Questions on How to wire a stove top / oven combo

Hello Guys,
I'd appreciate some advice, as I'm trying to get most of my kitchen done this weekend. I'm thankful that several of you have already helped me out with this project.

I am adding a new stove top and single oven and can't figure out how to wire them properly. I ran a new branch from my main panel-- 60 amp breaker with 6/3 nm cable. Now I have to split it between the 40 amp stove top and the 20 amp oven. I have 8/3 and 12/3 respectively for this.

But I'm confused. The wires coming from the appliances are stranded and silver in color-- could the be aluminum? I'm not sure how to safely mix my copper run to these wires, with the change in metal and the change in wire gauge. There must be a technique for this, but I can't find it. Seriously, thanks again.

 

You can't do this. You should've run two separate circuits. You can not have #8 or #12 cable on a 60A circuit. The only (unconditional) way to do it with the one feed is to install a subpanel with a 20A and a 40A breaker.

What you should do is bring the #6 directly to the cooktop, and put a 40A breaker on it, then run a new #10 run to the oven and put the 20A breaker on it. Are you sure it's 20A? Wall ovens are normally 30A. Even if it's 20, use #10 to allow for a future upgrade.

IF - and ONLY IF - neither appliance's installation instructions call for a dedicated circuit, AND neither appliance's nameplate list a "MAX FUSE AMPACITY" of smaller than 50A, you can split off #6 to both appliances, and install a 50A breaker to protect both. If the oven has a 30A or 40A max fuse rating, then you can't do it because a 40A breaker will be cutting it too close for the demand load of both appliances. 30A won't work at all as the cooktop alone could trip it.

 

Thank you guys, could you provide a little more info?

I thought that the NEC allowed for a oven / cook top combo on one circuit, hence I ran one cable. It's not too late to add another cable. I would have to re-arrange my panel, since there currently aren't enough openings to add another double pole.

The oven is only 3250 watts, so 13.5 amps.
The cook top is 7200 watts, so 30 amps.

That's 43.5 amps total for the circuit.

I guess your right, JerseyMatt, a 50 amp breaker would work, if I derate the appliances slightly. Neither instructions calls for a dedicated circuit. I'll have to dig the appliances out of their boxes to see if their nameplate indicates a max fuse ampacity.

My knowledge of electricity is lacking here... of course you don't want too little conductor. But I'm talking about sharing the circuit between two appliances. How would one do this?

I'm glad that I'm not mixing metals Ray 2047, but I still can't see how to connect these different gauge wires. Is it okay to do this or is it not?
As alway, appreciative...

 

I didn't catch that part. You can't run the smaller gage wire. Please see JersyMatts post. All house wiring must be sized to the minimum required for the breaker used.

 

Thanks, I'm going to run another branch with 10/3 for the oven and use a 20 amp breaker. I'll connect the 6/3 already in the wall to a 40 amp breaker and hook it to the cook top.

There was so much talk on the net about the NEC rule that permits both oven and cook top on one circuit that I thought it would be straightforward... Thanks for the help.

 

Your new plan sounds good and it future proofs the oven install in case you ever need 30a for a different oven.

 

It's better that you've decided to run the separate circuit, because as Ray said it gives you an upgrade path for both the oven and cooktop later on (the new magnetic induction cooktops are serious power hogs and most of them do require dedicated 50-60A circuits).

But to answer your questions NEC220.55 does allow for the cooktop and oven to be on the same circuit, as long as certain conditions are met. The manufacturer must permit it (hence the two IFs from above), the wiring must be sized correctly for the breaker throughout the circuit (you must bring the #6 to both appliances, you can not reduce it to #8 or #12), and you must calculate the demand load for both appliances to find the breaker requirement (and that can not exceed a manufacturer's specified Max Fuse Ampacity for either appliance).

In your case neither appliance exceeds 8750W, so you use the demand load factor of 0.65. Your demand load for the circuit would be 6800W or 28.4A. You could then use a 40A breaker (I used a higher wattage assumption in my calculation for my other post) to protect both appliances but a 30A would be cutting it too close. The reason NEC allows this is because ovens and cooktops are not constant loads. They cycle on and off, and it is rare that all of the elements are all on at the same time. It's not 'derating' per se, it's just taking into account the real-world operation of the units.

For example the oven only runs at its full wattage (both elements on maintaining a very high temperature) in self-cleaning mode. Under normal circumstances you wouldn't be running the cooktop full-bore (ie trying to cook a large dinner) while the oven is cleaning. I don't know about you but when I'm cooking a large dinner my oven gets used for cooking things and keeping things warm.

So yes while there is an exception that allows running both off one circuit, it's not really the best way because it paints you into a corner if you or your better half decide you want something bigger down the road. It also puts you in trouble if the manufacturer specifies a max fuse ampacity or a dedicated circuit requirement - because manufacturer's instructions trump NEC.