Standard Breaker and Fuse Sizes

Standard ampere ratings of fuses and circuit breakers, with the 80 percent continuous-load rule.

Circuit breakers and fuses are made only in certain standard ratings, so after you work out the current you must pick the next standard size up. This table lists the standard ratings from 15 to 6 000 A (NEC 240.6) with the largest continuous load that a breaker of that rating may carry (80 percent of the rating) and the power that load is at 400 V three-phase and 120 V single-phase.

The table

37 rows

Standard rating (A)	Largest continuous load: 80 percent of the rating (A)	Three-phase power at 400 V at that load, power factor 1 (kW)	Single-phase power at 120 V at that load (kW)
15	12	8.3	1.44
20	16	11.1	1.92
25	20	13.9	2.4
30	24	16.6	2.88
35	28	19.4	3.36
40	32	22.2	3.84
45	36	24.9	4.32
50	40	27.7	4.8
60	48	33.3	5.76
70	56	38.8	6.72
80	64	44.3	7.68
90	72	49.9	8.64
100	80	55.4	9.6
110	88	61	10.56
125	100	69.3	12
150	120	83.1	14.4
175	140	97	16.8
200	160	110.9	19.2
225	180	124.7	21.6
250	200	138.6	24
300	240	166.3	28.8
350	280	194	33.6
400	320	221.7	38.4
450	360	249.4	43.2
500	400	277.1	48
600	480	332.6	57.6
700	560	388	67.2
800	640	443.4	76.8
1000	800	554.3	96
1200	960	665.1	115.2
1600	1280	886.8	153.6
2000	1600	1108.5	192
2500	2000	1385.6	240
3000	2400	1662.8	288
4000	3200	2217	384
5000	4000	2771.3	480
6000	4800	3325.5	576

Source and notes: Standard ampere ratings of fuses and circuit breakers in NFPA 70 (National Electrical Code) Section 240.6(A); fuses are also made in 1, 3, 6, 10, 601 A and other special ratings. IEC miniature breakers use 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A. Columns are computed: the 80 percent rule (continuous loads on a standard breaker) and power = sqrt(3) x V x I (three-phase) or V x I.

How to use this table

Work out the load current (for a continuous load, multiply by 1.25).
Choose the next standard rating at or above that figure.
Check the cable ampacity against the breaker rating; the breaker protects the cable.

Worked example: a continuous 32 A load

Continuous loads (running three hours or more) are sized at 125 percent: 32 × 1.25 = 40 A.
The next standard rating is 40 A, which carries a continuous load of up to 32 A (80 percent of 40).
At 400 V three-phase that is 22.2 kW; at 120 V single-phase 3.84 kW.

Notes and cautions

The 80 percent rule applies to a standard breaker that is not rated for 100 percent continuous duty; 100 percent rated breakers exist.
Breakers also have an interrupting rating (the largest fault current they can break) that must be at least the available fault current.
These sizes are NEC ratings. IEC miniature breakers use 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A.

FAQ

What are the standard breaker sizes?

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and so on up to 6 000 A in the US code.

What is the 80 percent rule?

A standard breaker should not carry a continuous load over 80 percent of its rating, equivalent to sizing the breaker at 125 percent of the continuous load.

Sources

NFPA 70, National Electrical Code, Section 240.6(A) (standard ampere ratings) and Section 210.20 (continuous loads); IEC 60898-1 (miniature circuit breakers).
Standard ampere ratings of fuses and circuit breakers in NFPA 70 (National Electrical Code) Section 240.6(A); fuses are also made in 1, 3, 6, 10, 601 A and other special ratings. IEC miniature breakers use 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A. Columns are computed: the 80 percent rule (continuous loads on a standard breaker) and power = sqrt(3) x V x I (three-phase) or V x I.