lights flickering revisited

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deckscrew

Member
A few weeks ago I asked for advise regarding lights dimming or brightening when the garbage disposal was activated. The thread was shut down. Perhaps the moderator thought I was a DIYer trying to fix his problem.

Anyway, I'm a GC in the SF Bay Area and I recently completed an addition and remodel. We upgraded the service. We have had this problem with the lights since the owner moved in. My electrician could not solve the problem.

I called PG&E and they put some kind of meter on the panel and found different voltages on the two incoming hot wires (a 6 volt difference). The PG&E rep said it was probably some loose wiring on their part, and that he would fix the problem. The problem still remained. I called PG&E again and met the rep at the site.

He told me that there were 3 buildings (1 single unit and 2 duplexes) serviced by a 10 kva transformer and that this should be enough to power these buildings. However, when he measured the voltage at each connection he told me that there was 120 volts at the transformer, but there was a voltage drop at each connection to the house. With a 14 volt drop at our connection.

I asked him if this could cause the lights to flicker, but he was evasive and said he have to take more readings and send the information to their engineer. Its been more than a week and PG&E still has not responded.

My question: can this voltage drop cause the lights to flicker? I'm not trying to fix the problem myself, but would like to become educated so I can get the parties who can fix this problem ( my electrician or PG&E) to fix this problem.

Thanks
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
V drop limits

V drop limits

I say absolutely YES. That's a 14/120V drop = 11.7% of a nominal 120V base.
NEC advises a maximum of 5% drop at the final loads, and your starting off at 12% with no room for drop associated with any inrush - as would be caused by a 3/4-1 HP disposal.


.. edited to add: the lights would flicker when the ballast voltage dips below tolerance during disposal startup


JM
 
Last edited:

basicbill

Member
Location
Western Canada
As a casual reader and from a purely theoretical view, a 10,000 VA transformer feeding three dwelling units? (or does two duplexes actually make it five?).

10,000 / 240 = 41.6 amps

for 3 (or 5) dwelling units?

I don't work for a utility but this seems a little under served.
Is this a normal application for a 10kVA transformer?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090123-1756 EST

deckscrew:

I believe in your thread that you say was removed I had asked for some quantitative data. There was no response.

Is this current question about that same location?

There is a logical approach to determine the cause of your problem. It may not be a solvable without changes that may be beyond your control.

A 10 KVA pole transformer would seem to be small. 10 KVA at 240 V is 42 A. Suppose the transformer impedance is 5%, then a 120 V open circuit source will drop to 120 * 0.95 at 42 A load. Or 120 V drops to 114 V at a 42 A load.

This does not include voltage drop in the wiring. How much inrush current does the garbage disposal require? Assume 1000 VA running at full load that is 8.3 A. Next assume inrush is 7 times this or 58 A. Then we should expect at least 8 V drop.

An experiment with a 75 W incandescent lamp. Voltage to lamp before adding a load is 122 V, with load 116 V, a 6 V change. Noticeable dimming of lamp. I consider the amount as too much.

Change to a CFL and there is no noticeable flicker from the same load at these same voltages.

.
 

deckscrew

Member
Yes, this is the same location. My electrician rewired the upper unit. The lower unit remained the same except for one added circuit. When the disposer is turned on the lights in both units flicker. As well as the lights in the refrigerator and the oven.

The GD is a 3/4 HP, 8.1 Amp unit (Insinkerator Evolution Compact)
on a dedicated 15 amp circuit (split circuit also for D/W) . I'm not sure what the inrush is.

My reason for asking so I can understand what might be causing the problem. As usually no one is going to voluntarily take responsibility, and as the GC I have to deal with both sides.

Thanks


.
 

deckscrew

Member
" by flickering do you mean slightly dimming or shutting off completly"


The lights quickly dim for a split second when the disposer is turned on
 

legato

Member
Location
Mountain View, CA
Occupation
Consulting Electrical Engineer
Was a load calc done on this? I'm surprised this thing was completed the way it is...Remodel or not, I'd want to make sure there's enough power for everyone before adding new appliances.

10kVA xfmr for 3 dwelling units does not sound right...What happens if the D/W, GD, and oven are all on...
 

electricalperson

Senior Member
Location
massachusetts
" by flickering do you mean slightly dimming or shutting off completly"


The lights quickly dim for a split second when the disposer is turned on

well if its dimming the way i think it is, it sounds like typical voltage drop caused by the motors inrush. have the electrician putting the lights on a different phase if its really bad. have him check connections in fixtures, boxes, panel and meter socket for good measures.

he can put the meter on the panel phase and and turn the disposal on to measure voltage drop down there with the min/max function
 

macmikeman

Senior Member
Was a load calc done on this? I'm surprised this thing was completed the way it is...Remodel or not, I'd want to make sure there's enough power for everyone before adding new appliances.

10kVA xfmr for 3 dwelling units does not sound right...What happens if the D/W, GD, and oven are all on...

I have had my 3/0 cu conductors out of the weatherhead connected to poco #4 al conductors before. I did my load calcs. It took a lot of complaining on the part of the homeowner before somebody down at the poco did theirs.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090123-2011 EST

deckscrew:

It seems my assumption of full load running current was close. I then estimated the startup inrush of 7 times this full load value.

Just the small size of the pole transformer and a reasonable estimate of internal impedance gives a voltage drop without any wire drop in a range that will definitely cause light flicker.

In this house pick an arbitrary location which is likely to be furthest from the main panel. Plug an incandescent lamp into that outlet. Get a 1500 W heater and plug into the same duplex outlet. You will see the light intensity drop. A 1500 W heater will be about 12 to 13 A.

I have a much stiffer system at home than what you are describing. A larger pole transformer, and lower resistance wiring. I do get a small noticeable flicker of my 75 W test lamp from starting my disposal, but not objectionable. On a circuit on the opposite phase there is nothing noticeable.

You can have your electrician run tests using a test incandescent lamp as the test tool because it is sensitive to small voltage changes. In particular, I believe I previously mentioned that you should connect two test bulbs at the main panel. One on each side of neutral. These should connect directly to the neutral bus, and the other sides should connect to the output sides of two breakers, and neither of these breakers should be the disposal circuit.

If the neutral from and including the neutral bus to the pole transformer center tap is good, then when you start the disposal one light should flicker, and the other should not be affected.

If one brightens and the other one dims, then you have a neutral problem somewhere between the neutral bus where the test lamps are connected and the pole transformer center tap. A very minor brightening may not imply a problem because we expect some voltage drop on the neutral from the transformer to the panel. On my home system at the main panel a 10 A load on one side of neutral does not affect the other side by more than 1/10 V if I remember correctly. My service drop wiring is very low resistance. My transformer has a much higher impedance than the service wires.

.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Gar, thanks for your logical approach and all other posts to this thread. Instead of all of this testing I want to cut to the chase. It appears the supply xfmr is large enough. If
no other units are experiencing the flickering then the problem is unique to this unit. It
sounds like a loose connection during construction (problem has always been there.)
I would shut down power at the incoming metering panel, follow the run and tighten all wiring connections in all splice boxes and subpanels down to the disposal outlet. If this doesn't find it then arrange a utility outage and check the incomming connections to the metering panel.
 

Karl H

Senior Member
Location
San Diego,CA
If the disposal circuit is on A phase and the lighting is also on A phase.

I would put either the disposal circuit or the lighting circuit on B phase.

BTW I also use a 1500w space heater to check for VD. My little space

heater has exposed a lot of VD problems that was created by an EC

that was too lazy to do the math or was too busy counting all the money

he would save by undersizing conductors.

Listen to Gar, he gives you a lot of his time and education for free.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090124-1651 EST

tonytonon:

I do not believe deckscrew has any significant electrical training. He is trying to get information to guide his electrician who appears to lack troubleshooting experience. This is sort of a roundabout path.

There are times when your method of checking all joints is a good approach. On the other hand in many cases it is important to identify the particular cause. In this case it might well be the neutral connection at the transformer center tap, and a more direct determination would be with measurements.

The transformer may be of adequate size for the average load, but not sufficient to prevent light flicker. I previously did a calculation in an earlier post with some assumptions on what might be the voltage drop from a 10 KVA transformer's internal impedance with the motor starting current. I also determined whether that might produce noticeable flicker, and it would.

I think in this case measurements will be the most productive approach.

My method of two light bulbs to evaluate a neutral problem has the advantage of being inexpensive and allows for simultaneous observation of the two voltages. If this seems to imply a neutral or hot line problem, then voltmeter measurements would be useful. You can start with voltmeter measurements if you desire and that may be the easiest. If you only have one voltmeter, then you can not make simultaneous measurements. Even with two voltmeters it is hard to read both simultaneously. It is a tossup of what to do.

At our shop we had a problem for a number of years that was quite random. This is a three phase open delta system. The problem might not occur for months. Typically we momentarily lost a phase. At least a sufficient voltage change to cause some of the CNC machines to drop out. At one point the power company, DTE, put a monitor on the line at the meter. This caught nothing. Later the problems increased, but still low probability. One day I noticed the lights dim and a portable fan speed up. We got the power company out to look for the problem. It was getting near dusk and one truck was there and he was sitting in the cab and saw arcing at the transformer terminations. Problem detected and solved. Had the power company monitor been on longer it would have caught the problem, but they were/are reluctant to leave it there for months at a time.

If I gave you a 5 tube radio to fix that had no sound output, but all tube filaments were lit, then would you first change every tube, or would you probe and logically test different parts of the radio to look for the cause? Also note that most ACDC radios had the filaments in series so if one burned out, then none would glow. In this case would you pull each tube out and test it, or would you make measurements on the circuit to look for the open point. It could be the on-off switch and not a tube.

.
 

iwire

Moderator
Staff member
Location
Massachusetts
FWIW many power comapinies will come out and do a load test at the meter if you just call and ask them. They pull the meter and replace it with a device that can load test the supply in just a few seconds. It is kind of like an "Ideal Suretest" for meter sockets.
 

deckscrew

Member
FWIW many power comapinies will come out and do a load test at the meter if you just call and ask them. They pull the meter and replace it with a device that can load test the supply in just a few seconds. It is kind of like an "Ideal Suretest" for meter sockets.

The Power Company did come out and pulled the meter and ran a test. He also checked for any loose wiring in the service panel. The test showed a difference of 6 volts between the two incoming hot wires. Furthermore when he tested the voltage at the tap at the power pole he told me there was a 14 volt drop at our tap. I'm assuming that means we are getting
106 volts at our power connection. Can this be OK?

I had my electrician out there and he checked all his connections and he said everything looked good. I have to trust him he is the electrical contractor.

The lower unit's lights also flicker when the upstairs GD is turned on. We did not touch the wiring in the lower unit. All the lights in all the units flicker when the upstairs GD is turned on, even the oven and refrigerator lights in both units.

Thank you Gar for your testing ideas. I will try them on Monday
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090125-1542 EST

deckscrew:

The information from the power company is inadequate. Voltage changes do not mean much without knowing the kind of current changes that occurred to cause the voltage changes, and to what part of the network are the currents associated.

How far is the transformer from the point where your service drop is connected to the wires from the transformer? These distribution wires from the transformer are apparently servicing other customers? Are you close to the transformer or far away compared to the other customers on this transformer?

What is the length of your service drop?

How many different customers are on this one transformer?

If you select a customer that is further from the transformer than you and operate their disposal, then does this cause your lights to flicker?

When you say all lights in the house flicker when the disposal is turned on, then does the word flicker also mean that some lights might momentarily brighten?

Does your electrician have a clamp-on ammeter?

Six volts difference between the magnitude of one phase vs the other at your main panel seems excessive. At this moment whatever my neighbor's and my residual loads and balance are I am only seeing a few tenths of a volt difference. However, if I were to add an unbalanced load of 12 A, as I did last night, then I might see a 1 V difference. By the way, when I put a 12 A load on one phase there is negligible change in voltage on the other phase, maybe 0.1 V.

It is possible there is a large unbalanced load on your transformer and this could account for the voltage difference between the two phases.

On the subject of the 14 V drop at your tap comment: What were the conditions when this was measured? Was it with no load from your service lines? What was this 14 V drop from --- A voltage at the transformer? Or if not where and under what conditions? If without any load from your service lines the 14 V drop exists, then it results from transformer impedance and the impedance of the wires from the transformer to your tap and your neighbors loads both before and after your tap.

What voltage change occurs at your tap when your disposal is started?

Do other customers further away from the transformer than you see their lights flicker when your disposal turns on? If there is high impedance from the transformer and the distribution lines to the various customers, then this should occur.

Without any loads on in your rentals how much do the voltages change on the two phases? If there is truely no load from you, then these changes are a function of the utility supply voltage stability, and varying loads from your neighbors.

In the last 10 minutes or so one of my phases has stayed within 0.8 V. Turning my disposal on caused less than 1.4 V change at the main panel.

.
 
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