Said It Here

Why does my dimmer switch read 100v instead of 120v?

Standard non-true RMS multimeters frequently measure around 100 volts at full brightness on fully functional dimmer switches, even though regular outlets on the same circuit register 120 volts. This occurs because standard meters cannot accurately read the waveform produced by the dimmer, causing confusion when troubleshooting normal circuits.

What people tried

Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.

  1. 1
    Using a true RMS meter to check if readings change
  2. 2
    Measuring voltage with and without loads attached
  3. 3
    Assuming the lower voltage reading is a normal artifact of how dimmers operate

In their words

Unedited, most upvoted first, each linked to the thread it came from.

“My ACTUAL question that's come up while investigating is why the other (working) dimmer of the same make/model in this room is metering at ~100v while at full.”source ↗

“Non-dim outlets that share the same circuit meter at ~120v.”source ↗

“I checked other rooms with the same dimmers (also functional) and have found the same result (~100v while at 100% from dimmer, despite ~120v at outlets).”source ↗

Ncraft · r/askanelectrician · 1 upvotes

“My meter is not true RMS.”source ↗

Technical-Role-4346 · r/askanelectrician · 1 upvotes

Where this came up

r/askanelectricianelectricaltroubleshooting

People with this problem also raised