Outdoor cord voltage drop
Companion estimate for 120V holiday light loads on an outdoor extension cord — AWG, length, and amps to approximate voltage drop (not a full feeder design).
V_drop ≈ 2 × length_ft × amps × R_ohm_per_1000ft ÷ 1000 · % = V_drop ÷ 120 × 100
When to use
Reach for the outdoor cord voltage drop calculator when a decorating decision depends on measurements you already have. You will enter light / load watts on the cord, supply voltage (V), and one-way cord length (feet).
Companion estimate for 120V holiday light loads on an outdoor extension cord — AWG, length, and amps to approximate voltage drop (not a full feeder design). Compare the panel result to the worked example, then adjust for spare strands, wind, or oddly shaped eaves.
Do not treat the output as an electrical-code stamp, install quote, or substitute for ladder safety and outdoor-rated gear.
Thicker wire = lower AWG number = less drop.
Worked example
Start with the calculator’s default entries: Light / load watts on the cord = 300; Supply voltage (V) = 120; One-way cord length (feet) = 50; Cord AWG = 16 AWG.
With those entries the calculator returns:
Load current: 2.50 A
Approx. voltage drop: 1.00 V (0.84%)
Approx. voltage at load: 119.0 V
Using ~4.02 Ω/1000 ft copper for 16 AWG. Flags: >3% warn, >5% high.
Formula used: V_drop ≈ 2 × length_ft × amps × R_ohm_per_1000ft ÷ 1000 · % = V_drop ÷ 120 × 100
Recompute the first line by hand from the formula. If your arithmetic and the result differ by more than rounding, the usual cause is a unit mix-up (inches vs feet, percent vs decimal) rather than the formula.
What changes if you change one input? Raise Light / load watts on the cord from 300 to 450 and leave everything else alone. The result becomes:
Load current: 3.75 A
Approx. voltage drop: 1.51 V (1.26%)
Approx. voltage at load: 118.5 V
Changing one measured value at a time like this shows which input drives the answer, so you know which value is worth measuring twice before you buy.
Limits / assumptions
Units: keep every length in one system (feet or inches end-to-end) and every electrical quantity in the units the labels name. Mixing without converting invents a wrong answer.
Where the formula fails: odd roof geometry, wind on inflatables, worn sockets, and conditions not listed on the page are outside the model. Zero or nonsense inputs should error — do not force a number.
Not a substitute: the outdoor cord voltage drop result is a planning aid. It is not an NEC determination, not a licensed electrician’s opinion, and not a manufacturer max-connect chart.
Estimate only — not electrical advice or code compliance. Use outdoor-rated cords, respect amp ratings, and do not daisy-chain long thin cords under heavy loads.
FAQ
What percent drop is concerning?
Many lighting guides aim to stay near or under about 3–5% on 120V. Large drops can dim bulbs at the far end.
Why multiply length by 2?
Current travels out and back, so one-way feet are doubled in this simple copper resistance model.
Is this for whole-house feeders?
No — it is a holiday-display companion for extension cords and light loads, not panel or feeder sizing.
How do I get amps?
Amps ≈ watts ÷ volts. You can also cross-check with the outlet load calculator.