3

Original situation:

I have a 14/2 circuit, where the circuit powered 7 lights and 3 outlets. One of those outlets was in the kitchen and was powering a microwave and sometimes an air fryer. During microwave operation, the microwave sound started to sound lower in pitch occasionally suggesting to me that the circuit is overloaded or maybe that the microwave is going bad. I want to start at the circuit, because it is indeed underpowered for the microwave.

Appliance Usage

I usually use one or the other appliance, but not both. However, I might sometimes use both, but I imagine it will be rare, and so I don't know if it will be worth to wire up the circuit to account for using both, or using just one at a time. Also the way my appliances are positioned, both of them are plugged into the same kitchen outlet (the outlet in the photos).

Power stickers on the appliances:

  • Air Fryer: 1700W, 120 VAC, 60Hz
  • Microware: 60Hz, 11.5 amp, 120VAC

Note, however that when measuring amps directly the ammeter gave a reading of 16.4 amps at the highest for quite a while, and then it went down to 14 amp reading otherwise. I don't know if going from 11.5 amp to 14-16.4 amp is within the normal operating range of the microwave or if it is drawing more current because it is reaching its end of life or is faulty otherwise. I do plan to replace the microwave eventually.

Current Situation

I want to wire up a new outlet with duplex receptacles probably with 12 gauge wire that can handle both appliances. Chances are I will not run both of them at the same time, but there might be exceptions to where I do.

How do I best wire up this receptacle?

  • what AWG do I use?
  • what amp and type of breakers do I use, and where do I position them relative to each other?
  • what type of outlet do I use?
  • anything else I need to consider?

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  • 1
    Most high-amp tools like power saws or appliances that generate heat like microwaves or hair dryers, etc. have bigger start-up (and often shut-down) amperage draw compared to normal running amperage, so that's normal. The rating stamped on the appliance is going to be the start-up/shut-down amperage, not the normal running amperage, if there is only one amp value listed (since that's the minimum needed to use it).
    – TylerH
    Jan 5 at 20:05
  • AC power measurement is also very complex. For anything except the simplest resistive load, power factor comes into play - volt-amps aren't watts. Plug-in "kill-a-watt" style meters can measure power factor, but clamp meters cannot.
    – nobody
    Jan 6 at 2:13

3 Answers 3

9

You've got a complicated issue here. You want to use a 20A circuit (which is required for new kitchen countertop circuits, so that is 100% correct) so 12 AWG wire. But you really want to be able to run two significant (> 10A) appliances at one time. You can't do that on a 20A circuit. There are two possible solutions:

  • MWBC = Multi-Wire Branch Circuit

    • Advantage: A little cheaper on wire, because you use one 12/3 cable instead of 2 12/2 cables.
    • Disadvantage: GFCI needs to be (practically speaking) at the breaker, which is more expensive than at the receptacle.
  • Two separate circuits.

    • Advantage: GFCI can be at the receptacle.
    • Disadvantage: Need two separate cables and two separate GFCI receptacles. (They can be in the same box side-by-side.)

An MWBC uses one neutral shared with two hots, together with a double-breaker (or two handle tied breakers, but double breaker is easier). The problem is that a shared neutral doesn't work well with ordinary GFCI receptacles, and a GFCI receptacle can't work with two hots anyway (you can't split it like a plain receptacle). But if you really want "a standard duplex receptacle with top and bottom each able to pull maximum current" then an MWBC is the way to do it.

Two separate circuits also uses two breaker spaces, but the breakers don't have to be next to each other. Each circuits needs GFCI but can use standard GFCI receptacles to do that. But you can't have the two receptacles of a duplex on two separate circuits unless you handle tie the breakers or use double breakers, and if you do that then you might as well save on cable by using 12/3 and then you have an MWBC.

Since you have easy access to run cables, I'd go with two separate 20A circuits, each with a standard 20A GFCI duplex receptacle. Mark one of them as "microwave". That way everyone knows "use the second receptacle of the duplex for small stuff like phone chargers but not for toaster or air fryer or other big stuff".

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  • 1
    Why use a 20A receptacle? Neither device has a 20 A plug.
    – Glen Yates
    Jan 5 at 20:01
  • 2
    @TylerH on another forum, we recently determined that buying small rolls at Home Depot is sheer madness. One guy needed a 10' roll of 12/3. Home Depot would cheerfully sell 15' for $45. I was lectured sternly that it's every American's duty to shop at big-box for the savings, and we must run all those overpriced mom-n-pop hardware stores out of business "since they've been taken over by ACE". So I price-checked my local very, very boutiquey "Tiffany's" hardware store. 10' of 12/3 was $23, half of Home Depot's price. Jan 5 at 21:13
  • 1
    @dennismv with MWBC, you'll use GFCI breakER (singular) at the panel of the 2-pole/240V persuasion. About $80 pre-COVID. That's because GFCIs compare current on all wires to make sure all current that goes out comes back. Jan 5 at 21:20
  • 1
    @Harper-ReinstateMonica Sure, I agree with you; unfortunately not every town has a "Tiffany's"... but they do typically have a Lowes or Home Depot or Menards. Luckily I live in Southwire's global HQ town so I have access to a lot of high quality electrical stuff at pretty great prices.
    – TylerH
    Jan 5 at 22:24
  • 1
    @manassehkatz, excellent answer, thank you!
    – dennismv
    Jan 6 at 5:00
6

Potential wiring faults

During microwave operation, the microwave sound started to sound lower in pitch occasionally suggesting to me that the circuit is overloaded

That's not how overloads work. Your circuit is probably short enough that it has <1% voltage drop at max amperage. If you overload it 200% (30A) then you have 2% voltage drop. A triple overload (45A) would only be 3% drop. None of that would even be detectable at the appliance.

If the appliance is struggling, either

  • a wiring fault is causing much worse voltage drop, meaning a connection is getting dangerously warm somewhere. Or
  • a house-wide Lost Neutral is causing big voltage swings. Lost neutrals are insidious; they can go months before displaying clear enough symptoms to identify them. Or
  • the appliance is dying.

Alien breakers

This is plainly a Siemens/Murray panel, noting the un-plated copper bus bars which are a signature.

Due to differences in bus bar spec, breakers not made for this panel should not be in this panel. The panel labeling lists all allowed breaker types, and I'm fairly sure GE THQL and Square D HOM are not on that list lol.

All modern 1" wide breakers will seem to fit each other's panels but won't engage to the bus bar properly, which can cause arcing and fire damage down the road. So the Square D and GE breakers (I assume there's an Eaton in there under a label, just to complete the collection) need to go and be replaced with Murray MP (now Siemens QP). We're talking $5 a breaker in normal pricing, so this won't break the bank.

Bringing enough power

I usually use one or the other appliance, but not both. However, I might sometimes use both, but I imagine it will be rare, and so I don't know if it will be worth to wire up the circuit to account for using both, or using just one at a time.

You CAN'T use both. I'd explain why (both appliances are over 10 amps) but you have the technical chops to know why.

This is true of almost all kitchen heat appliances, as almost all are 12 amps (they run at absolute limit of 1500W @ 125V rating).

If you are "unable to control yourself" LOL, then you could install a 1-socket receptacle (simplex) so you'd be forced to unplug one to use the other. However, my view is that we're the humans! Electricity is our servant not the other way 'round, and *electrical should Serve Our Needs. Therefore if you want to run both at once, why be limited? Run 2 circuits. Easy peasy. Copper is cheap.

Well, not right now. If I owned plenty of 12/2 but had to buy 12/3, I would run two individual circuits and double-split the receptacle (or change to a 2-gang box). A MWBC on 12/3 is alright, but will require a two-pole GFCI breaker (not two singles).

Note that if 2 separate circuits feed the same receptacle yoke, they must be handle-tied, or a 2-pole breaker. This is the one case where you can handle-tie two single GFCI breakers.

Note, however that when measuring amps directly the ammeter gave a reading of 16.4 amps at the highest for quite a while, and then it went down to 14 amp reading otherwise.

Because it is sharing the circuit with 3 other outlets and 7 lights. The extra 2-4 amps are coming from other lights or plug-in loads on the circuit, which are doing their own things.

Your questions

what AWG do I use?

You have no choice. Kitchen receptacles MUST be on 20A circuits.

So you must use 12 AWG copper (#10 if you have that).

#10 aluminum is also legal, but a lot of local inspectors will give you a fight on that.

what amp and type of breakers do I use, and where do I position them relative to each other?
what type of outlet do I use?

Kitchen counter receptacles require GFCI protection. That could be one 20A breaker and one GFCI repeptacle. Except that you have aspirations to run both appliances at once, which requires 2 circuits somehow. \

If you want to fit this into a 1-gang receptacle box, that means splitting the receptacle so each socket has a full 20A. (giving each a full 15A seems sensible, but is illegal per above). So these are our choices:

  • A 2-pole (240V), 20A GFCI breaker, feeding a MWBC (Multi-Wire Branch Circuit) via 12/3 cable. To a single receptacle with the hot side "split" and fed from each hot wire.
  • Two individual 20A GFCI breakers, with a handle-tie, placed adjacent since the handle-tie requires it. Dual 12/2 cable running to the receptacle. On the receptacle, both hot and neutral sides are "split". Each socket is powered from a separate cable.

If you are willing to blow out the box into a 2-gang box (or two 1-gang boxes), then you can use two GFCI receptacles, fed either from an MWBC or two independent circuits. In the latter case, the breakers can be anywhere. The MWBC breakers need to be handle-tied, so they will be adjacent. The cheapest way to get 2 handle-tied breakers is to use a 2-pole (240V) breaker.

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    No comment on the alien invasion that has taken place in this panel??
    – nobody
    Jan 6 at 0:10
  • @nobody holy smoke yeah, I didn't spot that. Jan 6 at 1:57
  • 1
    The "alien invasion" is a good reason to not put stickers over the breaker bodies like that. Is that actually allowed in code or is that verboten, too?
    – FreeMan
    Jan 6 at 14:41
  • you've mentioned I can use 12 AWG, but #10 if I have it.... you mean 10 AWG? I don't have it but since I will be going through the troubles of installation ... should I use 10 AWG? It seems like it's not required, but why do you mention possibly using it? Will it give any significant benefit over using #12?
    – dennismv
    Jan 9 at 20:39
3

I would:

  • Put a 15A GFCI into the existing electric box
  • Install an old work box a few inches from the existing box
    • Just make sure it's in the same stud bay and you should be able to re-use the existing wire's chase-way

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Heck, go ahead and put in a double-gang old-work box and run two 12/2 wires to it so you can have two dedicated 20A GFCI outlets.


what AWG do I use?

I always run 12 AWG for outlets, period.

what amp and type of breakers do I use

The 14 AWG circuit can only use a 15A breaker, period.

The 12 AWG circuit can use either 15A or 20A breaker.

and where do I position them relative to each other?

It doesn't matter

what type of outlet do I use?

I believe the 15A circuit requires a 15A outlet as well. You don't want to put a 20A outlet on it and have people think it's a 20A circuit.

I recall some code that if there is only a single receptacle on a 20A breaker then it has to be a 20A receptacle. Multiple 15A receptacles can go on a 20A breaker.

Why does US Code not allow a 15A single receptacle on a 20A circuit?

anything else I need to consider?

Verify the circuit is off before touching wires =)

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  • "if there is only a single outlet on a 20A breaker then it has to be a 20A outlet." That's NEC 210.21(B)(1): "A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit." (note the word there is receptacle, not outlet) It isn't just for 20A but anything (there are special exceptions for a motor of less than 1/3 hp or an arc welder). Why this is the case was asked and answered in this DIY.SE thread: diy.stackexchange.com/q/210537
    – TylerH
    Jan 5 at 23:09
  • @TylerH Excellent! Added to my answer.
    – MonkeyZeus
    Jan 6 at 16:40
  • question - why put a 15A GFCI into the existing electric box? I actually did have a 15A GFCI there, which I took out intending to convert it to a proper 20A GFCI, because I felt that 15amp was not a proper circuit for the appliances over 10A that I've been keeping & using there for a while. I also see in another answer that "kitchen receptacles MUST be on 20A circuit", which I understand must be due to the NEC.
    – dennismv
    Jan 6 at 22:51
  • Ignoring NEC for a moment, I think a dedicated 15A circuit might work fine for each appliance in a real world application, but in my case almost half of all lights in my house are located on that 15A circuit, and plugging in a microwave in addition to that circuit is probably not advisable .... despite me having already done so for the last 10+ years, err.
    – dennismv
    Jan 6 at 22:51
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    @dennismv Nowhere in your post did you mention the desire to decommission the 14/2 wire so maybe I'm the confused one. Anyways. If you're going through the trouble of fishing new wire then I would run 2 wires so that you can have 2 dedicated outlets.
    – MonkeyZeus
    Jan 7 at 18:58

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