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The four-step sizing chain
Every correctly sized circuit passes through the same four steps, in this order:
- Design current (Ib). Convert the load — watts, kVA or nameplate amperes — into amperes at the actual system voltage.
- Load character. Decide whether the load is continuous (NEC: 3 hours or more) or noncontinuous. IEC practice handles this through utilization factors and diversity rather than a blanket 125%.
- Device rating (In). Apply the continuous-load factor, then select the next standard rating at or above the result. Never invent a 37 A breaker.
- Conductor check (Iz). Confirm the cable's corrected ampacity is at least the device rating, and that the device still protects the cable.
The IEC version of step 4 is compact: Ib ≤ In ≤ Iz. The NEC reaches the same place through Article 210.20(A), 210.19(A) and 240.4.
Formulas
| Quantity | Single-phase | Three-phase |
|---|---|---|
| Current from power (W) | Ib = P / (V × PF) | Ib = P / (√3 × V × PF) |
| Current from apparent power (VA) | Ib = S / V | Ib = S / (√3 × V) |
| Continuous-load device rating | In ≥ 1.25 × Ib | In ≥ 1.25 × Ib |
| Conductor ampacity (NEC) | Iz ≥ 1.25 × Ib (continuous) | same |
| IEC coordination | Ib ≤ In ≤ Iz | same |
| Three-phase power from current | P = √3 × V × Ib × PF | — |
√3 = 1.732. For a 400 V three-phase supply, √3 × 400 = 692.8, so each ampere carries about 693 VA at unity power factor.
Standard ratings you are allowed to select
A calculated 37.4 A does not become a 37 A breaker. Both systems restrict you to preferred values.
| NEC 240.6 standard ratings (A) | IEC preferred ratings (A) |
|---|---|
| 15, 20, 25, 30, 35, 40, 45, 50 | 6, 10, 13, 16, 20, 25, 32, 40 |
| 60, 70, 80, 90, 100, 110, 125 | 50, 63, 80, 100, 125, 160 |
| 150, 175, 200, 225, 250 | 200, 250, 315, 400 |
NEC 240.6 also permits the next higher standard rating when the calculated value does not match a standard size, but only if conductor ampacity is not exceeded. IEC 60364-4-43 has no such allowance in the same form; the device must satisfy Ib ≤ In ≤ Iz directly, subject to the standard's tolerances.
Worked example 1 — 120 V continuous heater
A 1,800 W space heater on 120 V, treated as a continuous load.
- Ib = 1,800 / 120 = 15.0 A
- Continuous factor: 15.0 × 1.25 = 18.75 A
- Next standard rating: 20 A
- Conductor: 12 AWG copper at 60 °C is rated 20 A in NEC Table 310.16, and 240.4(D) limits 12 AWG to a 20 A device — so 12 AWG and a 20 A breaker is a valid pair.
Note the trap: the load current equals 15 A, which is exactly a standard breaker size. Using 15 A would mean a device rated at 100% of a continuous load, which 210.20(A) does not allow for this case.
Worked example 2 — 230 V water heater
A 3,000 W resistive water heater on 230 V, noncontinuous in the IEC sense.
- Ib = 3,000 / 230 = 13.04 A
- Next IEC preferred rating: 16 A
- Cable: 2.5 mm² copper with PVC insulation, two loaded conductors in conduit, is typically tabulated near 20 A under IEC 60364-5-52 method B2 — comfortably above 16 A.
- Check: 13.04 ≤ 16 ≤ 20 ✓
If the same heater were treated as continuous under NEC rules, 13.04 × 1.25 = 16.3 A would push the device to 20 A, and the cable would need to carry 20 A after correction factors.
Worked example 3 — 400 V three-phase load
A 30 kW balanced load at 400 V, power factor 0.90.
- Ib = 30,000 / (1.732 × 400 × 0.90) = 30,000 / 623.5 = 48.1 A
- NEC continuous factor: 48.1 × 1.25 = 60.1 A → next standard 70 A
- IEC selection: 48.1 A → 63 A preferred rating
- Cable: 16 mm² copper clipped direct, three loaded conductors, is typically in the high 60s of amperes under IEC 60364-5-52 — sufficient for 63 A but marginal for 70 A, so the NEC branch would usually need 25 mm².
The two codes land on different devices for the same load — expected, since they set continuous-duty margin differently.
Worked example 4 — motor branch
An 11 kW, 400 V motor, efficiency 0.90, power factor 0.85.
- Input power = 11,000 / 0.90 = 12,222 W
- Ib = 12,222 / (1.732 × 400 × 0.85) = 12,222 / 588.9 = 20.8 A
- Conductor sizing (NEC 430.22): 20.8 × 1.25 = 25.9 A → 4 mm² copper
- Short-circuit protection (NEC 430.52, inverse-time breaker): up to 250% of FLA = 51.9 A → a 50 A breaker is the largest standard rating allowed without engineering supervision
Motors are the case where the breaker is not sized to the conductor. The branch breaker protects against short circuits; overload protection is a separate function, provided by the overload relay. Sizing a motor branch like a heater branch is one of the most common design errors we see in imported drawings.
Common mistakes
- Sizing the breaker to the cable, not the load. The breaker must carry the load and protect the cable — both conditions, always.
- Forgetting 125% on continuous loads. A 20 A continuous load needs a 25 A device before any other factor, not a 20 A device.
- Choosing a non-standard rating. 37 A, 55 A and 110 A breakers exist in some ranges but are not NEC standard ratings for the general case.
- Ignoring ambient and grouping. A 40 °C panel room or six circuits in one trunking can cut conductor ampacity by 20% or more, which breaks the In ≤ Iz link.
- Treating an AC unit's MCA as FLA. Use the nameplate Minimum Circuit Ampacity for conductors and the Maximum Overcurrent Protection for the device; they are different numbers for a reason.
- Using the wrong voltage. 230 V single-phase and 400 V three-phase differ by 1.73; a decimal slip here produces a breaker one or two sizes off.
From calculation to a checked panel
A breaker size is only correct in context. The panel needs total load, diversity, busbar rating, and selective coordination between main and branch devices. GorkyCAD runs the load calculation and the 41-rule check on the same model that produces the single-line diagram, so a changed circuit updates the schedule and the drawing together. See the documentation for the rule list, or read how watts to amps conversion feeds this chain.
Try it on your own panel: open the demo and size three circuits end to end.
Verify against the current edition of NFPA 70 and your local amendments, or the applicable IEC 60364 part, before issuing a design. Breaker selection also depends on the device's listed rating and the manufacturer's coordination tables — this article is guidance, not a certification.
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Open demo See pricingFrequently asked questions
How do I calculate breaker size from watts?
Convert watts to amperes first: single-phase Ib = P / (V × PF), three-phase Ib = P / (√3 × V × PF). Multiply by 1.25 for a continuous load under NEC, then pick the next standard rating. Finally confirm the conductor's corrected ampacity is at least that rating.
Is a 20 A breaker enough for 2,000 W at 230 V?
2,000 W at 230 V and PF 1 is 8.70 A, so a 20 A device carries it easily. In IEC practice a 10 A or 16 A device would normally be selected instead, provided cable and load characteristics allow it. In NEC practice, 8.70 A noncontinuous allows a 15 A device; continuous duty would require 10.9 A and still allow 15 A.
What is the 80% rule for breakers?
It is the practical consequence of the 125% continuous-load requirement: a device loaded to no more than 80% of its rating for 3 hours or more. A 20 A breaker therefore serves a 16 A continuous load. The rule applies to the load calculation, not to the breaker's magnetic trip.
Does breaker size have to match cable size exactly?
No — the cable's corrected ampacity must be at least the device rating, and the device must be at least the design current. Oversizing cable is allowed; undersizing it relative to the breaker is not. Small-conductor rules such as NEC 240.4(D) impose additional caps.
Can I use the next standard breaker size above my calculation?
NEC 240.6 permits the next higher standard rating in specific cases when the calculated load does not match a standard size, provided conductor ampacity is not exceeded. IEC practice requires Ib ≤ In ≤ Iz directly. Always check the exact clause in the edition you are using.
How do I size a breaker for an air conditioner?
Use the nameplate: Minimum Circuit Ampacity for the conductors, Maximum Overcurrent Protection (or the listed branch-circuit selection current) for the device. Do not recalculate from cooling capacity — the nameplate already includes the compressor and fan currents.
Sources
- NFPA 70, National Electrical Code — Articles 210, 215, 240, 310, 430; Table 310.16; 240.6 standard ratings.
- IEC 60364-4-43, Protection against overcurrent — the Ib ≤ In ≤ Iz coordination rule.
- IEC 60364-5-52, Selection and erection of wiring systems — current-carrying capacity tables and correction factors.
- IEC 60947-2, Low-voltage switchgear and controlgear — Circuit-breakers.
- UL 489, Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures.
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