WiremanWireman · Voltage drop test under load: troubleshooting for electricians · 2023 NEC ·
Voltage drop test under load
Compare voltage with no load and full load to find long runs, small wire and bad connections.
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How it works
Wire and connections have resistance, and current through resistance drops voltage (V = I × R). With no load, a circuit with a burned connection can still read a perfect 120 V because no current flows. Put real current through it and the bad spot shows up as voltage lost. That’s why the test compares the voltage at the load with no load against the voltage with a known load on.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- How will you load the circuit?
- Receptacle analyzer with a load testTest: Run the analyzer’s loaded testExpect: A few percent or less at the receptacle.
- Drop of about 5% or lessVoltage drop is acceptable
- More than about 5%, or a big jump at one outletTest: Find where the voltage is lostExpect: Near 0 V across any good connection. Drop spread evenly along the wire, split about equally between hot and neutral.
- Most of the drop is across one connection or deviceA bad connection
- Most of the drop is on the neutral sideLoose or failing neutral
- Spread evenly along the wireWire too small for the distance
- A real load (heater, the equipment itself)Test: Measure with no loadExpect: About the same at both ends, near nominal (roughly 114–126 V on a 120 V circuit).
- Normal at both endsTest: Measure with the load onExpect: About 3% or less on a branch circuit. That’s about 3.6 V on 120 V.
- 3% or lessVoltage drop is acceptable
- More than 3%Does the measured drop match the calculated drop for this wire?
- Close to the calculated dropWire too small for the distance
- Much more than calculatedTest: Find where the voltage is lostExpect: Near 0 V across any good connection. Drop spread evenly along the wire, split about equally between hot and neutral.
- Most of the drop is across one connection or deviceA bad connection
- Most of the drop is on the neutral sideLoose or failing neutral
- Spread evenly along the wireWire too small for the distance
- Already low at the panelLow voltage at the source
- Normal at both endsTest: Measure with the load onExpect: About 3% or less on a branch circuit. That’s about 3.6 V on 120 V.
- Receptacle analyzer with a load testTest: Run the analyzer’s loaded testExpect: A few percent or less at the receptacle.