Voltage Drop Calculator

How to Calculate Voltage Drop

Use this free voltage drop calculator to get instant, accurate results for your construction project. Simply enter your measurements above and the calculator will automatically compute the result using industry-standard formulas.

Pro Tip: Always add 10-15% extra material to account for waste, cuts, and job-site conditions. This is standard practice recommended by experienced contractors.

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💡 Pro Tips — Voltage Drop

NEC 310.15: Wire ampacity depends on insulation type, temperature, and conduit fill.
3% voltage drop rule: Max recommended drop is 3% on branch circuits, 5% total.
Aluminum needs anti-oxidant: Apply compound to all aluminum wire connections.
GFCI required: Kitchens, bathrooms, garages, outdoors, unfinished basements.
Always pull an extra conductor — having a spare wire in conduit saves future work.

📊 NEC Wire Ampacity (Copper, 75°C)

AWG	Max Amps	Breaker	Common Use
14	15A	15A	Lighting
12	20A	20A	Kitchen/bath outlets
10	30A	30A	Dryer, water heater
8	40A	40A	Range, large AC
6	55A	50-60A	Subpanel feeder
4	70A	70A	Large subpanel

❓ Frequently Asked Questions

+What wire size for 20 amps?

12 AWG copper for runs under 50ft at 120V. For longer runs, upgrade to 10 AWG to minimize voltage drop.

+What is the 80% rule?

NEC says continuous loads must not exceed 80% of breaker rating. A 20A breaker carries max 16A continuously.

+Do I need a permit?

Yes for new circuits, panel upgrades, or adding subpanels. Simple switch/outlet replacements typically do not need a permit.

+Can I use aluminum wire?

Yes for large feeders (4 AWG and up). Requires anti-oxidant compound on all connections. Must be 2 sizes larger than equivalent copper.

How to Calculate Voltage Drop

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💡 Pro Tips — Voltage Drop

📊 NEC Wire Ampacity (Copper, 75°C)

❓ Frequently Asked Questions