Cable power dissipation check to handle load

ronyk212

Joined Nov 7, 2017
2
Hi

I have a 25A, 250W, 5V Low voltage power supply delivering a load of 85A over two sets of cables:
16mm2 of length 30m from Low voltage power supply to a filter box with capacitors and from there 3m of 2.5mm2 cable to the load of 85A.
the copper linear resistivity Rho = 17. 10e-9 W.m,

Can anyone tell me what criteria or information i need to check if the power dissipation in cables is ok to handle the 85A load?

LesJones

Joined Jan 8, 2017
4,171
You figures are very confusing. 5 volts at 25 amps is 125 watts (NOT 250 watts) A 25 amp power supply cannot supply 85 amps. You are quoting resistivity in units of watts metre. the units of resistance are ohms.

Les.

ronyk212

Joined Nov 7, 2017
2
Sorry let me correct.
The previous question had specs of old LV power supply, and is 8V not 5V.

I have a new 110A, 8V Low voltage power supply delivering a load of 85A over two sets of cables:
16mm2 of length 30m from Low voltage power supply to a filter box with capacitors and from there 3m of 2.5mm2 cable to the load of 85A.
the copper linear resistivity Rho = 17. 10e-9 ohm.m

Is it clearer now

Picbuster

Joined Dec 2, 2013
1,047
You have two options:
1: look in a table for sq mm current per meter.
2 : calculate
Work out the voltage lost you will accept at full load.
Measure the distance over the cable ( 2 x hence up and dwn).
Calculate the resistance.
Find the resistance per sq mm per meter for that cable material.
Calculate the needed sq mm for that length.

I will use for 5V 25 Amps length 1 meter a cable of 6mm sq.

Picbuster

LesJones

Joined Jan 8, 2017
4,171
Without considering the heating effect you would not get 85 amps with the ends of the 30 metre run of 16 mm^2 cable (60 metre of conductor.) shorted together. the current would only be about 72 amps. I know that 2.5mm^2 cable (PVC twin and earth) is only rated at about 27 amps (From memory.)

Les.