Max Current Through Multiple Harnesses - Page 1
Forum section: electronics / beginners

author: omiks3
author_rank: Newbie
timestamp: September 30, 2021, 09:57:05 am
content: Hi,I was having a discussion with a friend and we both had differing opinions, so I would like to hear what other people have to say.When calculating the max current through a wire that travels through multiple connected harnesses as shown below. Should you only calculate for each harness run i.e. A→B, or for the entire run of cable i.e. A→D?Depending how you look at it you can arrive at a big difference in max current using the following formula:I = (A*V)/(L*r)Where I = Max Current (Amps)A = Wire Size (mm²)V = Voltage Drop (0.6V)L = Length (m)r = Resistivity of Copper (0.0175)If you plug these values in for the below circuit using 0.5mm² wire you get a max current of 5.714A per 3m harness. But if you take the entire circuit as a whole A→D the max current is 1.9AMy friend is under the impression that because the terminals in the connectors are rated for say 15A, that this allows the max current to be calculated in sections. I'm of the opinion that the entire wire run should be taken into consideration.Looking forward to seeing what people think and bonus points if you can link to some standards/guidelines.Thanks,

author: golden_labels
author_rank: Super Contributor
timestamp: September 30, 2021, 10:56:07 am
content: Both of you may be right, depending on a situation and the limiting factor. The current in a wire is limited by two factors: permitted voltage drop and heating.The voltage lost on a run of wire can’t be too large, because the load needs some minimum voltage. If the input is 14V and 5V is lost on a wire, the load sees 9V. If it needs 12V, it’s not ok. Therefore the first limiting factor is: Imax= ΔVmax·Rwire, where:ΔVmaxis the maximumallowedvoltage drop,Rwireis the total resistance of the wire. Since connecting resistances in series increases the total resistance, multiple harnesses have lower maximum current.The current carrying capacity determined by heat dissipation is a property of a conductor and is not directly dependent on its length. A 3× longer conductor dissipates 3× more power, but it does that over 3× larger area, so it doesn’t matter. In that case only the wire gauge and resistivity limit the current. You may look those up in various tables or datasheets from wire manufacturers. Looking from the perspective of that limit, it doesn’t matter if you have one or a hundred harnesses connected.HOWEVER, there is a catch. The tables/datasheets for current carrying capacity assume specific conditions. Those usually are: each piece of wire is thermally independent of any other. If you coil the wire, it can’t dissipate heat as well and that reduces the maximum current. If you have a bunch of harnesses that you tangle and pack tightly together in a tiny space, that will definitely limit how much current they can carry.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 02, 2021, 12:01:01 pm
content: Worse still, it depends on how cables are configured inside the harness. There is no correct answer.https://connectorsupplier.com/derating-wire-harnesses-can-achieve-weight-space-and-heat-goals/