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Why does running hairdryer increase voltage on other circuits?

Why does running hairdryer increase voltage on other circuits?

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Old 08-26-19, 05:51 AM
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Why does running hairdryer increase voltage on other circuits?

I have a guest bathroom next to my office in my house. I am pretty sure both are on the same circuit. Whenever a hairdryer is plugged in and switched on in the guest bedroom I notice that my APC UPS in my office is getting into a non-normal mode. I thought that the voltage is dropping and it gets into battery mode. However, I was wrong. It actually switches to trim mode. I measured that the voltage (with a plugin meter) actually increases by 2V from ~ 123V to 125V.
Why would the voltage go up by that much if a hairdryer is plugged in?
 
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Old 08-26-19, 06:11 AM
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In a residential split-phase service, you have incoming 240V that is split into two 120V halves by the neutral wire. The sum of the voltages on either side of the neutral remains constant. What is happening in your case is that the voltage on the side of the service with the hair dryer is dropping 2V and the other side of the service is increasing by 2V.

I consider 2V within normal limits, but if the voltage change is excessive (enough to visibly brighten lights) this is an indicator of a major problem with the service neutral.
 
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Old 08-26-19, 06:12 AM
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Classic symptom of a loose neutral on your main supply.
 
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Old 08-26-19, 06:44 AM
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I am pretty sure that the bathroom and my office are on the same circuit (breaker). Of course, I will triple check. Also, my socket tester doesn't indicate an issue.
 
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Old 08-26-19, 07:16 AM
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I am pretty sure that the bathroom and my office are on the same circuit (breaker).
The data you produce doesn't support that both loads are on the same circuit. 2V rise (or drop) with a 12A load corresponds to a 0.17 Ohm neutral wire. That doesn't sound like a defect to me.
 
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Old 08-26-19, 11:10 AM
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I will check whether both outlets are on the same circuit.

Another check I did: plugged in powermeter at bathroom location. Hair dryer plugged into it.
Switched off: 124V Switched on 117V
 
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Old 08-26-19, 11:21 AM
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That's all within normal limits. See if your UPS has a sensitivity setting. They sometimes ship with it set to high which causes lots of unnecessary transfers to battery. Medium or low should be just fine in a residential application.
 
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Old 08-26-19, 12:20 PM
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The voltage should never go UP on a circuit. The only cause of that is a loose neutral.

Here is a demonstration.

https://youtu.be/HsSlQnGCvdg
 
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Old 08-26-19, 12:32 PM
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Adding additional 120V load on leg A, will ALWAYS increase the voltage on leg B. There is always some neutral R.

Up until a breaker trips, anyway.
 
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Old 08-26-19, 06:58 PM
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An update. The bathroom and my office are not all on the same circuit. The outlets shared with the bathroom wall are on the same circuit. However, the outlet where UPS is plugged in is on a different circuit.
The circuit breakers in the panel are on top of each other for the two circuits. Therefore, I am concluding on two different phases.

Based in this information, is the voltage drop on my UPS outlet considered normal?

Hairdryer outlet: 124V to 116.5V when on
UPS outlet (without UPS plugged in): 123V 126V
UPS outlet (with UPS plugged in): 121.6V 125.4V
(voltages fluctuate slightly; i.e. assuming what UPS draws can vary). Also, voltages appear to be now (at night) slightly lower. In the morning tend to be higher. My UPS doesn't go to AVR trim while I am testing now.

My UPS is set for 130V transfer. So, I am not sure why it at any point started to trim. Looking at the data log, the highest it sensed was 128.1V. Also reading a bit higher than my voltmeter.
 
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Old 08-26-19, 07:02 PM
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Since the breakers are one above the other..... more than likely you have a MWBC. Multi wire branch circuit. That's two circuits sharing one neutral. It will respond exactly like you describe. There is no problem there.

You can confirm MWBC by checking the two breakers. One should have a black wire and the other a red wire.
 
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Old 08-26-19, 07:04 PM
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All your reading are normal. Your homes voltage will also vary at different times of the day based on the load in your neighborhood. Your neighbors are all part of the same electrical grid and/or transformer.
 
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Old 08-26-19, 07:28 PM
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While the voltage drop/gain is not fully symmetrical I guess this explains it.

If a perfectionist upgraded the bathroom wiring from 15A to 20A, woudl he he see less of a gain/drop?

Next I need to find out why my APC UPS transfers below 130V.
 
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Old 08-26-19, 07:39 PM
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A 20A line would have less voltage drop but there would still be some.
 
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Old 08-27-19, 06:15 AM
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What you're seeing is the difference between textbook and reality. Every connection has a little resistance, wire isn't made 100% consistent, the grid fluctuates, meters have different calibration, etc. It's a hundred fractions here and there which adds up to a couple volts variation at the end of a circuit. Nothing to worry about. Consumer goods easily tolerate 10%, you're at well less than 5%.
 
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Old 08-27-19, 06:20 AM
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It looks like that not every circuit has a dedicated neutral. Just looking at the panel I am assuming that there is likely a shared neutral for some of the breakers in question. Possibly even for circuits on different phases.
Is this code?
 
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Old 08-27-19, 06:37 AM
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Yes, it's called a multiwire branch circuit (MWBC). In fact the two hot legs must be on different phases. Today's code requires a handle tie between the two breakers of the MWBC, but it was legal in the past to have two completely separate breakers. They are also less common now because AFCI and GFCI protection are more complicated on this type of circuit.
 
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Old 08-27-19, 11:14 AM
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However, code does not require to run a dedicated neutral with both of the circuits (on two phases)?
 
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Old 08-27-19, 11:53 AM
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When the hots are on different phases, they can share a neutral because the neutral current cancels. The current on the neutral is equal to the absolute value of the difference of current in the hots, which means it will not exceed the largest hot.

If the hots were on the same phase (not allowed), the shared neutral current is additive and could end up as twice the breaker value which is an unsafe overload.
 
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Old 08-27-19, 06:48 PM
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Added note: Your home is fed with a multiwire circuit from the power companies transformer. The two hots are sharing one neutral.
 
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Old 08-27-19, 06:57 PM
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Thanks. When I get around I will open the panel and inspect the "pairs" neutral/hots feeding into which conduit to make sure no neutral is used unbalanced.
 
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