Load & Breaker
Enter the load (W or VA), voltage and number of phases and I compute the current, the minimum breaker by the 80% continuous-load rule and the recommended standard rating (IEC/NEC).
The breaker must sit above the load current with headroom.
Series ratings
| Rating | Max continuous load (80%) | Status |
|---|
How it is computed · current, 80% and rating
1. If the load comes in W I
turn it into apparent power by dividing by the power factor:
VA = W / PF. If it already comes in VA, it
goes straight through.
2. Load current. Single phase:
I = VA / V. Three phase (V = line-to-line):
I = VA / (√3 · V).
3. The 80% rule for a continuous load
(runs ≥ 3 h): the breaker must not carry more than 80% of its
rating, so breaker ≥ I / 0.8. For a non-continuous load
breaker ≥ I is enough.
4. I pick the next standard
rating above that minimum, from the IEC 60898 or NEC 240.6
series. Also check the conductor can carry that current.
Runs locally in your browser · no sign-up · nothing leaves your browser. Design reference; verify against your local code and the conductor.
How it works
The calculator sizes the thermal-magnetic breaker for a load: it converts power into current and suggests the standard rating. If the load is entered in W, it first converts it to apparent power with VA = W ÷ PF; the current is I = VA ÷ V for single-phase or I = VA ÷ (√3 × V) for three-phase, with V line-to-line. Typical power factors are 1.0 for resistive loads, 0.9 for mixed loads and 0.8 for motors.
For continuous loads (operating 3 hours or more) it applies the NEC 80 % rule: the breaker must not be loaded beyond 80 % of its rating, so breaker ≥ I ÷ 0.8; for non-continuous loads breaker ≥ I is enough. It then picks the next standard rating from the IEC 60898 series (6, 10, 13, 16, 20, 25, 32, 40, 50, 63 A…) or NEC 240.6 (15, 20, 25, 30, 35, 40, 50, 60 A…) and shows the utilization percentage. It is a design reference: the conductor must be verified separately and local code prevails.
Example: breaker for a continuous 3 kVA load at 230 V single-phase (IEC series)
- Current:
3,000 VA ÷ 230 V = 13.0 A. - Continuous load, 80 % rule:
13.0 ÷ 0.8 = 16.3 Aminimum. - Next IEC rating above 16.3 A: 20 A.
- Resulting utilization:
13.0 ÷ 20 = 65 %of the rating, with proper headroom.