Basic Residential Electric Question

Why is that lights get brighter briefly when our fridge or some other appliance kicks on? We have 100 amp service and will be upgrading but I’m curious as to why the lights don’t dim rather than brighten when the draw increases.

after the device starts it puts a little bit of higher voltage into the circuit because of its electrical characteristics (inductance).

There’s a good chance that your home’s neutral is just slightly “loose” - in broadly over-simple terms, the neutral is the zero volt reference between the two 120-volt lines that make up 240-volt service. If there’s a poor connection in the neutral wiring, you can get higher voltage.

What’s happening is when the fridge kicks in, it’s drawing so much amperage for that split second that the neutral’s reference to zero volts shifts slightly - instead of your power being two 120-volt lines, you get something more like one at 110 and the other at 130.

When you get your service upgraded, the connections should all be new and good, and you shouldn’t see fluctuations like that any more.

I’m a little hazy on the physics, but…

When an electric motor starts, often the lights will blink as the newly opened circuit draws down the voltage - since it’s a motor or similar magnetic device: the current surges through the newly closed ciruit into the coil, creating a growing magnetic field; the magnetic field then back-induces a slowdown in the current, and a corresponding voltage surge. Once the system settles down, everything is back to normal, except the added induction load has changed the phase of the current a bit.

Fancier devices have a resistor or capacitor to limit the initial surge when the circuit is closed.