12 is rated only for 20 amps. You should examine the plate at the back of the oven that specifies the voltage / amperage to be sure, but I have yet to see an electric oven that is happy with 20 amps. Perhaps the lighting, digital controls and even rotisserie operate on that, while the actual heating elements want something larger? It's not uncommon for ovens to want two feeds, one for controls and features (15 - 20A @120V) while the other is for generating heat (40 - 50A @240/208V).

Going past #10 AWG, you can't use typical connectors (aka wirenuts). You need to use something called 'bugs' to splice a length of #8 AWG to your oven, replacing the #12 whip.

My advice, as an electrician is .. call an electrician. In short, you'll need an additional 40 amp circuit installed, and you have to get rid of that #12 whip to the oven (replacing it with a whip of the same gauge). You may also need an appliance receptacle to power the oven controls, as it probably wants two feeds.

What you should end up with is (at least) two range receptacles so that both the oven and the cooktop have a suitable means of disconnect, unless they are in direct sight of the breaker box (panel).

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I'm guessing the oven and cooktop are hardwired, so if a receptacle was used you'd have to uninstall the cooktop/oven to get to it. +1 for call an electrician. –  Tester101 Oct 30 '12 at 14:21
    
@Tester101 He'll need a bit of SJ cord and the appropriate cord cap, but .. that kind of falls into the "call an electrician" bit of advice :) Both appliances should be easily removed for service with a simple means of disconnect. When you hit that kind of mess, it's time to call someone. –  Tim Post Oct 30 '12 at 14:26
    
NEC 422.33(A) says "For cord-and-plug-connected appliances, an accessible separable connector or an accessible plug and receptacle shall be permitted to serve as the disconnecting means.". If the cooktop is installed in such a way that the receptacle is not accessible, you'll have to provide a disconnecting means in accordance with 422.31. –  Tester101 Oct 30 '12 at 19:27
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It doesn't matter if it's backwards. It's wrong!!!

If you have a feed from the panel of some gauge, you cannot split it into several smaller gauge wires. All branches must be able to properly handle full current because the breaker will happily supply it in the case of a short.

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Yes, and no. "Fixture wires (NEC402)", and "Flexible Cords and Cables (NEC400)" are both covered in separate sections of the NEC because they are special situations that need their own rules. It's entirely possible that this wiring is correct. Though impossible to be sure if it is correct, without actually being there (or at least reading the manufacturers documentation on the devices). –  Tester101 Oct 30 '12 at 13:51
    
@Tester101 Correct, but .. have you seen an electric oven that is happy with 20 amps? I'm pretty sure what the OP needs is two range receptacles. Also, both need a means of disconnect unless in direct sight of the panel. –  Tim Post Oct 30 '12 at 14:13
    
@Tester101, true. I'm thinking of wires that are fixed, like those that run through walls. –  Brian White Oct 30 '12 at 14:56
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Fixture Wires

What you're looking at is a special type of wiring known as Fixture Wire. Fixture wires are conductors used for wiring fixtures and control circuits, they are not branch circuit conductors. There are special requirements and uses for fixture wires, which are covered in article 402 of the National Electrical Code (NEC). Article 240.5(B)(2) also addresses fixture wires.

NEC 2008
240.5 Protection of Flexible Cords, Flexible Cables, and Fixture Wires. (B) Branch-Circuit Overcurrent Device. (2) Fixture Wire. Fixture wire shall be permitted to be tapped to the branch-circuit conductor of a branch circuit in accordance with the following:
(1) 20-ampere circuits — 18 AWG, up to 15 m (50 ft) of run length
(2) 20-ampere circuits — 16 AWG, up to 30 m (100 ft) of run length
(3) 20-ampere circuits — 14 AWG and larger
(4) 30-ampere circuits — 14 AWG and larger
(5) 40-ampere circuits — 12 AWG and larger
(6) 50-ampere circuits — 12 AWG and larger

In cooktop installations where fixture wires are used, you may see #8 or #6 wire from a dedicated breaker to a junction box behind the fixture. You'll then see #12 or #10 wire from the junction box to the cooktop. Because of articles 402 and 240.5(B)(2), this is a valid installation technique.

Mean of Disconnect

When dealing with permanently connected appliances, you'll want to check article 422 of the NEC. Article 422.31(A) tells us if the appliance is rated less than 300 Volt-Amperes or 1/8 Horsepower, the branch-circuit overcurrent device can be the disconnecting means. If the appliance is rated over 300 Volt-Ampheres or 1/8 Horsepower, the circuit breaker can be the disconnect if:

  • The circuit breaker is within sight from the appliance.
  • Or the circuit breaker is capable of being locked in the open position.

422.31 Disconnection of Permanently Connected Appliances.

(A) Rated at Not over 300 Volt-Amperes or 1⁄8 Horsepower. For permanently connected appliances rated at not over 300 volt-amperes or 1⁄8 hp, the branch-circuit overcurrent device shall be permitted to serve as the disconnecting means.

(B) Appliances Rated over 300 Volt-Amperes or 1⁄8 Horsepower. For permanently connected appliances rated over 300 volt-amperes or 1⁄8 hp, the branch-circuit switch or circuit breaker shall be permitted to serve as the disconnecting means where the switch or circuit breaker is within sight from the appliance or is capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means shall be installed on or at the switch or circuit breaker used as the disconnecting means and shall remain in place with or without the lock installed.

NOTES:

  • When installing devices like cooktops, always follow the manufacturers installations instructions.
  • If you are unfamiliar with proper wiring methods, please contact a qualified Electrician (which will also be stated in the manufacturers installation instructions).
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