What Size Breaker Does a 4.5 kW Wall Oven Need?
A 4.5 kilowatt wall oven at 240 volts draws 18.75 amperes. That calculates to a 20 ampere branch circuit on 12 AWG copper. But before you wire it that way, read the installation instructions, because a large share of wall ovens are marked for a 30 ampere circuit and 110.3(B) makes that marking enforceable.
The branch circuit calculation
Section 422.10(A) requires the branch circuit rating for an individual appliance to be not less than the marked rating of the appliance. Where the appliance is not marked, you calculate: 4500 watts divided by 240 volts equals 18.75 amperes. Section 210.19(A) requires branch circuit conductors to have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load. A household oven is not treated as a continuous load, so the conductor needs 18.75 amperes of ampacity.
Twelve AWG copper THHN is 30 amperes in the 90 degree C column, derated to the 75 degree C termination value of 25 amperes under 110.14(C), then capped at 20 amperes by 240.4(D)(5). That 20 ampere cap covers the 18.75 ampere load with room, so a 20 ampere two pole breaker on 12 AWG is the calculated minimum. A 3 wire circuit with a neutral is only needed if the oven has 120 volt controls, lights, or a clock, which most do, so plan on 12-3 with ground or three conductors plus a ground in raceway.
Where the demand factor fits and where it does not
Table 220.55 is the source of a lot of confusion here. It applies to household cooking appliances rated over 1.75 kW, and Note 3 directs you to Column B for ranges rated over 1.75 kW through 8.75 kW, where a single appliance carries an 80 percent demand factor. For a 4.5 kW oven that gives 3.6 kW, or 15 amperes, for the service and feeder calculation.
That demand factor is for sizing the service and the feeder only. It never sizes the branch circuit. The branch circuit sees the full 18.75 amperes because the oven can actually draw that much. Applying an 80 percent demand factor to the branch circuit and landing on a 15 ampere breaker would be a violation of 210.19 and a nuisance tripping problem on day one.
Section 210.19(C) also matters for anyone tempted to undersize: branch circuit conductors supplying household ranges, wall mounted ovens, and counter mounted cooking units shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served, with the neutral of a 3 wire branch circuit permitted to be smaller than the ungrounded conductors but not smaller than 10 AWG for a range rated 8 3/4 kW or more.
Two field notes. Wall ovens rated 240 volts should be verified against the actual supply, since a 208 volt commercial or multifamily service drops a 4.5 kW nameplate rating to about 3.4 kW and changes the current to 16.2 amperes. And 422.16(B)(3) permits cord and plug connection for a wall oven only where the appliance is identified for that use and the receptacle is accessible, so most wall ovens are hard wired through a junction box behind the cabinet.