Cable current rating advice

You are potentially looking at two different scenarios here. The tables in BS7671 are based around a 40 year service life at full load. Automotive products are usually rated for 3000hrs (about 4 months continuous use). For commercial vehicles different values may be used.


When a cable carries a current the ohmic heating causes the temperature to rise. This may take some hours to reach it's maximum. The degredation of the insulation due to the maximum temperature sets the published service life.


If the cable carried a higher current for a shorter time it will not reach it's maximum temperature. The starter cables on a typical vehicle probably only carry current for 30 seconds every hour. Thermal degradation of the insulation is not going to be a problem. Volt drop is probably the key factor for selecting the size. The traction motor cables on an electric truck will only carry full rated current when climbing a gradient, that will probably be for only a few minutes followed by a decent (a decent is interesting as if regenerative braking is used the cables will still be under quite a high load) or level running at a lower current allowing the cables to cool.


Under given conditions a 16mm2 wire may be suitable for 125A or even 150A but the conditions would have to be specified.

You are potentially looking at two different scenarios here. The tables in BS7671 are based around a 40 year service life at full load. Automotive products are usually rated for 3000hrs (about 4 months continuous use). For commercial vehicles different values may be used.


When a cable carries a current the ohmic heating causes the temperature to rise. This may take some hours to reach it's maximum. The degredation of the insulation due to the maximum temperature sets the published service life.


If the cable carried a higher current for a shorter time it will not reach it's maximum temperature. The starter cables on a typical vehicle probably only carry current for 30 seconds every hour. Thermal degradation of the insulation is not going to be a problem. Volt drop is probably the key factor for selecting the size. The traction motor cables on an electric truck will only carry full rated current when climbing a gradient, that will probably be for only a few minutes followed by a decent (a decent is interesting as if regenerative braking is used the cables will still be under quite a high load) or level running at a lower current allowing the cables to cool.


Under given conditions a 16mm2 wire may be suitable for 125A or even 150A but the conditions would have to be specified.