Permanently microwave on 1991

[Esthy]

"If the lights/microwave are on an improperly installed MWBC, simply switching #10 or 14 breaker's wire to #12 breaker, to use both legs, may solve the problem"

JFletcher, I don't get the above ...

 

[Esthy]

My apologies, maybe I am confusing thing out. Notice the MWBC, see breakers (2-4), (6-8) and (10-14: same phase) and in (12) a single dark grey #12 in a 15 amps breaker that goes to the kitchen light and where the micro is taped. BUT

something that don't make sense to me is that having all those multiwiring why a single circuit went to those kitchen lights? If I figure it out, maybe I find an easier solution. MAYBE that circuit is a dedicated one, AND when the kitchen was

remodeled, THEN they used that "dedicated circuit" and TAP the lights (those lights are new cans in a concave ceiling" see the photo in the first page). It is obvious that I will transfer the conductor on breaker 14 to the breaker 12, so this

MWBC will be in a different phase and transfer the single circuit from breaker 12 to breaker 14.

AGAIN: common sense tells me that that single circuit was a dedicated one for something in 1991, and the remodelers used it for the lights.

 

[JFletcher]

"If the lights/microwave are on an improperly installed MWBC, simply switching #10 or 14 breaker's wire to #12 breaker, to use both legs, may solve the problem"

JFletcher, I don't get the above ...

If a MWBC (eta: of a 120/240V split phase/residential system) has 2 ungrounded (hot) conductors originating from the same leg, the amperage on its neutral will be the sum of the currents, and potentially overloaded. If the ungrounded conductors originate from opposite legs, the amperage on the neutral is the difference in currents and it's impossible to overload the neutral. There'd be less voltage drop as well.

I've no idea how this house is actually wired, the fact that MWBC are on the same leg would make me wanna rip it all out and start over. The lighting doesnt need a dedicated circuit. Yes, it's possible it has been wired that way for 26 years and hasnt burnt down. The NEC is pretty conservative. I wouldnt try to put 30+A on #14 neutral for any amount of time, and perhaps there has never been overcurrent on the neutral.

 

[kwired]

If a MWBC (eta: of a 120/240V split phase/residential system) has 2 ungrounded (hot) conductors originating from the same leg, the amperage on its neutral will be the sum of the currents, and potentially overloaded. If the ungrounded conductors originate from opposite legs, the amperage on the neutral is the difference in currents and it's impossible to overload the neutral. There'd be less voltage drop as well.

I've no idea how this house is actually wired, the fact that MWBC are on the same leg would make me wanna rip it all out and start over. The lighting doesnt need a dedicated circuit. Yes, it's possible it has been wired that way for 26 years and hasnt burnt down. The NEC is pretty conservative. I wouldnt try to put 30+A on #14 neutral for any amount of time, and perhaps there has never been overcurrent on the neutral.

You could just put the two ungrounded on opposite lines instead of ripping it out, unless you believe the it has been compromised by overloading or other issues not mentioned are present.

For no longer then many microwaves are loaded at a time I really doubt the neutral has been overloaded enough to cause it much damage either though. Not saying it makes the install correct, but I don't think that neutral is as compromised as it would be with a much more continuous load of same amperage.

 

[mjf]

I didn't read through all the posts.

Get the make and model of the microwave and see if the manufacturer calls for a dedicated circuit.

 

[kwired]

Someday that microwave just might be replaced with one with different instructions - if cord already reaches a receptacle is it still the original electricians fault it doesn't comply with the instructions?

Many of us run a individual circuit to such appliance locations regardless of what model may get installed - generally is seen as good practice regardless.

 

[gadfly56]

My apologies, maybe I am confusing thing out. Notice the MWBC, see breakers (2-4), (6-8) and (10-14: same phase) and in (12) a single dark grey #12 in a 15 amps breaker that goes to the kitchen light and where the micro is taped.

Is "dark grey" acceptable as a hot leg? Couldn't it reasonably be mistaken for a neutral in the field?

 

[Esthy]

Tomorrow I will reconnect the mwbc in the correct way and see what happens and from there I will propose the work to be done unless the reconnection solve the problem (I don't think so, but ...)

 

[JFletcher]

You could just put the two ungrounded on opposite lines instead of ripping it out, unless you believe the it has been compromised by overloading or other issues not mentioned are present.

For no longer then many microwaves are loaded at a time I really doubt the neutral has been overloaded enough to cause it much damage either though. Not saying it makes the install correct, but I don't think that neutral is as compromised as it would be with a much more continuous load of same amperage.

"rip it all out" was a bit dramatic on my end; I meant to convey I would check and redo the panel to make sure any and all MWBCs were on opposite legs, which in Esthy's pictured panel, should not take much time, even if every single circuit was MWBC and wired wrong.

If a 1550W microwave is jumped off a 15 (or even 20)A lighting circuit, the lights are gonna dim until they are separated. LEDs might be less susceptible to dimming due to voltage drop, but the voltage drop is still there.

 

[mjf]

Someday that microwave just might be replaced with one with different instructions - if cord already reaches a receptacle is it still the original electricians fault it doesn't comply with the instructions?

Many of us run a individual circuit to such appliance locations regardless of what model may get installed - generally is seen as good practice regardless.

Personally, I have never not run a 20 amp individual circuit to an over the range microwave.

 

[kwired]

Personally, I have never not run a 20 amp individual circuit to an over the range microwave.

I'm sure one can get away with not running one, but on new installations, many of us "overwire" kitchens as a general rule anyway.

Not too many times does one need more then the 2 required SABC's in the kitchen, yet many new homes or major kitchen remodels get at least 3 or 4, plus individual circuits to such a microwave or even the refrigerator.

 

[gadfly56]

Personally, I have never not run a 20 amp individual circuit to an over the range microwave.

OP states that the fixed in place microwave is 1550 watts. Too high to comply with the max 50% for fixed in place equipment on even a 20 amp circuit. It would have to be dedicated, either 15 amp or 20 amp.