Electrical question

I wonder how much inrush current for a microwave oven on startup, the heater for the magnetron tube, filling the high voltage capacitor, plus the high voltage transformer. The magnetron tube is directly heated so it's warming up right away.

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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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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A quote from Vinnie Rossi audio site see below:

’’With active power factor correction (PFC) and superior regulation and stability, the power supply ensures maximum performance regardless of AC mains quality. Housed in a fully enclosed compartment precision machined into Brama’s solid aluminum body, it is fully grounded and isolated from the audio circuitry, performing like a highly-responsive, well-tuned engine.’’ See Vinnie Rossi here.

Mike

Regulated power supplies are rarely used in amplifiers and the voltage on the rails is direclty proportional to the voltage on the AC.

Having said htat, that's an unusually high drop.  Your house's age may have something to do with this but modern code has a separate circuit just for the microwave.  If you are still seeing that kind of drop, measure the neutral to earth.  If it's more than 2V you have a separate issue which is a bad neutral.  I use these because they do both voltages at the same time.

HJaving said all of that I use a Furman voltage regulator to keep my AC within 5V at all times, but it's not for bad wiring.  It's for the 2 heat pumps turning on and off and seasonal variations. 

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.  

 

and varying amounts of capacitance...the dynamics of music don't have a linear draw on the power supply either 

Interesting how no one seems aware that their gear have regulated power supplies. Regulated means that DC fed to the actual circuitry remains exactly the same whether the microwave is on or off.

Of course, regulation stages can’t handle severe surges or brownouts, but temporary voltage drops from appliances are fine.

Dedicated circuits direct from your gear to the panel (aka home runs) are never a bad idea.

In my state, the microwave, refrigerator, disposal and outlets have to be on separate circuits.  Also, a typical home microwave should NOT draw that much amperage to mess that much with the voltage.  If your microwave is pulling that much power there might be another problem and you might want to get it checked out by a licensed electrician.

 

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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Just put in a new circuit with heavy gauge wire, like 20 amp.  And use a hospital grade outlet.  Thank me later.

@boomerbillone I usually have the Panamax where I can’t see the Volt meter which is on the same line where some stuff is plugged in when she turned on the microwave on it freaked me out. It’s been doing this for quite a while didn’t notice it before. Probably just get the microwave on a different line. 

Hello charles007100!  Normal voltages in the USA run 110 - 120 volts AC. Do you take showers when the washing machine is running? Have you felt the surge of cooler water on your delicate hide? Did you survive? Don't sweat the small stuff! A surge is much worse that a drip of 10%. If your gear has reguated power supplies, it won't even notice a small fluctuation. We had a fire that damaged the power lines. The bulb in the bathroom night light actully scorched the wall paper behind it. I measured surges of 165 volts. All my gear survived. Relax! Enjoy the music.

At the least need to get the microwave off that line..with the Pass XA-25 amp and First watt sit-4 going with the Pass pre-amp Dac and tv it maintains 119 to 118 volts…when my wife turns on the microwave and I am yelling at her don’t do it if falls to 113 to 112..

Once again there seems to be a lot of people who can't read or, and this is more likely, want to act like they know what's best and post info that isn't necessary nor asked for. Every, dang, time.

Sounds like your microwave and audio system are sharing the same circuit.   Less than ideal, but the temporary voltage drop should not cook your audio gear.   If this is prolonged, and your audio gear has a weak power supply, then it could cause premature failures of sensitive integrated circuits (chips).

Sonically, I would expect you would hear it, and that would annoy me to no end.   You could try AC regenerators, but the real solution is that both the microwave and your audio system should be on independent dedicated electrical circuits.

@charles007100  congrats on your Lucera.

Pass gear Sit-4…

Maybe reach out to First Watt.  Personally, I wouldn’t risk it until I know it’s safe

No.

Here's the thing. I recently switched my home insurance to State Farm. They sent me a free gadget called Ting. You plug it into any outlet that doesn't get much use and it monitors your whole house electrical circuit. You get real time supply voltage and it monitors your circuit for high frequency excusions that may be indicative of arching that could lead to fires. The incoming voltage swings from about 108 to 122v due to loading on the main distribution circuit. Your microwave is a noisy culprit but a dedicated circuit will not prevent voltage swings on the incoming power. These swings are not harmful to most appliances but all electronic gear is designed around a particular voltage (usually 112,115, or 120v). It will operate just fine within a certain range but unless your voltage is consistent with the design voltage of your gear, you will not be operating at optimum regarding sound. 

Have the electrician put the audio equipment on the opposite phase from the microwave , if you get a funny look from them get another electrician 

The house is 30 years old, best to get an electrician in here put the microwave on a separate line and a couple lines for the hifi gear. 
 

side note I drove 631 miles day before yesterday and brought back the Volti Lucera, holy cow it sounds so good..screw my ringing ears…

Get a couple dedicated lines installed.  It’ll solve your problem and likely improve your overall sound as well, and in the relative scheme of things it’s not all that expensive either. 

The electrical code only requires a dedicated appliance circuit if the appliance draws more than 50% of the circuits capacity.  120 x 20 equals 2400 watts.  They microwave ovens draw is what is on the name plate not “cooking watts”.  Most counter microwave ovens do not need a separate circuit by code.  
   Is the house you live in built after 1980?  

You need dedicated line. The lower voltage won't hurt things b ut the sudden surge when microwave turns off could harm things in long run. 

More troublesome are things with motors that frequently and randomly come on and off, i.e. refrigerator’s compressor, furnace’s fan, lights on/off, all making unsteady supply and noise on the line.

One thing you can do is use a unit like the Furman PFR 1800

https://furmanpower.com/products/p-1800-pfr?srsltid=AfmBOorgkUijQxvj1W3EA1RrrglykQvVOmOOQ8ny0fQJZLceBtKRYDO6

 

 

There maybe be some equipment that suffer from under-voltage situations, but thst means far less than 112.  

You may get better sound with a dedicated AC line for your audio system, or at least separated from high current drawing appliances such as a microwave.  But I don’t think the lower voltage or limited current would damage your equipment.

Put the microwave on a "Jackery" battery back-up or similar.

My microwave and frig are on the same 20 amp circuit and my Jackery (1,500 continuous/3,000 peak) can run them both.  The time you run the microwave won’t deplete it and it continually recharges from the wall.

Plus, if you ever have a power failure, you have the Jackery for back-up.

And, they are nice for camping and anytime you may want to be able to use a small appliance where there is no power available.  Even nice for things like soldering in the yard without a long extension cord trailing along.

You can even get solar panels to charge them.

If my electric falls to 112 volts am I croaking my gear until I get it resolved…Otherwise I will cut the microwave cord because my wife won’t quit using it in the meantime..definitely don’t want my Pass gear Sit-4 damaged…

A microwave should be on a separate circuit!

Microwave Oven Circuit

The microwave oven needs a dedicated 20-amp, the 120/125-volt circuit to feed it. This will require 12/2 NM wire with a ground. Microwave ovens come in different varieties and sizes. Some are countertop models, and other microwaves mount above the stove.

Although it's not uncommon to see microwave ovens plugged into standard appliance outlets, larger microwave ovens can draw as much as 1500 watts, and these need their own dedicated circuits.

In the meantime am i croaking my gear

Ya I know, have to figure out the best route to go, put microwave on its own circuit or run plugin’s for the system…

Technically the microwave should be on its own circuit. 

The micro wave is not used too often but is the long term not good?..I have a old Panamax with the tv and the modem router plugged into for convenience on the same line and has a volt meter on the face plate…it shows 120 to 122 constantly unless she turns on the microwave on for a few minutes…other wise it would be a pain to run a separate line in, there is room in my electrical panel…

‘might be putting a strain on my Dac and streamer too..