Voltage Increases when I add a load

Question

I live in a rural area of Costa Rica. In my rented house I like to use electrical devices in the kitchen, but I always get a large voltage drop when I energize one.

For instance, if I turn on my 1400 watt air fryer the voltage drops from 118v to 105v. If I add a second device, say a 1000 watt toaster oven it drops to 94volts.

In trying to track down this problem I did some checking. The transformer on the high voltage tower is 600 feet away from my house, and the wires are #4, which I had assumed was the problem. The wires from the breaker box to the kitchen are also undersized for a 20 amp circuit, being #14.

Now here is the strange thing and my question: If I turn on my air popper, I get the same drop from 118v to 105v. However if I turn on my electric shower head in the bathroom (another resistance device) at the same time, which draws from it's own 20 amp dedicated circuit, the voltage in the kitchen circuit goes up to 115v. WHY?

Answer 1

Costa Rica uses El NEC. It's the American code, but in Spanish... and probably a few editions behind (heck every US state is at least 1 edition behind - NEC started piling on very costly requirements starting in 2014, and states are dragging their feet more than ever.)

Wire break? Call your power company right now and report an outage

This is a standard failure mode in North American split-phase power. The overhead wire whips in the wind for 30 years, the carrier/neutral takes the brunt of it, and aluminum has no fatigue limit. (nor does copper). This service drop wire is the power company's responsibility, so they should fix it fast, for free. Assuming they aren't overwhelmed with repairing damage from an ice storm :) Ours came out in an hour on Sunday.

The "Some circuits going above 120V" is a dead giveaway for Lost Neutral. Here's an example of the behavior there, rearranged a bit as a demonstration. (while it uses DC, it's designed to show how AC power systems work, and it's accurate). The asterisk is that a properly installed house will have local grounding rods, giving neutral current an alternate path through the dirt to other homes with ground rods or the supply transformer. Ground rods may be Ufer or water pipe connections.

Another failure is a Lost Hot. This is an odd one, because it almost completely breaks 240V appliances, and it should break 120V loads on the broken phase/pole. However in fact, it connects all your 240V loads together in series with all your 120V loads on that phase/pole. So voltage will mysteriously recover when a 240V load cycles on; the water heater being a major influence here.

Your reported 13-24V voltage drop cannot be attributed to the 600' run of #4 wire. The 1400W load you mention should cause only about 3.4% drop (4 volts), and the 1400 + 1200 watt loads together should only cause about 5.7% drop (6 volts). So no. It's not the voltage drop.

Your next step, if you're comfortable opening up your panel, is to take your voltage readings at the top of your main breaker (as close to the meter as you can get without breaking seals). That will most accurately reflect the delivered power.

If the power drop is local in your panel, look for loose connections. It was recently discovered screw torques are critical even on small circuits, and NEC 2014 now requires use of torque screwdrivers on every device which specifies a torque. An often overlooked place is the neutral bar on your panel. A suspect connection is far beyond torquing it now. It needs to be pulled apart, inspected, cleaned up if necessary, and cut back to copper which has not been annealed by heat.

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There is no such thing as "not subject to Code". There is such a thing as "no pre-accident inspections" but the two shouldn't be confused. Costa Rica uses El NEC, the continental code book and that is a matter of Federal law. Thus there is a positive reference for "correct wiring" that conforms to USA practice (except for being in Spanish :)
There are many "Give em plenty of rope" jurisdictions" where inspection and enforcement are thin. That seems good, but it's the opposite. it means selective and politically motivated enforcement. Somebody offends a local official or someone dies or just says the wrong thing on Youtube, and they're out there with the codebook and a micrometer. Thus, taking advantage of local political conditions to evade compliance is a dangerous game - and a private decision between you, insurer, mortgage lender, AHJ, prosecutor, the bereaved, and your deity.

Answer 2

In a split phase system, ( i don't know what system is in Costa Rica) if two loads are connected in parallel to the same phase as in Figure 1, then the currents sum in both the Hot and Neutral wires.

If a single load is connected to one phase and a second load is attached to the second phase, then each Hot wire carries current for a single load. The current is reduced in the neutral wire as I1-I2. Thus the resistance of the neutral wire is not used for equal loads resulting in an increased voltage drop across the load.

Essentially HA does double duty. It is the supply for R1 and also the return for R2, Likewise HB is the supply for R2 and the return for R1. If the loads are not equal then the difference in current passes along the neutral wire.

So the increase in voltage corresponds to the decrease of current in the neutral.

^{simulate this circuit – Schematic created using CircuitLab}