Cable Gauge / Ampacity
Definition
The current a cable can carry continuously, its current-carrying capacity (called ampacity in North America), depends on the conductor cross-section, the insulation, the ambient temperature and the installation method. Cross-sections are given in mm² under IEC 60228 and in AWG (American Wire Gauge, ASTM B258) in North America, where a smaller gauge number means a larger conductor: 12 AWG is about 3.3 mm² and 14 AWG about 2.1 mm², so 2.5 mm² lies between them. Capacity tables are published in IEC 60364-5-52 and its national versions (NF C 15-100, BS 7671) and in NEC Table 310.16. Grouping cables or running them in conduit lowers their capacity, because heat escapes less easily, and long runs must also be sized for voltage drop.
In the field
On long power runs, the cable is sized for voltage drop as well as for current. IEC 60364-5-52 recommends a drop of at most 3% for lighting and 5% for other uses in installations supplied from the public network, and the NEC suggests 3% for a branch circuit and 5% in total in an informational note. A 100 m single-phase run in 2.5 mm² copper carrying 16 A loses about 22 V, close to 10% of 230 V, which can make fixtures at the far end dim or restart.
Sources
- IEC 60228 : sections normalisées des âmes de câble en mm²
- American wire gauge (Wikipedia, pointeur secondaire) : ASTM B258 ; 12 AWG = 3,31 mm², 14 AWG = 2,08 mm² · en.wikipedia.org
- cabledatasheet.com, Maximum allowable voltage drop according to IEC 60364-5-52, annexe G : 3 % éclairage, 5 % autres usages (réseau public) · cabledatasheet.com
- EEPower, NEC Basics: Computing Voltage Drop, note informative 210.19 : 3 % circuit terminal, 5 % au total, recommandation non obligatoire · eepower.com
- Electrical Installation Guide, General method for cable sizing : IEC 60364-5-52, facteurs de groupement et de température · electrical-installation.org
- Calcul : 2 × 100 m × 0,0172 / 2,5 = 1,38 Ω ; 1,38 × 16 A = 22 V ; 22 / 230 = 9,6 %