lights dim when ising blow dryer

[jkddel]

when a customer plugs in a blow dryer & turns it on, the lights dim in the same room. I checked all breakers, they read 120v. I checked all wires to breakers, they are secure. what could be the cause of this issue.

thank you

 

[kwired]

when a customer plugs in a blow dryer & turns it on, the lights dim in the same room. I checked all breakers, they read 120v. I checked all wires to breakers, they are secure. what could be the cause of this issue.

thank you

Sounds like normal voltage drop on the branch circuit conductors to me, will typically be worse if the circuit is 14 AWG conductors then if it is 12 AWG, but I expect to see some effect with either, the longer the circuit length the more effect there will be.

 

[Fulthrotl]

when a customer plugs in a blow dryer & turns it on, the lights dim in the same room. I checked all breakers, they read 120v. I checked all wires to breakers, they are secure. what could be the cause of this issue.

thank you

voltage drop. an 1,800 watt blow dryer is 2 1/2 horsepower's worth of
power.

when you turn on an electric motor, the lights will flicker
and then come back up to full brightness usually. a motors
starting spike is up to ten times the running power.

but a resistive load will be a constant dimming influence
as long as it is on. incandescent lamps are most strongly
affected by this. LED luminaries not nearly as much.

 

[growler]

when a customer plugs in a blow dryer & turns it on, the lights dim in the same room. I checked all breakers, they read 120v. I checked all wires to breakers, they are secure. what could be the cause of this issue.

thank you

Was the blow dryer on when you checked voltage at breakers?

Make sure which circuit you are dealing with and then pull that breaker and check for signs of heating or arcing where it plugs onto the buss. Hook that circuit up at a different location and see if the light still dims.

 

[growler]

Sounds like normal voltage drop on the branch circuit conductors to me.

It may be a normal flicker or it may be more. It's really hard to tell how much dimming he's talking about.

I had one like this last summer and it turned out to be a bad service neutral. The only lights dimming that the owner had noticed was in the bathroom but that's not what I measured with the meter.

Need to know what type of load was present when he measured that 120 volts.

 

[peter d]

Before the 1996 NEC, it was common to have one 15 amp circuit that served all the bathroom receptacles. This could cause easily cause voltage drop that is noticeable.

 

[gar]

170529-1756 EDT

jkddel:

Many light sources have an output intensity that is fairly dependent upon lamp voltage. Plots of this intensity variation for some bulbs and different excitations are shown in my photos P9 thru P18 at http://www.beta-a2.com/EE-photos.html .

Many electrical circuits can be simplified to a constant voltage source V (means it does not change with load current changes) with some impedance Zint (impedance internal to the voltage source), and a series connected load of Zload.

For simplicity for your problem consider said simple circuit, and assume the impedances to be resistors.

View the real world in this simplified fashion. At your pole transformer we can come close to assuming the input voltage to that transformer is constant for short times and barely changes when you apply your load within the home. Thus, assume zero source impedance supplying the pole transformer. The transformer and the wires to your main panel, including the meter, will have an internal impedance that is measurable. In the main panel will be a main breaker, bus bars, contact resistance, and a breaker to your circuit. Following this is the wiring to the destination load.

In my home I ran some tests. My source transformer is 50 kVA, and it only supplies two homes, and two street lights. Wimpy wires from the transformer part way to my main panel, heavier the rest of the way. The main panel is a 200 A QO with main fuses. The circuits I made measurements on are 120 V from a 20 A QO, and a 15 A QO breakers. The long branch circuit wire is #14 copper.

Measurements were with a Kill-A-Watt EZ on a 6' #12 extension cord. Test load was a 1500 W portable heater. Test light was a 100 W 120 V standard incandescent. The light was plugged into 1/2 of a duplex receptacle, and the meter and heater into the other half.

Tests I have made in the past indicate that I can detect flicker (short pulse) from an incandescent with less than 1 V change. With the nature of this test I was not as sensitive.

A load change of 12 A produced about 0.1 V change across the QO 20, or a QO resistance of about 0.01 ohm. This includes some bus bar resistance and the contact resistance from the QO to the bus bar. But most is internal to the QO in its heating element.

At an outlet at the main panel my heater produced a load current of 12.6 A. Voltage changed from 122.8 to 121.0 or 1.8 V. Thus, Rint at this location, includes the transformer and everything from there thru the main panel and the branch circuit breaker, is 1.8/12.6 = 0.14 ohms. Consider how bad the voltage drop would be at 200 A.

You have to wait a short time for the heater resistance to stablized. I did not clock it but possibly 30 seconds. During this wait time you hope the power company source voltage does not change, and no other loads change in the home. Repeated tests and looking at voltage before and after turning on the heater allows you to make a judgement on whether or not the power company voltage to the pole transformer primary was stable during the test, and that you had no other load variations.

About 15 minutes later. Note: power company voltage will likely change by possibly 1 V in this time frame.

On an outlet at the end of a branch circuit of #14 copper about 40' long (loop length 80 ft, possibly longer) the measurements were: my heater produced a load current of 12.02 A. Voltage changed from 123.1 to 118.8 or 4.3 V. Thus, Rint at this location, includes the transformer and everything from there thru the main panel and the branch circuit to this location, is 4.3/12.02 = 0.36 ohms.

For #14 copper wire at 80 ft I would calculate about 0.20 ohms. To this wire resistance we have to add the impedance measured at the main panel. 0.20 + 0.14 = 0.34 ohms total.

This may or not be the same phase as the original main panel measurement, but both of my phases are very close to each other in voltage and impedance. Actually a very good correlation considering the errors that are possible when making difference mrasurements like this.

For any tests you run use a meter that resolve 0.1 V at 120 V.

What you can do is put two voltmeters at the main panel, one from each phase to neutral. If two meters are not available, then you have to work with one, and live with non-simultaneous readings between the two phases. Read values before applying the hair blower load, apply the load, measure the current if possible, with the load on and stablized , then read both meters. Remove the load and reread the meters.

The meter on the phase with the load will drop X V (change in reading), and the other should increase by about X/2.

At yout light that dims measure the change in voltage resulting from the load.

Report back.

.