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I’m looking to have a LG Model #: LWC3063ST microwave/oven installed which will require running a new branch circuit.

The installation requirements state it draws 32.5amps on a dedicated 240v circuit… but also says:

A Microwave Combination Wall Oven can consume up to 7,800 W at 240 VAC. A Hinge 50 Amp circuit breaker with wire gauge #8 AWG must be used.

Two questions:

  1. Why would it call for a 50 amp breaker if it says it only pulls 32.5… wouldn’t a 40amp be sufficient?
  2. Everything I have researched says that you should use #6 wire for a 50amp breaker… not #8. What is correct?
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  • That's extremely bizarre that they'd require a 50A breaker on a 7.8kW wall oven -- the NEC would happily permit this appliance on a 40A breaker any day of the week! (Table 220.55 Note 4 says that the nameplate rating is the branch circuit load for a single cooking appliance) Apr 11, 2023 at 4:31

3 Answers 3

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The derating for continuous use in areas using the NEC is 125%. Which means:

  • 12A -> 15A (which is why you have so many 1500W (125V x 12A) space heaters)
  • 24A -> 30A (typical for clothes dryers)
  • 32A -> 40A (typical for large ovens)

and

  • 32.5A -> 40.625A, which rounds up the next standard breaker size of 50A

which looks like a very poor design decision by LG. Maybe they originally thought it would be only 32A. Impossible to guess.

As far as wire size, that depends on what kind of wires you are using. Based on this ampacity chart, the options for 50A include:

  • Copper, 60C rating and NM-B (a.k.a., Romex - the typical cable type) = 6 AWG
  • Copper, 75C rating and THWN (individual wires in conduit) = 8 AWG
  • Aluminum, 75C rating and THWN = 6 AWG

So 8 AWG is the minimum, but code requires larger wire depending on your specifics. Many people think only in terms of "cables", but conduit + individual wires can make a lot of sense, depending on a number of factors.

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    Thanks… very helpful. I understand the 50 amp breaker requirement, but should I interpret their stated requirement for #8 AWG as driving me to have to use copper 75c THWN? Normally I would think I could up size to #6 but I understand code calls for compliance to manufacture specs right?
    – Ndogg
    Apr 10, 2023 at 22:22
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    You can pretty much always upsize wires. Arguably you could say "manufacturer says use 8 AWG" and try to use 8 AWG cable, but that is likely to fail inspection because of the 50A breaker. So 8 AWG copper wires (larger if you want to, but I wouldn't bother) or 6 AWG copper cable (larger if you want to, but I wouldn't bother). 6 AWG (or larger) aluminum wires is also an option, unless your local jurisdiction restricts use of aluminum wire (mine does). Apr 10, 2023 at 22:28
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    Got it thanks… appreciate the clarity
    – Ndogg
    Apr 10, 2023 at 23:06
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Ranges and ovens get really weird calculations.

NEC specifies the Load Calculation in NEC 220.55, and UL's rules for integrated appliances reflect that.

As such, it's not uncommon to see oddball wire and breaker specs.

8 AWG wire is legal for 50A as long as it's anything but NM or UF type. SER type cable gets installed pretty much the same as NM cable (and for a short time they floated an argument that therefore it should have the same current limits, but cooler minds prevailed).

Also, 6 AWG aluminum is allowed 50A, again as long as it's not NM or UF type. SER is again a winner.

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That 0.5A ensures that a 50A is required, due to derating.

A 32.0A "continuous" load (may run more than 3 hours at a time) can run on a 40A breaker, because 32.0A/0.80=40.0A. A 32.5A load comes out 40.625A, and by the rules of the NEC, that means a 50A breaker is required, because it's more than 40.000.

The other reason a 50A breaker is required is that following manufacturer's instructions is part of code, and they tell you to use a 50A breaker. The smart money is on aluminum wiring, not copper, at an appropriate size for 50A.

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