NEC recommends max 3% VD for branch circuits, 5% combined feeder + branch. Formula: VD = (2 × K × I × L) / CM where K=12.9 copper / 21.2 aluminum.
NEC 310.16 Ampacity Table — Copper Conductors (75°C / 90°C in Conduit, ≤3 Conductors, 30°C Ambient)
Important: Use the 75°C ampacity column for most applications — equipment terminals are typically rated at 75°C per NEC 110.14(C). The 90°C column applies only when all termination equipment is rated 90°C. Apply correction factors for ambient temperature above 30°C and adjustment factors for more than 3 current-carrying conductors.
AWG / kcmil
Circular Mils
Cu 60°C (A)
Cu 75°C (A)
Cu 90°C (A)
Al 75°C (A)
Al 90°C (A)
Cu Ω/kft (75°C)
Typical Use
Temperature Correction Factors (NEC Table 310.15(B)(1))
Ambient Temp °C
Ambient Temp °F
60°C Insulation
75°C Insulation
90°C Insulation
10
50
1.29
1.20
1.15
16–20
61–68
1.22
1.13
1.11
21–25
70–77
1.14
1.08
1.05
26–30
79–86
1.00
1.00
1.00
31–35
88–95
0.91
0.94
0.96
36–40
97–104
0.82
0.88
0.91
41–45
106–113
0.71
0.82
0.87
46–50
115–122
0.58
0.75
0.82
51–55
124–131
0.41
0.67
0.76
56–60
133–140
—
0.58
0.71
61–70
142–158
—
0.33
0.58
Adjustment Factors — More Than 3 Current-Carrying Conductors (NEC 310.15(C)(1))
Conductors in Conduit
Adjustment Factor
Example: 20A Wire becomes...
1–3
100% (no reduction)
20A
4–6
80%
16A
7–9
70%
14A
10–20
50%
10A
21–30
45%
9A
31–40
40%
8A
41+
35%
7A
Voltage Drop Formula
Single Phase: VD (V) = (2 × K × I × L) / CM Three Phase: VD (V) = (1.732 × K × I × L) / CM
K = 12.9 for copper, 21.2 for aluminum
I = Load current in amps
L = One-way length in feet
CM = Circular mils of conductor
VD% = (VD / System Voltage) × 100