Circuit Breaker Formulas: Rating and Trip
Circuit breaker formulas: rating from design current and cable capacity, the 1.45 overload rule, trip ranges of B, C and D curves and breaking capacity.
The formulas
| To check | Rule |
|---|---|
| Rating against load and cable | Ib ≤ In ≤ Iz |
| Overload protection | I2 ≤ 1.45 × Iz, where I2 = 1.45 × In for a miniature breaker |
| Magnetic (short-circuit) trip | B: 3 to 5 × In, C: 5 to 10 × In, D: 10 to 20 × In |
| Breaking capacity | Icu ≥ prospective fault current at the breaker |
| Fault protection | Zs ≤ U0 / Ia, where Ia is the current that trips in the required time |
Ib is the design current, In the breaker rating, Iz the cable current capacity and I2 the current that makes the breaker trip within the conventional time. The trip ranges are those of IEC 60898 miniature breakers. Moulded-case and air breakers have adjustable settings, so use the data sheet.
Worked example: a 63 A circuit
The design current is 63 A and the cable can carry 72 A after correction factors.
- Ib ≤ In ≤ Iz: 63 ≤ 63 ≤ 72, so the rating is acceptable
- I2 = 1.45 × 63 = 91.35 A, and 1.45 × Iz = 104.4 A, so I2 ≤ 1.45 × Iz
Worked example: instantaneous trip of a 32 A breaker
- Type B trips between 96 A and 160 A
- Type C trips between 160 A and 320 A
- Type D trips between 320 A and 640 A
Magnetic trip range by rating and type
| Rating | Type B | Type C | Type D |
|---|---|---|---|
| 16 A | 48 to 80 A | 80 to 160 A | 160 to 320 A |
| 32 A | 96 to 160 A | 160 to 320 A | 320 to 640 A |
| 63 A | 189 to 315 A | 315 to 630 A | 630 to 1,260 A |
Frequently asked questions
Which curve should I choose?
Type B suits resistive loads and long cables. Type C suits most mixed loads, and type D loads with a high inrush, such as transformers and some motors.
Why is there a limit of 1.45 times Iz?
A breaker can carry up to 1.45 times its rating for a long time before it trips. The cable must survive that current, so the limit protects the insulation.
Does a bigger breaker mean a safer circuit?
No. A breaker larger than the cable allows will not trip before the cable overheats. The rating must not exceed the cable current capacity.
What is the difference between Icu and Ics?
Icu is the largest fault the breaker can interrupt once. Ics is the share of it that the breaker can interrupt and still stay in service. Ics is the more conservative figure.
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