WiremanWireman · Voltage too high or too low: troubleshooting for electricians · 2023 NEC ·
Voltage too high or too low
Measured voltage outside the normal range, or sagging under load. Service, feeder, branch circuit or a system mismatch.
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How it works
Utilities aim to deliver voltage within ANSI C84.1 Range A at the service: plus or minus 5% of nominal, which is 114 to 126 V on a 120 V system and 228 to 252 V on 240 V. Range B is a wider band allowed less often. Voltage that is high or low everywhere, at light load, is a utility issue they fix with transformer taps or regulators. Voltage that sags only under load comes from resistance: in the utility transformer and drop, the service conductors, the feeder or the branch circuit. Equipment rated 230 V running on a 208 V system is a different problem: the system is fine, the equipment doesn’t match it.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Where is the voltage wrong?
- Everywhere in the buildingTest: Record the service voltageExpect: Within ANSI Range A: 114 to 126 V to neutral on a 120 V system at all times, legs within a few volts of each other.
- Above 126 V, highest at light loadUtility voltage running high
- Below 114 V even at light loadUtility voltage running low
- Normal at light load, sags a lot under loadTest: Find where the drop happensExpect: A few volts of drop at full load. NEC informational notes suggest about 3% on a branch circuit or feeder and 5% total, as a design guide.
- Most of the drop is at the mainResistance between the transformer and the main
- Main holds, the load end sagsToo much drop in the feeder or branch circuit
- Legs unequal and shifting with loadA loose or open neutral
- One circuit or one areaTest: Measure drop on the circuit under loadExpect: A drop of a few volts at most, and hot to neutral about equal to hot to ground.
- Drops more than about 5% (6 V)Too much drop in the feeder or branch circuit
- Holds steadyThe circuit holds voltage
- Equipment says 230 or 240 V; the building is 208Y/120 VEquipment rated for 240 V on a 208 V system
- Everywhere in the buildingTest: Record the service voltageExpect: Within ANSI Range A: 114 to 126 V to neutral on a 120 V system at all times, legs within a few volts of each other.