Cable Sizing Mistakes to Avoid
Cables fail from heat, and heat comes from current in a cable that cannot shed it — because it was sized for the wrong current, the wrong conditions, or the wrong route. This guide lists the sizing mistakes that show up as hot cables, tripping protection, or voltage drop at the far end, and the checks that catch them on paper.
Sizing for nameplate current alone
The first mistake is sizing for the load's nameplate current without correction factors: ambient temperature, grouping with other cables, burial or trunking, and installation method all reduce what a cable can safely carry. A cable that is fine on a table at standard conditions can overheat in a hot, crowded trunking run. Apply the correction factors for your actual installation before choosing a size, and document which factors you used.
Forgetting voltage drop and starting current
A cable that is thermally adequate can still fail the installation on voltage drop, especially on long runs and motor starting. Check voltage drop at the farthest load under starting conditions, not only at steady load, and remember that a motor that cannot start because voltage sagged is a cable problem, not a motor problem. Short-circuit withstand matters too: the cable must survive a fault until protection clears it, which is a separate check from continuous current.
Grouping, routes and future loads
Cables grouped together heat each other, and a route that later gains more circuits derates every cable in it. Plan routes with spare capacity and record the grouping assumptions, so a future addition does not silently overload existing cables. And size for the load you will actually have, including foreseeable growth, rather than the load on the day of installation — replacing a cable in a finished building costs far more than one size up at design stage.
A sane cable sizing sequence
Determine design current including starting and future load, apply correction factors for temperature, grouping and installation method, check voltage drop at the farthest load under starting conditions, check short-circuit withstand, then choose the next size up if any check is marginal. Document each step so the next engineer can see why the size was chosen and what would change it.
If you want a second pair of eyes on your own numbers or drawings before you decide, see the remote transformer services — documents only, fixed price, fixed turnaround, no site visit. For the wider testing picture, start with the transformer testing and commissioning guide.