As the winter months get colder, taking action to keep warm can produce similar results as in the summer when people try to keep cool. Once such incident occurred recently at the residence of an architect, who designed and helped build an addition to his house 20 years ago.

The master carpenter who taught me that trade often said that part of an architect’s training should be to spend a year as a carpenter’s apprentice to see what it takes to turn drawings into stable structures. A similar statement could be made about turning electrical schematics into functioning wired circuits and spending time as an electrician’s helper.

It happened in the laundry room

Back to the story: One of the two rooms in the addition was a first-floor laundry room off the family/TV room. The laundry room had no HVAC and was isolated with a solid door to keep the noise down. The homeowner decided to add a small electric heater, and when running it, the lights momentarily blinked or dimmed in the room. Was it safe or a sign of a problem lurking?

The laundry room was wired according to the National Electrical Code version adopted by the local and state government at the time of construction. This meant a dedicated 20A circuit for the laundry. Remembering the NEC’s 80% safety rule, this means the capacity was 16A or 1,920W. The amount of current required depends on the load. In this case, it was a high-efficiency, front-loading washing machine and a gas dryer.

Power requirements on washing machines can vary considerably, from a low of 6–10A on compact front loaders to 12–18A on some high-efficiency machines. The newer machines use adjustable speed drives (ASD) instead of plain induction motors, so the inrush current characteristics are different, as well as current draw during various stages of washing. In this case, it was a 10A washer. The last piece in the puzzle is the heater that was added, a 500W electric heater, which draws approximately 4A without much inrush current when it kicks on.

No matter what the season, Ohm’s and Kirchhoff’s laws still prevail. Without having access to a PQ monitor, it became a bit of conjecture as to what sequence of events caused the light to dim. Adding the currents of 10A for washer, 4A for dryer and 4A for heater, this put the maximum current possible over the 80% rule. It is still below the trip point for the 20A breaker, and the 12-AWG wires aren’t going to melt, but it isn’t a recommended operation.

Though the resistance of 50 feet of 12-AWG wire is just 0.03 ohms, a more typical overall number to use would be 0.3–0.5 ohms for the source impedance. Easy math says take 0.5 ohms multiplied by 18A, which equates to a 9V drop. Assuming ideal 120V at the breaker, there is just 101V left for the three loads. 

While the heater doesn’t mind, the ASD and its control circuitry in the washing machine may require more current to keep a constant power level. That would increase the voltage drop. The resulting residual voltage may then get below the operating point of the washer or dryer, and they could either malfunction or trip offline, especially the newer high-efficiency units.

What about the light that blinked?

The conclusion is that it isn’t a good idea to run the heater off the dedicated circuit, or off another circuit with an extension cord. But neither explain the light that blinked.

Typically, a blink or dim in a light for a fraction of a second is caused by the inrush current of a load, such as a motor, that exceeds the steady-state current. Incandescent bulbs are particularly vulnerable, but with the newer lighting technologies, the rules have changed. This room did have an incandescent recessed spotlight. But if the wiring was all up to Code, the light shouldn’t have blinked during operation of the three loads, as it is not supposed to be on the dedicated circuit with the three loads. For that matter, neither should the heater.

The reason the Code changed many years ago to require dedicated circuits for bathrooms and laundry rooms was not likely driven by the power quality concerns of voltage sags that resulted in a light blink. Rather, it was more likely based on the classic case of a hair dryer tripping the breaker for the entire bathroom and leaving the person with wet hair in the dark. In our case, the only logical explanation is that the light switch was connected to the dedicated circuit. Had it been wired to another circuit, the voltage drop from the three loads would not have any significant effect on the other parallel circuit.

If all NEC articles had been followed from design through construction, and provisions made for “in case” heating or other appliances (such as an iron, assuming some people still use them), then there wouldn’t have been a basis for this article—good for homeowners, not so good for me.

stock.adobe.com / Igor