• Amperage is a measure of the electrical current flowing through a circuit. Current is measured in amperes or "amps". You must use the correct size wire for the amperage requirement of the circuit to prevent the wire from over heating.
  • The number and type of electrical devices connected to a circuit determine the amperage requirement of the circuit. Usually, a general purpose house circuit is designed for 20 amps. Lighting circuits may be designed for only 15 amps.
  • To calculate the amperage for a circuit, first add up the wattage of all the electrical devices that will be on the circuit. Then, divide the total wattage by the voltage of the system, 110 or 220, and that will give you the expected current or amps.
Wire Size and Amp Ratings
Wire Gauge Size
Copper
Aluminum
60°C
(140°F)
75°C
(167°F)
90°C
(194°F)
75°C
(167°F)
90°C
(194°F)
NM-B
THW
THWN-2
THW
XHHW-2
UF-B
THWN
THHN
THWN
THHN
 
SE
XHHW-2
SE
THWN-2
 
USE

 USE-2
USE
  
 
XHHW
  
XHHW
  
14
15
15
15
---
---
12
20
20
20
15
15
10
30
30
30
25
25
8
40
50
55
40
45
6
55
65
75
50
60
4
70
85
95
65
75
3
85
100
110
75
85
2
95
115
130
90
100
1
---
130
150
100
115
1/0
---
150
170
120
135
2/0
---
175
195
135
150
3/0
---
200
225
155
175
4/0
---
230
260
180
205
250
---
255
290
205
230
300
---
285
320
230
255
350
---
310
350
250
280
500
---
380
430
310
350
600
---
420
475
340
385
750
---
475
535
385
435
1000
---
545
615
445
500
WARNING! Installation of electrical wire can be hazardous, if done improperly, can result in personal injury or property damage. For safe wiring practices, consult the National Electrical Code® and your local building inspector.