Power Requirements for an Added Oven

Question

Existing condition is one oven off a 40 amp, 120/240 V dedicated circuit. We are looking to replace both the existing oven and also add a new 120/240 V speed oven as well. So the new wall ovens would be as follows:

• "Speed Oven" - 120/240 V, 30 Amps (6.3 kw power rating at 240V)
• "Convection Oven" - 120/240 V, 20 Amps (4.2 kw power rating at 240V)

I’m wanting to explore how to get this to work without requiring extensive electrical work assosciated with adding a new dedicated circuit and wiring between the panel and the ovens, which is only a 35-50 foot run but would require sheet rock work and have to be run exposed in the garage… Is there any reason I can't take both ovens off the existing oven branch circuit and just rewire from that existing circuit to include taps to both ovens? The current circuit is 2-pole 40 amp and is wired with 6 AWG wire labeled XHHW-2. I have plenty of available load on my circuit breaker to expand the 40 amp service to 50 amps and it appears the 6AWG XHHW-2 wire is rated to 55 amps in accordance with NEC Table 310.15(B)(16). The NEC seems to generally allow wiring multiple ovens off the same branch circuit as evidenced in 210.19(A)(3) Exception 1.

Alternatively, I am toying with the idea of utilizing the existing 6AWG XHHW-2 wiring to supply a new 50 amp sub panel where I can divide the 30 amp breaker and 20 amp breaker separately, but I'm not sure if there is anything to consider that I'm not already considering regarding this approach. The manufacturer installation instructions seem to prefer a dedicated circuit (although they use softer language like "should" rather than "shall" and seem to have some conflicting information regarding whether or not it needs to be on a dedicated circuit between the instalaltion instructions for the two different appliances). The main benefit of this approach would be to provide a dedicated circuit to each appliance, although I'm not sure if this is truly required or if there are practical or theoretical advantages to this approach worth the additional effort.

Here are links to the ovens intended for use if someone finds this useful:

Link to Standard Convection Oven: https://www.cafeappliances.com/appliance/Cafe-Professional-Series-30-Smart-Built-In-Convection-Single-Wall-Oven-CTS90DP4NW2

Link to Speed Oven: https://www.cafeappliances.com/appliance/Cafe-30-Smart-Five-in-One-Wall-Oven-with-240V-Advantium-Technology-CSB923P2NS1

Any help would be greatly appreciated as I've spent a tremendous amount of time trying to understand how to solve this issue and am having trouble getting an electrician to make a house call. Ovens are ready for delivery and I'm wanting to have a gameplan for the electrical before accepting delivery of the items.

Answer 1

The main advantage of a sub-panel is that if anything goes wrong, there's less headroom before a breaker trip. Something I appreciate more after hearing a funny noise from my oven, turning around, & watching my oven element die, and arc, and build a little volcano of yellow and white-hot slag on itself, while my 50A range breaker said "Hey, no problem, not 51 amps, burn, baby, burn!"

The main thing you may not have considered which can be a downside of a sub-panel is that it requires clear working space, 36" out in front, 30" wide, 6-1/2 feet high, and it can't have anything stored in it (must be kept clear - usually managed by putting it beside a walkway/hallway where that doesn't create a huge waste of storage space. The 30" needs to include the panel, but does not need to be centered on it. Your picture seems to show plenty of space.

If, as seems to be implied in comments, you have individual 6AWG XHHW-2 wires and no conduit, you'll be ripping the walls open anyway to fix that before you get an inspector to sign off on this. That should have been failed when it was installed, which implies bootleg uninspected work... That will be costly (if hired out) or difficult (if DIY) to remedy, but it's required, and it should not be "unsightly" once the drywall repair is done correctly. You have a cable, so that's not an issue.

Answer 2

You need to do a range/oven Load Calculation

NEC Load Calculations are in Article 220.

Your range and oven load calculation happens in NEC 220.55. This sends you over to Table 220.55 (on another page; it's huge) which you will need to calculate for yourself since you haven't told us anything about the cooktop which is I presume is part of this circuit.

If it's not, we can proceed. You have two appliances of 6.3 kW and 4.2 kW, so they come out of Table 220.55 column B (both). Sum is 10.5kW, demand factor is 65% giving 6.825 kW. That's a shave under 30A actually, so the 40A breaker should be able to carry it.

This favorable derate is due to the presumption that both ovens won't be at 100% duty cycle at the same time for any length of time. Breakers are highly tolerant to mild overload, so a 10.5kW load on a 9.6kW breaker (10%) for long enough to warm up one oven probably will be tolerated by the breaker. If not, I'd consider it a trivial nuisance trip, reset it and carry on.

... and a Load Calculation for the service.

You are adding something fairly big. So now you use the rest of Article 220 to do a Load Calculation for all the other stuff on the service.

You can use the above oven calculation straight if you want to. But you're also welcome to grab the cooktops also and any other >1750W cooktops and ovens elsewhere in the house, and combine them all with a second pass through NEC 220.55. That will be advantageous.

That will tell you whether your panel/service can handle this or you need a service upgrade (or more realistically: some sort of load-shed strategy e.g. interrupting a tanked water heater when the oven or dryer is on).

If you have surplus in the Load Calculation, take note of that - home buyers make better offers when there's a 240V EV charge outlet in the garage, so throw that circuit in there right before you list. EVs can charge off 240V/15A, 20A, 30A, 40A, 50A or 60A circuits (just a matter of DIP switch settings on the wall-mount unit, or ordering a different dongle plug for a travel unit).

How about two ovens sharing a circuit?

That's fine per eithrt NEC 220.55, or as you say, 210.19(A)(3) Ex. 1. It's pretty much the only place that the tap rules are allowed in residential.

Now the only trick you have is if that is aluminum wire, how do you splice to it safely? (it would help if it was not costly and did fit in that box).

The best answer I have seen is the ILSCO Mac Block Connector or competitor. It has two ports. A port can accept a #6 aluminum wire. And the other port can accept up to two #10 copper wires. (and many other combinations). They're a bit hard to find, but a lot smaller and cheaper than 3-port Polaris connectors. And unlike Alumiconns they go up to #6.

The purple "wire nuts" supposedly rated for aluminum wire do not perform well.

Answer 3

Here is a previous question addressing a similar situation. The conclusion seems to be if the instructions do not require dedicated circuits, and particularly if the nameplate does not specify a max breaker size, you can use the tap rules to add both ovens to a single circuit.