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Voltage Drop from High Current Load

04/08/2014 9:16 AM

My stepdaughter is concerned about her wiring. The dining room chandlier visibly dims when the microwave is running. She thinks this is a problem. I think it's probably just a minor annoyance.

I've also noticed this in our house. I installed a 50 amp tankless heater in the bathroom for the shower. I also installed some incandescent rope lighting along the ceiling for pleasant low level lighting. I noticed that the ropelight dims when the tankless is heating. The dimming is just barely noticeable, and lets me know the heater is working and how often. (I'll never buy Bosch tankless heaters again, but that's another story) I know the tankless is on a separate 240v circuit and should have no relation to the lighting circuit, so assume that the heater load is going back all the way to the breaker panel. (The house was completely rewired about eight years ago) (I recently replaced the ropelight and the new one isn't quite as sensitive to voltage fluctuations as the old one was.)

I don't know if my stepdaughters circuits are separate or not, but I would hope the microwave is on a dedicated circuit. The microwave is a typical 120v model mounted over the stove. I haven't done any current or voltage readings on it yet. She is planning to remodel the kitchen eventually and mentioned that she wants this "fixed" when that happens. Her house is about ten years old and seems to be consistent with modern Florida shoddy construction standards.

Any idea on whether this is a real rpoblem or not?

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#1

Re: Voltage drop from high current load

04/08/2014 9:19 AM

Not.

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#2

Re: Voltage Drop from High Current Load

04/08/2014 10:17 AM

No idea at all. It could be a problem. It could not be a problem. You should hire somebody with proper test equipment and power distribution experience (an electrician) to test the house wiring when this heater engages.

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#3

Re: Voltage Drop from High Current Load

04/08/2014 12:07 PM

Sounds like it's just loading of that leg. Not a major deal, but can be annoying.

My microwave's fan and light would power on for about one second whenever my Aero Garden would turn on its grow lights. It took a few times before we realized our house wasn't haunted. A little circuit detective work tracked down the effect.

Your tankless is probably not on a different circuit, but I don't know what country you are in. A 240VAC appliance in the US uses both legs of incoming power, so is essentially sucking juice from the whole house (like you assumed). Depending on how sensitive to incoming power your rope lights' power supply was, they were just indicating a sag better than anything else (like you thought).

Depending on if her kitchen is up to code (NEC), there are specific requirements for certain appliances, the number of receptacles per area, how many on a branch, shapes of counter tops(!), etc.

NFPA 70 (the NEC), Article 210.50 under section III, Required Outlets gives all of the details.

You can view the NEC for free (as you can most NFPA documents) here. You will need and account, but it's well worth the effort.

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#4
In reply to #3

Re: Voltage Drop from High Current Load

04/08/2014 12:15 PM

By dedicated, I meant that it has its own 240v breaker in the main panel, so it is probably causing a slight voltage drop on the incoming power upstream of the main panel.

I suspect that it is just such a relatively large current load that is regularly switching on and off(usually 3 or 4 second cycle) that it is more noticeable than other large loads.

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#5

Re: Voltage Drop from High Current Load

04/08/2014 12:34 PM

Dimming lights indicates undersized wire, a poor connection, or overloaded circuits....as long as your circuit breakers are properly sized, and in proper working condition, they should trip if a dangerous rather than just annoying situation presents itself.....The average load for a microwave is around 10-12 amps, and should be on a separate 20amp circuit, as well as the toaster oven and refrigerator....I always like to oversize the capability of the wire in reference to the circuit breaker, in this case rather than 12 single strand I would use 12 thhn, stranded higher temp wire...this allows some leeway for degradation of connection over the years...doesn't hurt to use some conductive grease as well.....You should have a minimum of 150 Amp service coming in to the house....

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#6
In reply to #5

Re: Voltage Drop from High Current Load

04/08/2014 12:57 PM

All good points.

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#7
In reply to #6

Re: Voltage Drop from High Current Load

04/08/2014 4:09 PM

Your problem has many potential sources. It could be anywhere from the outlets and lighting fixture boxes all the way out to the pole or ground mounted junction box. A bad spice on the pole would cause the voltage on your service to drop because of the resistance from the splice. Any connection from there to the circuit breaker panel would affect most everything in the house. A bad connection, splice or outlet could cause part of the house to suffer. Likewise too small of wiring could also cause it. If you have a motor or incandescent lamps on circuits or the same circuit they have very large surge currents when they start which will cause a temporary lowering of voltage, noticeable especially in lights. An example of this would be when a central a/c unit starts up. A 25 amp unit on a 30amp circuit can have as much as a 60amp or more surge current causing a very noticeable momentary dimming of lights until it gets up to speed.

The solution to the problem is to have a professional do any work entailing manipulation of wires or other potential contact with electricity tasks. What you can do is see if there is any/more matches of cause and effect within the circuits on the main panel. Some things you can do with this is change the circuits that effecting/effected units share. Is it worse? Is it better? Does the circuit breaker trip? If you have access to a clamp-on amp meter with the device to clamp onto without open access to individual wires try that on individual units and watch/listen to others.

What you have is not an unusual situation. My garage lights are on the same circuit as a 300w halogen flood shining on the driveway. The garage lights dim slightly when the flood comes on and the total of all is 700 watts on a 20amp circuit. Chances are you can find and solve the problem yourself or at least cut down on an electrician's time considerably.

Good Luck, Old Salt

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#9
In reply to #5

Re: Voltage Drop from High Current Load

04/08/2014 4:22 PM

You have mentioned all possible causes . If there is loose contact in the main service cable connection from the OH distribution to the meter or under sized cable is used for the service connection similar problem of voltage drop can happen. So needs through check up by experienced professional.

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#8

Re: Voltage Drop from High Current Load

04/08/2014 4:16 PM

Take a look at a Kill-O-Watt meter. Very handy tool for the price.

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#10
In reply to #8

Re: Voltage Drop from High Current Load

04/08/2014 4:24 PM

I have one of those. It doesn't work on high voltage and/or amp devices. I have a clamp on ammeter for that.

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#12
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Re: Voltage Drop from High Current Load

04/08/2014 10:43 PM

If your heater is on it's own breaker in the panel then you either have a loose terminal on the panel bus or an undersized drop wire from your utility.

You have a meter...measure the voltage on the panel bus while the heater is doing its thing.

Then tighten all the terminals and repeat measurement.

If the voltage drop disappears or diminishes significantly then you are good to go.

If there is no change and the drop is more than say 3% call the utility to come and have a look.

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#16
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Re: Voltage Drop from High Current Load

04/08/2014 11:26 PM

An understandable comment until one realizes that the possibly loose terminal screw maybe might be connected to the power grid with no disconnect between screw and power grid. To tighten this screw might require working hot.

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#17
In reply to #16

Re: Voltage Drop from High Current Load

04/09/2014 12:12 AM

....and your point is?

OP can protect himself by repeating this mantra..

"One flash and you are ash" repeat continuously as you meditate on the job.

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#22
In reply to #17

Re: Voltage Drop from High Current Load

04/09/2014 6:44 AM

I can assure you that I do have enough knowledge to be aware that terminal screws are potentially connected to the grid. (I was an industrial electrician several decades ago. I've forgotten a lot, but some of the more important basics have stuck in my brain so far. If I've forgotten that kind of detail, I might as well check out. Reminds me of an elderly acquaintance who was afraid of Alzheimer's. She considered putting a bottle of poison on the mantle with a note that said, "If you can't remember what these are, take them.")

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#24
In reply to #17

Re: Voltage Drop from High Current Load

04/09/2014 6:49 AM

My point is that I'm not going to recommend to somebody that is clearly not knowledgeable on how to work hot to work hot. You may not give a $#!t about human life that's not your own but I do.

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#25
In reply to #24

Re: Voltage Drop from High Current Load

04/09/2014 7:09 AM

Thanks. I do appreciate your position. If I get zapped, it's not because I don't know any better!

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#28
In reply to #25

Re: Voltage Drop from High Current Load

04/09/2014 10:31 AM

...recite the mantra.

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#29
In reply to #8

Re: Voltage Drop from High Current Load

04/09/2014 10:37 AM

LOL

You kill me sometimes

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#11

Re: Voltage Drop from High Current Load

04/08/2014 6:03 PM

According to your description, most of the voltage drop is in the utility system. The transformer is too small or it is too far away and the secondary conductors are too small. The flicker you are seeing is more of a nuisance than a problem. You should get the utility to check it out.

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#13

Re: Voltage Drop from High Current Load

04/08/2014 10:44 PM

One thing you can check is to see where that drop might be coming from.

I'd first look at the AC at the main lug/breaker before and after the microwave comes on, Major drops there suggest problems before the panel.

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#14

Re: Voltage Drop from High Current Load

04/08/2014 10:47 PM

Maybe an IR thermometer to check for poor connections?

That's all I've got.

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#15

Re: Voltage Drop from High Current Load

04/08/2014 10:59 PM

Upgrade your connection from utility.

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#18

Re: Voltage Drop from High Current Load

04/09/2014 12:41 AM

The reasons to both the problems i.e. your and your step daughter are similar but may be/ may not be the same. Over all issue seems to be Votage drooping called regulation. May be this is due to the lower incoming voltage from source it self. Might be you have low voltage issue in your area ( It is common in INDIA). It yes then that is the issue and whenever yoiu switch on heavy power drawing appliances there will be a drooping effect . The gravity of effect depends on electrical vicinity of the two circuits - affecting and addected. If that not be the case then it could be the faulty wiring or shorting of wires somewhere. The leakage is not heavy enopugh to trip the MCCB or blow the fuse but in the longrun i.e in the corse of time this may develop and result in burning of wores and tripping of MCCB/ Blowing of fuse. Get them checked. If fault is SMALL in source the problem settles after a few secas or max a minute ( TOO MUCH) OF SWITCHING ON THE heavy lpower appliance.. else it may sustain through out the period appliances is in service. In any case better get it attended - at least by measuring source input voltage to your premises.

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#19

Re: Voltage Drop from High Current Load

04/09/2014 1:43 AM

This problem exists in many countries.When applying to utility for power connection the load is normally small but later additions like water heater/AC etc increases the load for which the voltage of utility drops may be due to small cable from utility.If utility lay larger cable making room for future additions it will be ideal.

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#20
In reply to #19

Re: Voltage Drop from High Current Load

04/09/2014 2:31 AM

yeah, This could also be a cause but chances are grim... can't be ruled out. This is because of high current in and resistance of the wire/cable. with thewires generally available in market, for house hold wiring ( of standard makes) - this isuue should normally not cropup.

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#26
In reply to #20

Re: Voltage Drop from High Current Load

04/09/2014 7:38 AM

Normally the load increases on every consumer's premises,therefore sometimes even transformers & feeeders(primary & secondary) need upgrading. Utility should take measurements of V,I,pf,Hz etc regularly to study the growth of load.

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#21

Re: Voltage Drop from High Current Load

04/09/2014 3:46 AM

you have to check your main supply cable may be it is under guage /under size

the problem is voltage fluctuation at your circuit.

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#23

Re: Voltage Drop from High Current Load

04/09/2014 6:48 AM

If your stepdaughter's lamp is on a dimmer, I suggest eliminating the dimmer for a test. Old dimmer switches can cause lamp dimming. It's likely the first and simplest test you can perform.

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#27

Re: Voltage Drop from High Current Load

04/09/2014 8:58 AM

It could be a problem for any electric motor such as a HVAC unit or refrigerator.

If the voltage is dipping too low especially during the starting sequence, the motor cannot produce enough torque/HP to drive the load.

This will result in excessively high amperage and can cause severe damage to the motor(s) over a short period of time if not corrected.

Have the voltage level checked by a competent electrician to find out what you are faced with.

FYI: Normally the voltage dip is caused by the utility transformer that supplies the dwelling being too smal and/or being overloaded.

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#30

Re: Voltage Drop from High Current Load

04/09/2014 8:11 PM

hey sharpstick,

you want to be of course careful with potentially lethal line voltage, but what's wrong with measuring the voltage drop between the fuse box and the outlet?

if yer high impedance DMM is giving you spurious or irrelevant readings, try parallelling it with a 5W 3K3 resistor long enough to get a real reading (it'll get hot rapidly)

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#31
In reply to #30

Re: Voltage Drop from High Current Load

04/09/2014 8:21 PM

...or just open a hot water tap and load the line with the heater's element.

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#32

Re: Voltage Drop from High Current Load

04/10/2014 5:53 AM

Could be a good thing. When the lights brighten, you know the food is cooked.

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