Derating — Electrical
Definition
Derating is the practice of using an electrical component below its rated capacity so that it stays within a safe operating range, taking account of load duration, ambient temperature and cable grouping. The rules differ by region. In North America, the NEC (NFPA 70, sections 210.19(A)(1) and 210.20(A)) requires the overcurrent device and the conductors of a circuit to be rated at no less than 125% of any continuous load, meaning a load that lasts 3 hours or more, unless the equipment is listed for 100% operation. A continuous load is therefore limited to 80% of the rating, for example 16 A on a 20 A circuit, and the Canadian Electrical Code (Rule 8-104) applies the same principle. IEC 60364 and its national versions, such as NF C 15-100 in France and BS 7671 in the United Kingdom, contain no 80% rule. They require the design current, the device rating and the cable capacity to satisfy Ib ≤ In ≤ Iz, with the cable capacity corrected for temperature and grouping under IEC 60364-5-52.
In the field
Grouping reduces the current a cable can carry in every system. Under the NEC, four to six current-carrying conductors in the same raceway are derated to 80%; under IEC 60364-5-52 (BS 7671 Table 4C1), two circuits bunched together take a factor of 0.80 and three a factor of 0.70. A breaker's thermal trip also reacts to ambient temperature. An IEC 60898-1 breaker rated 16 A carries its full rating continuously at 30 °C, yet a 15.5 A continuous load in a warm dimmer rack can eventually trip it. Leaving a margin of about 20% is therefore common practice outside North America too, even where no rule requires it.
Related lessons
Sources
- NFPA 70 (NEC) 210.20(A) et 210.19(A)(1) : protection et conducteurs ≥ 125 % de la charge continue (≥ 3 h, Art. 100), soit 80 %, sauf matériel listé 100 % · iaeimagazine.org
- Code canadien de l'électricité (CSA C22.1), règle 8-104 : même principe de charge continue · iaeimagazine.org
- NEC Table 310.15(C)(1), plus de 3 conducteurs actifs : 4–6 → 80 %, 7–9 → 70 %, 10–20 → 50 % · helukabel.us
- IEC 60364-4-43 §433.1 (= NF C 15-100 §433.1, BS 7671 Reg. 433.1.1) : Ib ≤ In ≤ Iz et I2 ≤ 1,45 Iz, pas de règle des 80 % · professional-electrician.com
- IEC 60898-1 : température de référence 30 °C, non-déclenchement à 1,13 In, déclenchement à 1,45 In en moins d'1 h · se.com
- IEC 60364-5-52 tableau B.52.17 (BS 7671 tableau 4C1), groupement : 2 circuits 0,80, 3 circuits 0,70 · electrical-installation.org