Most electricians size wire based on ampacity tables and stop. That works fine for short runs. A 6 AWG copper wire is rated for 65 amps at 75°C, so you use it for a 60-amp circuit and move on. But once you get past 100 feet, ampacity stops being the limiting factor. Voltage drop takes over.
Voltage drop is the loss of voltage between the source and the load due to resistance in the wire. A 60-amp load 200 feet away can easily see 8-10 volts of drop on a 6 AWG circuit. That's 3-4% at 240V, which is technically within NEC limits (5% total for feeders and branch circuits combined). But many motors, electronics, and LED drivers perform poorly below 95% of rated voltage. The equipment might work, but it won't work well.
Voltage Drop vs Ampacity: Two Different Problems
Ampacity is about safety. It's the maximum current a wire can carry without overheating and damaging the insulation. The NEC provides ampacity tables in Article 310 based on conductor size, insulation type, and ambient temperature. If you exceed the ampacity, the wire gets hot, insulation degrades, and you risk a fire. This is a hard limit.
Voltage drop is about performance. It's the loss of voltage between the panel and the load due to the resistance of the wire. Voltage drop doesn't cause fires, but it causes motors to overheat, lights to dim, and electronics to malfunction. The NEC allows up to 5% total drop (3% for feeders, 2% for branch circuits), but that's a recommendation, not a code requirement. Many engineers design for 2-3% total drop to avoid performance problems.
On short runs (under 50 feet), ampacity is almost always the binding constraint. The wire size needed to carry the current safely is larger than the wire size needed to keep voltage drop under 3%. But once you get past 100 feet, voltage drop starts to dominate. A 60-amp circuit might need 4 AWG or even 2 AWG to stay under 3% drop at 200 feet, even though 6 AWG is rated for 65 amps.
This is the key insight: ampacity and voltage drop are independent constraints. You have to check both, and you have to use the larger wire that satisfies the tighter constraint. For long runs, that's almost always voltage drop.
The Voltage Drop Formula
The voltage drop formula for single-phase circuits is: VD = (2 × K × I × D) / CM, where K is the resistivity constant, I is the current in amps, D is the one-way distance in feet, and CM is the circular mil area of the conductor. The factor of 2 accounts for the round trip: current flows out on the hot wire and back on the neutral.
The resistivity constant K depends on the conductor material and temperature. For copper at 75°C, K = 12.9. For aluminum at 75°C, K = 21.2. These are the values you use for most residential and commercial work because 75°C is the standard rating for THHN/THWN wire.
Circular mils (CM) is an odd unit, but it's what the formula uses. For standard wire sizes, you can look up CM in NEC Chapter 9, Table 8. For example, 10 AWG is 10,380 CM, 6 AWG is 26,240 CM, 2 AWG is 66,360 CM. Bigger wire = more CM = less resistance = less voltage drop.
Let's work an example. You have a 50-amp load at 240V, 200 feet away. You're using 6 AWG copper (26,240 CM). VD = (2 × 12.9 × 50 × 200) / 26,240 = 9.8 volts. That's 4.1% drop at 240V. If your target is 3%, you need to go up to 4 AWG (41,740 CM), which drops VD to 6.2 volts (2.6%).
VD = (2 × K × I × D) / CM
Voltage drop (three-phase):
VD = (1.732 × K × I × D) / CM
K = 12.9 (copper, 75°C)
K = 21.2 (aluminum, 75°C)
I = current (amps)
D = one-way distance (feet)
CM = circular mil area (from NEC Chapter 9, Table 8)
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NEC Derating: The Part Most People Skip
Ampacity tables in NEC 310.16 assume three conditions: conductors are in free air or in a raceway with no more than three current-carrying conductors, ambient temperature is 30°C (86°F), and the conductor insulation is rated for 75°C. If any of these conditions change, you have to derate the ampacity.
Temperature correction is in NEC 310.15(B)(2)(a). If the ambient temperature is higher than 30°C, you multiply the ampacity by a correction factor. For example, at 40°C (104°F), the correction factor for 75°C wire is 0.88. A 6 AWG conductor rated for 65 amps at 30°C is only good for 57 amps at 40°C. Attics in summer can hit 50°C (122°F), where the correction factor drops to 0.75. Your 65-amp wire is now a 49-amp wire.
Conduit fill adjustment is in NEC 310.15(B)(3)(a). If you have more than three current-carrying conductors in a raceway, you multiply the ampacity by an adjustment factor. For 4-6 conductors, the factor is 0.8. For 7-9 conductors, it's 0.7. This is where multi-circuit conduit runs get tricky.
The two derating factors multiply. A 6 AWG conductor in a 40°C attic with four conductors in the conduit is derated to 65 × 0.88 × 0.8 = 46 amps. If your load is 50 amps, 6 AWG isn't enough. You need 4 AWG, which gives you 85 × 0.88 × 0.8 = 60 amps after derating.
Copper vs Aluminum: A Real-World Comparison
Aluminum is cheaper than copper by about 40% per foot for the same ampacity. But aluminum has higher resistance, so you need 1-2 wire sizes larger to get the same voltage drop. For long runs (100+ feet), the cost difference narrows, and for very long runs (300+ feet), aluminum often wins despite the larger size.
A 2 AWG copper wire costs about $2.50/foot and has an ampacity of 115 amps at 75°C. A 1/0 AWG aluminum wire costs about $1.80/foot and has an ampacity of 120 amps. For a 100-foot run, that's $250 for copper vs $180 for aluminum. The aluminum is 28% cheaper.
But aluminum has different termination requirements. You need anti-oxidant compound on the terminations to prevent corrosion. You need to use the correct torque (aluminum is softer than copper, so over-tightening deforms the conductor). And you need to verify that your breakers, panels, and equipment are rated for aluminum conductors. Many residential panels are marked "CU/AL" (copper or aluminum), but some are copper-only.
The practical rule: for feeders and service entrances longer than 100 feet, aluminum is worth considering. For branch circuits, copper is usually easier unless you're doing a large commercial job. For short runs under 50 feet, copper is the default.
Common Mistakes on Long Runs
The most common mistake is sizing wire based on ampacity alone without checking voltage drop. This works fine for short runs but fails on long runs. A 10 AWG wire is rated for 35 amps, which is fine for a 30-amp circuit. But if that circuit is 150 feet away, you're looking at 7-8% voltage drop. The equipment might work, but motors will run hot and lights will dim.
The second mistake is using indoor-rated wire outdoors. THHN is rated for dry or damp locations. THWN is rated for wet locations. If you run a feeder to a detached garage or barn, you need THWN or direct-burial cable (UF). Using THHN in conduit exposed to rain is a code violation.
The third mistake is undersizing the neutral. For single-phase loads, the neutral carries the same current as the hot, so it needs to be the same size. For three-phase balanced loads, the neutral carries little to no current. But for three-phase loads with significant single-phase components, the neutral can carry close to the full phase current.
The fourth mistake is not accounting for future loads. If you're running a feeder to a barn and you size it for the current load, you'll regret it when you want to add a compressor or a heater. The incremental cost of going from 6 AWG to 4 AWG on a 200-foot run is maybe $150. The cost of pulling new wire later is $2,000+.
The fifth mistake is not applying the NEC 125% rule for continuous loads. A load is continuous if it runs for 3 hours or more. HVAC, well pumps, and some lighting qualify. For continuous loads, you have to size the wire and breaker for 125% of the load.