Lights dimming when dishwasher cycles

angus1

Member
Location
Boston
Having an issue with recessed lights dimming when dishwasher cycles
Kitchen wired by others
Lights on separate 15 amp circuit
Dishwasher on separate 20 amp circuit
Just did service change as part of basement renovation
All connections are tight
Any thoughts?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Try swapping either circuit to other phase, even if temporarily.

If the lights now get brighter, you have a service neutral issue.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Is the dimming happening just from the pump motor running (and/or starting) or us it related to the fairly high current drain of the heating element?

Sent from my Pixel 4a using Tapatalk
 

tom baker

First Chief Moderator
Staff member
Check the open circuit voltage, then use a VM - preferably one that has a min max function, check the voltage when the DW is starting/running.
Measure at the service panel and at the DW. Then as pointed out, it may be a POCO isssue
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The first thing you need to determine is if you have a true voltage drop issue.

In the old days, if you saw an incandescent light change brightness then you knew the voltage changed.

With dimmable LEDs, very small voltage fluctuations can trigger the dimming circuitry and cause much larger brightness fluctuations than the actual voltage change should cause.

Plug an incandescent lamp into the circuit and see how much flicker you get.

Remember that you will always have _some_ voltage drop when you have current flow (and if the voltage drop is on the neutral, then you will see voltage _rise_ on the opposite leg of a split phase service). What you need to determine is if you have an unexpectedly high voltage drop.

Others above have described how to measure and localize the source of the voltage drop. You need to compare the measured voltage drop to the voltage drop anticipated from things like service conductor gauge and length.

There is a good chance that if you have significant voltage drop, that you will find a loose connection somewhere.

-Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
There is a good chance....

Possible sources of excessive voltage drop are:

POCO transformer with high impedance

Loose connections at the POCO transformer.

Long or undersized service drop conductors.

Loose connections at the customer side.

-Jon
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Master Electrician - 2017 NEC
There is a good chance....

Possible sources of excessive voltage drop are:

POCO transformer with high impedance

Loose connections at the POCO transformer.

Long or undersized service drop conductors.

Loose connections at the customer side.

-Jon
Ill tack on - Partially compromised neutral service lateral. ;)
 

Flicker Index

Member
Location
Pac NW
Occupation
Lights
What's the voltage at the lighting circuit with, and without the dishwasher on? The heater on dishwasher is around 5A. If you have a stiff feeder, the effect on the voltage at the panel should be almost too small to measure, but in the rural area fed from a small transformer with a high impedance, this isn't necessarily the case.

The first thing you need to determine is if you have a true voltage drop issue.

In the old days, if you saw an incandescent light change brightness then you knew the voltage changed.

With dimmable LEDs, very small voltage fluctuations can trigger the dimming circuitry and cause much larger brightness fluctuations than the actual voltage change should cause.

Plug an incandescent lamp into the circuit and see how much flicker you get.
-Jon
I second this. You can probably recall a nearby speaker picking up spike and making a popping sound when you turn something on/off nearby. Such interference can travel into different circuits and can disrupt the peaceful state between LED ballast and dimmer. The severity of reaction and resistance to such impulse is greatly dependent on each dimmer and ballast. For example, first generation CREE LED bulb was very susceptible to spike that sometimes a spike from turning off the bathroom/laundry room fan would cause the lamp to turn off and back on. This wasn't due to a voltage drop. It was the bulb's ballast acting up due to poor design.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I think I've been spoiled by our almost constant 122vac PoCo service. :)
They've also cut down so many branches that I'm no longer shopping for a gen but it does uglify the neighborhood.
I’m lucky in that aspect as well. My closest residential neighbor is fed from a different direction and mine taps from POCO primary upgraded for summertime irrigation loads. There is no VD problem at my house.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
How is that possible?
A high resistance at the power pole?
Undersized/overloaded transformer, excessive voltage drop on the service drop or lateral, and maybe some other line side issues, such as what you suggested.
The 50kVA in front of my house feeds 10 houses.
 

gar

Senior Member
210109-1338 EST

Not all of my post was entered. This is a re-save.

The problem here should be relatively easy to troubleshoot, but you need a reasonable understanding of electrical circuit theory.

For flickering of an incandescent bulb to be detected by an individual you need about at least a 2 V change in the voltage at the bulb. Under signal known exactly conditions it can probably be much less, and if you are not paying attention to the bulb it may be more.

How do you troubleshoot a light flicker problem generated by some load change?

First --- use a fast response min-max meter at the bulb to monitor the change in voltage. Fast response probably means no longer than 1/10 second. Knowing this change in voltage is useful when checking change in voltage at other places in the system.

Also it may be useful to use your own test load, such as a 1500 W heater, about 12 A, to generate a load change.

Second --- you need to understand the system you are working on. This starts with the power company distribution transformer, and an assumption that it is driven by a zero impedance constant voltage primary supply. Which for most purposes is normally a reasonable assumption.

We assume the transformer has a single coil primary, and a center tapped secondary coil. Under no load conditions the two halves should be of almost identical voltage.

Internal impedance of the transformer may be approximated as some in the primary, and the remainder in the secondaries. The primary reflected impedance might be on the order of 1/4 to 1/2 of the impedance of either half of the secondary. This means that a change in load on 1/2 of the secondary produces less voltage change in the other secondary voltage than in the loaded half of the secondary.

Then there is impedance in the service wires from the pole transformer to and thru your meter to the main lugs in your main panel.

In general you can only make current and voltage measurements at and beyond these main lugs.

If all the service wires are of the same size and material, then the voltage drop on any one of these loaded wires is the same for a given amount of load current on 1/2 of the secondary.

If voltage measurements are made at the main input lugs of the main panel, then the only additional voltage drop on the hot wires is from the internal impedance of the power meter. If the measurements are made after breakers, then the hot circuit has the additional drop of the bus and breakers.

Some measurements at my main panel. These measurements were made at outlets near the main panel . Thus, after breakers.

Phase A is the phase being loaded with a 1500 W space heater. Approximate load current 12 A. Voltage changes measured are A to N, B to N, and A to B.

Phase A to Neutral is -1.2 V.
Phase B to Neutral is +0.3 V
Phase A to B is -0.6 V

There is no current change on Phase B, thus the expected rise in voltage resulting from the voltage drop on the Neutral from the Phase A load current on the Neutral conductor. If you do not understand the rise on the unloaded phase, then draw the circuit and study it.

The voltage drop on the hot line of phase A is more like about 1.2 - 0.3 = 0.9 V. This is not too far from the 0.6 V change on the Phase A to B measurement.

Using a 15 W incandescent bulb as a voltage measuring device, flicker detection, I saw no obvious flicker from my 1500 watt load change on the same duplex outlet.

Getting late now. More later.

..
 
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