Why Is My Voltage Low at the End of a Long Run?

The outlet in the garden shed reads 112 V while the panel reads 121 V. The pressure washer bogs, the lights dim when the compressor kicks on. Nine volts vanished somewhere — here is how to find out where, and what to do about it.

Three suspects

1. Honest voltage drop. Wire has resistance; current through resistance loses volts. The loss is proportional to current and distance — which is why everything looks fine with a meter (no load, no current, no drop) and sags the moment a motor starts. 2. A bad connection. A loose wirenut, corroded splice or backstabbed receptacle adds concentrated resistance at one point. It mimics wire drop but also makes heat — warm devices and brown discoloration are the tell. 3. Utility sag. If the voltage is low at the panel too, the problem is upstream of you.

The 10-minute diagnosis

Measure at the panel and at the far outlet, first with the circuit unloaded, then with a known load running (a 1500 W heater is perfect — about 12.5 A). Unloaded, the readings should match within a volt. Under load, the difference between panel and outlet is your circuit's drop. Compare it with the prediction from the voltage drop calculator for the wire size and distance: if the measured drop roughly matches the math, the wire is just undersized for the run. If it is dramatically worse, hunt for a bad connection — check every junction box, and feel for warmth.

The fixes, ranked

• Reduce the current: run the heater at 240 V instead of 120 V, or split loads across circuits — halving amps halves the drop.
• Upsize the conductor: each AWG step cuts resistance ~21%. The size solver gives the gauge that meets 3%.
• Raise the voltage: a 240 V circuit to the outbuilding with local 120 V breakers beats heroic copper at 120 V.
• Repair connections: free, and often the real culprit in older work.

Persistent low voltage is not cosmetic — motors draw more current to compensate, run hotter, and die younger. Diagnose once, fix once.