I live in a rural, small cabin (aprox. 50 yrs. Old) on a co-op electrical grid. The main panel with circuit breakers was recently installed (before I got here, 100 amp service) but the wiring was not replaced. It's the old two-wire (no bare copper ground).

When I run certain circuits (220 well pump, microwave (20a)) I noticed that the ceiling light in the kitchen will "dim". As a test I turned off everything in the cabin (except the refridgerator) and plugged a voltmeter into a kitchen outlet. It read ~126v. If I turn on the well pump or the microwave the voltage will drop to 115-120v.

That explains the light dimming but I've never seen voltage drops in previous residences before.

Is this normal? Or a symptom of a problem? Anyone seen anything like this? Anything I can test or do?

Thanks,
David...

This is going to be difficult to explain to a lay person.

Voltage drop is a result of long or small wires ,and the load placed on the wiring.

The 126 volts tells me the utility apparently try to plan for voltage drop by raising the volts to the maximum allowed for 120 volts, 120 V + 10% is 126 volts.

However this does not change the wire size and length, of course the load.

The recommended allowable voltage drop at the end of a branch circuit is 5 % of nominal voltage, as per the National Electric Code. So 120 volts - 5% is 114 volts.

Can you explain:
100 amp service) but the wiring was not replaced. its the old two-wire (no bare copper ground).
Does this mean you have a new 120/240 volt panel, but only two wires feeding it? What is your impression of "bare ground"?

This could be a symptom of small wires feeding the service, or long wires or both. Could also mean there is a loose or poor connection someplace.

Testing or troubleshooting this will require a great deal of knowledge of electrical systems, including several formulas of Ohm's Law, and voltage drop calculations.

An accurate formula for Vd is:

Voltage Drop= 2 x Length of circuit x Resistance of wire per 1000 foot x amp load / 1000.

Not really sure what your abilities are and what you can test, more of collecting the various values and performing the calcs to determine the size wire required for the load and length.

There are several free Vd calculators on the net, enter certain values to determine the drop in volts for a certain load , volts, and length of wire, and material of wire.

The utility is required to deliver a certain nominal voltage, usually +5% to -3% to the meter. You can requestr the utility to come out to test the volts delivered. What they do is remove the meter, and insert a 1500 watt load and measure the volts. They may need to increase the size of their wires due to the small size, length, material, and age, and also may have poor or loose connection in their system.

Hope this helps some, again not sure what you can do besides calling in the utility. All wire and conncetion on your side is your responsibility, so you can have the same conditions contributing to the situation.

thank you for the quick reply.

this is a very small cabin. 28ft x 28ft, single story, one bed, one bath (in other words, no single circuit in the place could be over 50ft.) the new 120/240 panel has a 240/100amp service disconnect, 5 15amp circuits (basically one per room, 2 in the living room), 2 separate 20amp breakers in the kitchen one used for the microwave the other a washing machine , a 240/50amp for the stove, 240/30 amp for the well pump, and a 240/30amp for the water heater.

the "two-wire" reference I made was about the actual wiring that runs through the cabin. The wire I buy today internally contains 3 wires (black, white & bare copper). The wire that runs through my cabin only has 2 wires (black & white). I assume this is due to the age of the cabin and when the wiring was done. All of my circuits have an open ground.

I am way out in the sticks, so its probably due to the "long" wires required to get power here. I don't mind the power dip as long as the cabin doesn't burst into flames because of it. But from what you have told me it sounds like this is just another "advantage" to living in the country.

thank you for your help.
david...