Voltage Drop Under Load


Old 04-13-05, 02:18 PM
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Voltage Drop Under Load

Is it normal for the voltage to drop ~6 volts at the same outlet when a heavy load such as a toaster is turned on?

I had an local electric company worker here this morning to install a recorder on my meter to see if we could determine why my entertainment center UPS is recording numerious "blackouts" in the morning hours. The house is new construction (Feb 1). The electrician asked to borrow my toaster, plugged it in to a wall outlet and measured the voltage before and after the toaster was on. The voltage was 125V with the toaster off and 119V with it on. He said this was a problem. I tested several outlets throughout the house and they each had the same voltage drop. Also, there is a small voltage drop when the toaster is across the room.

Is this normal? If not, what could be the problem?

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Old 04-13-05, 02:38 PM
Join Date: Sep 2000
Location: United States
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Six volts out of 120 is a 5% drop. It's generally considered too much, but it's not outrageous, and 119 volts is sufficient to operate most appliances. However, the only legitimate reason for that much drop is a long run, but it only takes about 75 feet of #14 carrying 15-amps to produce this much drop (longer if your toaster uses less than 15 amps).

If the wire distance from the panel to this outlet is probably less than 75 feet, then I'd be concerned about the voltage drop coming from a poor connection somewhere. Poor connections cause heat, and heat causes fire.

If it was me, I'd start double-checking the connections on the circuit.
Old 04-13-05, 02:49 PM
Join Date: Oct 2004
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That 125 volts may not be accurate without a load.
You need a small load for the first test.
First check the voltage with, say 100 watt load, light bulb.
Then check the voltage with just the toaster.
Let us know what you get.

What's the wattage of the toaster ?
Old 04-13-05, 04:43 PM
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Thanks for the quick responses!

I've re-tested with a better meter and these are the results:

121.9 drops to 121.4 with a 75 watt light bulb.
121.9 drops to 113.1 with a 1600 watt toaster.

I know for sure that the wiring is #12. It's a 2700 Sqft two story house so, 75 ft runs are not unreasonable.

I would look for a loose connection but I can't isolate the circuit. I get the same results in different rooms.

Any suggestions?
Old 04-13-05, 05:49 PM
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Is the voltage drop only on the toaster receptacle or on other receptacles that the toaster is not plugged into?
Old 04-13-05, 05:50 PM
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Yikes, you've got 8% voltage drop. If there is too much resistance in the circuit, you will experience the voltage drop. The voltage drop will be seen under load only. #12 AWG copper wire should do the job, but really long runs should be upsized.
The national electric code has recommendations for limiting voltage drop to 5% when measured at the receptacle under load (sometimes at the service for no load, then at the receptacle under load). These are recommendations, not requirements (called fine print notes).
Not a lot of solutions, other than try to keep the heavy loads off sensitive circuits, or replace the wire with larger.
Old 04-14-05, 12:48 AM
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1600 watts / 120volts = 13.3 amp toaster
12 ga cable 75 feet long with a 13.3 amp load will produce a voltage drop of about 3.8 volts.
14 ga cable 75 feet would be about 6.3 volts.
Aluminum wire would have a higher voltage drop.

Try making your test at the closest outlet to your main panel
maybe that will eliminate the question of a large voltage drop do to the length of your wire.

I would think if you have a lose connection some lights will flicker if you use a good working vacuum cleaner ?
If your neighbors are connected to the same power lines, ask them if they have any problems.
Old 04-14-05, 06:36 AM
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Since mjourney said that he got simular results by moving the toaster to different rooms I would expect that the problem isn't a specific run inside the house. I would be approaching the problem as follows: Plug in the toaster, and measure the voltage with an accurate meter at the breaker panel. Measure the voltage on the main buss. Perhaps the problem is with a loose connection on the main lugs. Sometimes all it takes is a loose connection there to cause problems in the whole house. I've seen connections that only required a quarter or half turn to fix the problem. That would be a quick, easy check to make. If the plugged in toaster causes a significant voltage drop on the entrance breaker there could be a loose connection on the drop cable outside the house. In any event you have to bracket the problem. Find out if it's inside or outside the house, inside the breaker panel, or just with a specfic wiring run in the house. Since this is new construction I would be more inclinded to suspect a problem with loose connections inside the breaker panel that have existed since it's installation. If there's minimal voltage drop at the breaker serving the outlet that the toaster is plugged into then the trouble can be attributed to the wiring inside the house. Divide up the problem to smaller & smaller zones then tighten or inspect and I'll be you can get some answers quickly.
Old 04-14-05, 07:46 AM
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I still wonder why your UPS is kicking in. Any voltage above 110 is condidered "normal" and should not cause that. Perhaps the drop is spiking low when the toaser first turns on. You would need a recorder to see that.

Based on everything you have said, the common element in the picture seems to be the main. Perhaps loose lugs as suggested, perhaps something wrong in the meter connection, etc.

Have you measured the voltage with a lot more load than just the toaster going on. Try the toaster, plus the microwave, kick on the washing machine, maybe a hair dryer in the bathroom. Then check voltage at the panel with all these running.
Old 04-14-05, 08:02 AM
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Join Date: Feb 2005
Location: Harrisonburg
Posts: 744
I saw something like this before....kinda...anyway in the end it was a bad transformer from the pole.....

Anyway... if you get a drop like that from the toaster....you need to read the drop on the main lugs from your stove or so on.....see if you get any drop with that test......if you do get a dramatic drop I would check the main lugs on the panel and the have the power company check their connections at the meter base....

Gotta start somewhere
Old 04-14-05, 09:01 AM
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You can usually adjust your UPS to avoid annoyance trips as well. They usually come from the factory in the most sensitive setting, something like +/- 5V to trip. Many units can be set to trip at up to +/- 15V if your problem simply can't be corrected. It doesn't solve your voltage problem, but it will keep the UPS from tripping 50 times a day.

Multiple back to back trips isn't good for the UPS. As soon as the UPS trips, a substantial load is removed from the line circuit which allows the voltage to return to normal. The UPS will then sense good line power and switch back. It's an on-going cycle that is damaging to the UPS.

You said it always happens in the morning? Perhaps there is a morning-only load that is problematic; like the water heater drawing full power after the whole family takes showers in the morning?
Old 04-14-05, 11:29 AM
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Location: Dallas, Texas
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What were the results of the recording voltmeter tests? Did the voltage vary at the meter when loads (toaster, electric clothes dryer) were applied? Remember that voltage drops through the transformer, any utility company secondary conductors and the service drop in addition to the wiring in your home. If the secondary and service are lengthy, you could be losing a couple of volts (or more) between the utility primary and the meter.

During peak loads periods, utilities regulate the voltage at the substation or at a regulation point (line voltage regulators) at 125 volts with a 3 to 4 volt bandwidth (+/- 1.5 - 2 volts). At other times the midpoint is set 1 or 2 volts lower so that they are not putting out 127 volts. During peak load perions, at no load (at the customer), if you were right on top of the substation or line regulators, you could theoretically have as much as 127 volts or as little as 123 volts. Downline from this, the utility has some voltage drop in their primary conductors so that the worst case scenario at the end of the line, a transformer could have as little as 120 volts at the secondary terminals and the could have as little as 114 volts at the service entrance. ANSI standards states that the nominal residential service voltage is 120 volts, the minimum utilization voltage is 108 volts, the favorable service voltage is 114-126 volts and the "tolerable" service voltage is 110-127 volts.

Last edited by txdiyguy; 04-14-05 at 04:58 PM. Reason: Bandwidth clarification/correction.
Old 04-14-05, 09:24 PM
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Thanks for all the great suggestions!

I will try and isolate the problem this weekend using the methods suggested and will post the results...

Thanks again!

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