Electrical question

Assume the microwave is in the kitchen above a countertop.

Kitchen, above countertop branch circuit wiring is 20 amp. #12awg copper wire minimum, 20 amp breaker(s). 

Assume, hard to believe, someone, other than an electrician fed a convenience outlet circuit to the room the audio system equipment is plugged into a wall outlet, off the circuit the microwave is plugged into. (Again assuming the microwave is located in the kitchen.)

Click on Voltage Drop Calculator

Fill in these areas to:

Wire material ........... Copper

Wire size ................... 12 AWG

Material of conduit ..... PVC (Romex)

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Distance one way ....... 65 feet

Load current ................ 20 amps

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Result:

Voltage drop: 4.52
Voltage drop percentage: 3.77%
Voltage at the end: 115.48

 

Load current. Plug in 25 amps (15 amps for an 1800 watt microwave. 10 amps for audio equipment.) 

Result;

Voltage drop: 5.65
Voltage drop percentage: 4.71%
Voltage at the end: 114.35

Breaker should trip after 2 to 3 minutes of constant load.

Lets push the 20 amp breaker to 30 amps load current.

Result:

Voltage drop: 6.78
Voltage drop percentage: 5.65%
Voltage at the end: 113.22

(15 amps for microwave, 15 amp for the audio equipment.) (One heck of a big power amplifier.)

If breaker does not trip after 2 to 3 minutes it probably never will.

While playing music I noticed my amp fell from 120 volts to 112 when the wife turned the microwave on…they must be on the same circuit..is that going to damage anything?…

8 volt VD.

Does the circuit breaker in the electrical panel, that kills the microwave power, kill the power to the wall duplex outlet the audio equipment is plugged into? Simple thing to check...

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Interesting how no one seems aware that their gear have regulated power supplies.

My ARC power amps do not have DC high voltage regulated power supplies. 

I suggest you check the archives for posts of @atmasphere how voltage drop affects the performance of a power amp.  

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The OP’s house was built in 1995. Minimum overhead fed electrical service is 100A @ 240V. That means each of the two legs 120V to neutral loads is good for up to 100A each.

200 amp overhead fed electrical services were common place in 1995 as well.

Underground  fed electrical services even in 1995 were required by many power companies to be 200 amp electrical services. 

    A 120V 15 amp load ain’t squat in the scheme of things.

FWIW our over the range microwave/ hood vent fan unit is rated at 17.7KW. 1700W / 120v = 14.17 amps. My 200A electrical service doesn’t even care when it’s running. My central air outdoors condensing unit draws more than that at 240V. Even that load is minuscule to my 200 amp electrical service.  

 

Correction:  

Post on 08-07-2025 at 06:29pm

FWIW our over the range microwave/ hood vent fan unit is rated at 17.7KW. 1700W / 120v = 14.17 amps.

17.7KW should be 1.7KW, 1700W. 

Brain fart. 

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17.7KW would be one powerful microwave unit. 

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Here is a great example of how a split phase secondary winding of a utility power single phase 240V/120v transformer works.

https://www.youtube.com/watch?v=eVamt9IdQd8

Note: Only the unbalanced load of one Hot leg, Line, through a load(s) to neutral , and the other hot Leg, Line, through a load(s) to neutral returns to the neutral center tap of the 240V/120V secondary winding of the Utility Power Transformer.

If both connected 120V loads are exactly the same, zero current (amps) will return on the service neutral conductor to the neutral leg of the transformer. The two identical loads are in series with one another and are fed by 240V.

If one 120V leg to neutral has a load of 20 amps and the other 120V leg to neutral load is 10 amps then 10 amps, the unbalance load (current), will return on the electrical service neutral conductor to the utility power transformer neutral leg.

Pay close attention in the video when the two loads are not equal and the electrical service neutral conductor connection to the neutral bus bar in the electrical panel to the connection of the neutral leg of the utility transformer is open/broken. (knife switch on the transformer neutral wire in the video.)

More common, instead of the connection being open/broken the connection is loose and or corroded and adds a series resistance in the loose and or corroded connection. (Heat in the poor connection caused by current adds more resistance) That will cause an uneven voltage at the electrical service panel from one Hot Leg to neutral and the other Hot Leg to neutral. The Hot leg to neutral with the greater of the two connected loads, the voltage will be lower than 120V nominal. The lesser loaded Hot leg to neutral voltage will be higher than 120V nominal. The voltage differences between the two depends on the connected loads, and constantly changing 120V loads connected to circuit breakers in the panel. 

Example the higher loaded Hot leg to poor connection of the electrical service neutral conductor at the main electrical panel neutral bar might measure 110V. The lesser loaded Hot leg will measure 120V nominal + 10V  = 130V. 

FWIW I have seen this happen more often with multi wire branch circuits where a neutral conductor is shared with two hot conductors. The 3 wire multi wire branch circuit neutral conductor has a poor/bad joint connection.  

(The two Ungrounded Hot conductors must be connected to circuit breakers on opposite legs. One to L1 and the other to L2.) ( Starting, 2008 NEC required a 2 pole breaker or two single pole breakers with a Listed handle tie to connect the two handles together) 

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