Whats this in the panel?

What is this little white round do-little?
Powdered Doughnut?
Flux Capacitor?
Coil arrestor?

Looks like some sort of inductor,such as the kind that prevents interference.

Sorry if this does not help, but it may be related to preventing buzzing noises in certain devises.

It appears to be a “donut” current transformer, used for monitoring loads for off peak control of certain appliances.

Most likel a CT.


Load Controller


That was part of an energy monitor system. It appears that someone didn’t trust their power co. so they failed to remove the c.t. from the lines they were trying to take a power reading (wrongly connected).

Bill S
master electrician/electrical inspector

I see these on older condos when they want to decentralize each of the sub-panels and start individual billing.


It’s a current transformer. It can be used for many reasons.
*to sense when power is on.
*to measure the amount of power used for usage charges. Either a utility company or even multifamily type of set up when there is only one utility meter at the service. The owner/manager could then figure out the cost to each unit.