Voltage Drop

How to use

1. Enter the supply rating and what it actually reads, the load, one-way length, conductor type, and temperature. When the run is outside the window, Options that pass at these conditions lists the combinations that land inside.
2. After a gauge is selected, Room left shows the maximum one-way length and maximum load that still pass, plus the volt margin to the bottom of the window.
3. A measured voltage at the device is compared to the expected voltage and implied loop resistance. Extra drop indicates something besides the copper that was entered.

Assumptions

• Solid annealed copper at 20 °C from the table in this app. Stranded class B multiplies resistance by 1.02. Fine-strand or tinned (1.08) is in the data table for review, not a third control. Temperature correction is then applied. These are not NEC 75 °C values.
• Paralleled conductors divide loop resistance by the number used per leg — the same move as doubling the spare pair in 18/4.
• AC circuits at these gauges and lengths are treated as purely resistive, so reactance is ignored.
• The round trip is two legs of the same gauge. Commonly stocked sizes in this trade are 22, 20, 18, 16, 14, and 12 AWG; every passing gauge is listed.
• The result is an estimate. Manufacturer data, the AHJ, and current applicable code govern material and whether a circuit is energized.

FAQ

Why does a hot attic run fail when the same run passes in winter?

Copper resistance rises with temperature. A 120 °F rooftop or attic run uses a higher R_T than the same length at 68 °F, so voltage at the device drops. A run that passes at 68 °F can fall outside the window at 120 °F.

Why does the tolerance window matter more than a flat percent drop?

The window is built from the nominal rating (for example 10.8–13.2 V on 12 VDC at ±10%), not a flat percent of whatever the supply happens to read. A supply sitting high can hide a long run; a supply sitting low can fail a short one.

What accounts for measured drop above the calculated drop?

Extra loop resistance is the first number to read. Conditions that produce it include a nicked or damaged conductor, a bad splice or termination, a partial short or ground, corroded connections, aluminum or copper-clad instead of solid copper, or a run longer than the value entered. The warning is a list of those conditions, not a diagnosis.