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We are upgrading our wall oven and cooktop. The old units were rated for 30 amps each and the new units require 40 amps. We have a 200 amp main panel with one 40 amp breaker for the oven/cooktop that feeds 600 volt 3-6 aluminum wiring to a subpanel. The subpanel currently has one 30 amp breaker feeding NM-B 10-3 to the cooktop and one 30 amp breaker feeding NM-B 10-3 to the oven. As both the new cooktop and wall oven each require 40 amps, what is the best way to proceed? Is the single 40 amp breaker in the main panel and the 3-6 wiring to the subpanel sufficent to meet the load demands of the new 40 amp units?

In response to the questions, the NM-B wiring can be easily replaced from the subpanel to the oven and cooktop. Replacing the 3-6 wiring would be much more difficult, as it is routed behind walls and over drywall cielings. The subpanel could be physically removed. The insulation for the 3-6 is grey plastic/vinyl. I have attached a couple of photos of the wiring and subpanel. I should also add that I plan to hire an electrician to do the work, but would like to know what to expect and, if possible, run some of the wiring beforehand to reduce the labor cost.

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EDIT: At the main panel the aluminum strand attached to the bus bar in the subpanel is attached to the bus bar in the main: enter image description here

EDIT: Thanks for all the responses. The subpanel is in the basement immediately below the kitchen floor underneath the wall oven. The main panel is on the opposite side of the house. I have attached additional photos of the 3-6 cable below.

The cooktop is a Kitchenaid downdraft (KCED606GBL). The oven/microwave combo is a Bosch Series 800 (HBL8752UC). Kitchenaid nameplate indicates 10.8 kw @240 v: enter image description here.

Bosch oven is 9.6kw @240 v: enter image description here

Bosch microwave is 3400 w @ 240 v: enter image description here

Additional 3-6 photos: enter image description here enter image description here

  • How much of this wiring can be replaced? Is removing the subpanel an option? – ThreePhaseEel Aug 10 at 12:58
  • What size is the conduit that the 3-6 al is in? What type of insulation? – JACK Aug 10 at 13:01
  • Hello, and welcome to Home Improvement. We'll need some more information; please edit it into your question. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Aug 10 at 13:24
  • Can you get us more photos of the labeling on the 6/3 Al NM? What make and model are the new appliances, BTW, and what are their nameplate kW ratings? – ThreePhaseEel Aug 10 at 15:06
  • What make/product line is that main panel of yours? – ThreePhaseEel Aug 10 at 16:54
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My first concern looking at the subpanel is if that bare #6AL is in fact a ground and the two appliances are straight 240V then the two white wires connect to the buss should have some green tape on them. If it's not a ground but actually a neutral, where are the grounds? You will need to ground everything.

I would try utilizing the #6AL XHHW since changing it out would be difficult. It is rated for 50amps according to my chart so the breaker in the main panel should be changed to a 50AMP. Since the two NM-B cables can easily replaced, change them to NM-B 3#8 and replace both breakers in the subpanel to 40 AMP.

Where actually is the subpanel?

News flash: The name plate on the oven states "four wire". Major game changer. The new wire you run will have to have a ground. This will probably require you to change the #6AL run to 3#6cu plus ground or just run two 3#8 with gnd, one for each appliance, add additional breaker in main panel. Both breakers would be 40AMP and eliminate the subpanel.

If your owner's manual states that you can hook up the four wire appliance to a three wire source, then it is ok to do it.

  • The subpanel is in the basement beneath the location of the wall oven. – Michael McCarter Aug 10 at 15:42
  • Thanks for updating your response. The manual indicates it can be installed with existing three wire setup by connecting the ground and white wires. This is how our old 4-wire Jenn Air oven was connected at the junction box. Although I understand 4 wire is preferred, isn't the 3 wire installation permissible? Does this change your thoughts? – Michael McCarter Aug 10 at 16:15
  • I also added a photo of the 3-6 as it is connected at the main panel, if that helps clarify the questions raised in your original response. Thanks. – Michael McCarter Aug 10 at 16:46
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The 3-6 appears to be aluminum so its ratings (distance and ampacity per diameter) are lower than for copper wire/cable. When you attach the ends of Aluminum wire to new breakers or other terminals, use a wire brush to clean off oxidation or dirt and coat with an oxidation inhibitor sold for the purpose. Be sure your terminations at breakers or devices are rated for aluminum wire. Tighten Al and Cu connections properly, not too much or too little.

For the two 40A runs to the appliances from the subpanel, you could use the ratings on this ampacity chart from Cerrowire. There are other makers and other charts; they should pretty much agree with each other, and be derived from some version of the National Electrical Code. Here is a search with more info about the NEC, and how to understand it.

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This is something your feeder could handle...

A 6AWG aluminum feeder landed on 75°C lugs (such as those found in loadcenters and on circuit breakers) can handle 50A. If we apply the Table 220.55 calculations, starting with Note 4:

  1. Branch-Circuit Load. It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch-circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance. The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range.

, we get an equivalent range nameplate load of 20.4kW for your new cooking setup, considering that the microwave's 3.4kW is included in the oven's overall 9.6kW load. We then apply the process from Note 1:

  1. Over 12 kW through 27 kW ranges all of same rating. For ranges individually rated more than 12 kW but not more than 27 kW, the maximum demand in Column C shall be increased 5 percent for each additional kilowatt of rating or major fraction thereof by which the rating of individual ranges exceeds 12 kW

, which yields a 45% increase (5% * 9) over the 8kW baseline value from Column C for a single range not over 12kW, or a factored load of 11.6kW. Dividing this by 240V yields just over 48A, which is just under the 50A your feeder is rated for.

But, you'll have to do something with it anyway

The other issue with your feeder is that it's a type SE style U cable, with only 3 wires (2 hots and a bare ground/neutral). This is no longer usable for a 120/240V circuit in new work, as neutral and ground must be separated now as per 250.24(A)(5):

(5) Load-Side Grounding Connections. A grounded conductor shall not be connected to normally non–current-carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article.

Note that you cannot apply 250.32(B)(1) Exception 1 in this case as the subpanel is in the same structure, nor can you apply the rules from the exception to 250.140, even if the subpanel was removed, due to having to replace the ungrounded 10/3 NM cable, as described below.

However, what you can do is remove the bonding screw in the subpanel, add an Eaton GBK5 ground bar to the subpanel, and run a bare or green 10AWG ground wire from the subpanel to the main panel by any route you can find, connecting it to the ground bar in the main panel; if you can't get to the main panel, then running it to the nearest grounding electrode conductor and connecting it using a mechanical T-tap (Ilsco GTT-2-2 or equivalent) works as well. This is within the spirit of NEC 250.140(C) (and its letter if you remove the subpanel's interior, even).

The 10/3 NM will have to go as well

The existing 10/3 NM cables will have to go the way of the dodo as well, as they are undersized for the new loads you are placing on them (10.8kW aka 45A for the cooktop circuit and 9.6kW aka exactly 40A for the oven circuit).

The good news is that since the overall circuit is 50A, we can apply the provisions from 210.19(A)(3) Exception 1 here, which means we don't need the subpanel. However, that doesn't get us much in terms of downsizing the wire since the cooktop requires 6AWG wire anyway.

(3) Household Ranges and Cooking Appliances. Branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8¾kW or more rating, the minimum branch-circuit rating shall be 40 amperes.

Exception No. 1: Conductors tapped from a 50-ampere branch circuit supplying electric ranges, wall-mounted electric ovens, and counter-mounted electric cooking units shall have an ampacity of not less than 20 amperes and shall be sufficient for the load to be served. These tap conductors include any conductors that are a part of the leads supplied with the appliance that are smaller than the branch-circuit conductors. The taps shall not be longer than necessary for servicing the appliance.

Given all this

I would get a spool of 6/3 W/G NM-B, sufficient to replace all of the wiring involved in this setup. If you want to retain the subpanel, that's fine albeit not necessary; you'll want two CH250 breakers if you're retaining the subpanel, as well as the CH250 you'll need for your main panel in any case. You may wish to let the electrician do the runs, especially for replacing the feeder back to the main panel, though.

  • I believe I might have made a mistake reporting the oven/microwave nameplate load. I believe the 9.6 kw may be for both, as the installation instructions specifically state to use a 40 amp breaker. If this is the case, would I apply Note 2? That calculation would provide 47.3 amps, which would mean the existing 6-3 wire to the main panel is acceptable, right? Or am I applying the wrong note? – Michael McCarter Aug 10 at 20:42
  • Yeah, I reviewed the installation manuals for your oven and it does appear that the 9.6kW nameplate load for the oven covers both appliances. You'll need something beefier than a 40A circuit for that cooktop, anyway, though. – ThreePhaseEel Aug 10 at 23:35

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