Online Calculators and Tables to Help You Determine Proper Wire Size


For ambient temperatures other than 30°C (86°F), multiply the allowable ampacities shown in the Maximum Allowable Ampacities table by the appropriate factor shown below.

Temperature Correction Factors for
Allowable Ampacities of Conductors

Source: NFPA 70, National Electrical Code, Table 310.15(B)(2)(a)
60°C
(140°F)		 75°C
(167°F)		 90°C
(194°F)		 60°C
(140°F)		 75°C
(167°F)		 90°C
(194°F)
AMBIENT TEMP TYPES
TW,
UF		 TYPES
RHW,
THW,
THWN		 TYPES
TBS,
SA,
SIS		 TYPES
TW,
UF		 TYPES
RHW,
THW,
THWN		 TYPES
TBS,
SA,
SIS		 AMBIENT TEMP
°C COPPER ALUMINUM °F
21-25 1.08 1.05 1.04 1.08 1.05 1.04 70-77
26-30 1 1 1 1 1 1 78-86
31-35 0.91 0.94 0.96 0.91 0.94 0.96 87-95
36-40 0.82 0.88 0.91 0.82 0.88 0.91 96-104
41-45 0.71 0.82 0.87 0.71 0.82 0.87 105-113
46-50 0.58 0.75 0.82 0.58 0.75 0.82 114-122
51-55 0.41 0.67 0.76 0.41 0.67 0.76 123-131
56-60 – 0.58 0.71 – 0.58 0.71 132-140
61-70 – 0.33 0.58 – 0.33 0.58 141-158
71-80 – – 0.41 – – 0.41 159-176

For more than three current-carrying conductors in a raceway or cable, multiply the allowable ampacities shown in the Maximum Allowable Ampacities table by the percentages shown below.

Number of Conductors Correction Factors for
Allowable Ampacities of Conductors

Source: NFPA 70, National Electrical Code, Table 310.15(B)(3)(a)
Number of Current-Carrying Conductors Percent of Values in Tables 310.15(B)(16) through 310.15(B)(19) as Adjusted for Ambient Temperature if Necessary
4-6 80
7-9 70
10-20 50
21-30 45
31-40 40
41+ 35

View wire size charts from the list below.

Maximum Allowable Ampacities for Conductors in Raceway, Cable or Earth (30°C) Maximum Allowable Ampacities for Conductors in Free Air (30°C) Maximum Allowable Ampacities for Conductors in Raceway, Cable or Earth (40°C) Maximum Allowable Ampacities for Conductors in Free Air (40°C)

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