Enter the cross-section, length, current and system voltage. The tool returns conductor resistance, absolute voltage drop, percentage drop and resistive loss — for copper or aluminum, at the operating temperature you expect.
Voltage drop is the quiet constraint. A bar can pass every temperature-rise check and still deliver too little voltage at the far end of a feeder, especially at 48 V or in long rack distribution. The calculation is simple; the discipline is in using the right length, the right temperature and the right return-path factor.
For a homogeneous conductor, resistance is R = ρ(T) · L / A. Voltage drop is the current multiplied by the resistance of the whole current path. In a two-wire DC or single-phase circuit the current goes out and comes back, so the path is twice the one-way length: ΔV = 2 · I · R.
In a three-phase circuit the return path is the other two phases, and the line-to-line drop becomes ΔV = √3 · I · R for a balanced load. Using the single-phase factor on a three-phase feeder overstates the drop by about 15%; using the one-way length understates it by half. Both mistakes show up in real specifications.
Resistivity is temperature-dependent. At 20 °C, copper is about 1.724 × 10⁻⁸ Ω·m and aluminum about 2.82 × 10⁻⁸ Ω·m. A bar running at 90 °C carries roughly 27% more resistance than its 20 °C value, which is the number that matters for a feeder sized at the temperature limit.
The tool returns the resistive drop of the conductor only. It does not include joint resistance, contact resistance at the terminals, or the drop across a connector interface. A bolted joint in good condition adds micro-ohms; a poor one adds milliohms and heat.
It also assumes a uniform cross-section and a single conductor carrying the whole current. Parallel bars share current according to their resistance and their position in the enclosure, so two bars in parallel rarely carry exactly half each.
Use the output to choose a cross-section, then confirm the choice against the temperature-rise limit, the short-circuit withstand and the installation clearance. If the drop is close to the limit, the answer is usually more copper or a shorter run, not a higher voltage class.
A 30 × 5 mm copper bar has a cross-section of 150 mm² and a resistance of about 0.115 mΩ per metre at 20 °C. At 200 A, one metre of two-wire path drops about 46 mV — negligible against a 48 V rail, but 0.1% of a 48 V system per metre adds up over a 10 m rack run.
At 800 V and 400 A, a 400 mm² bar drops about 0.19 V per metre of two-wire path, around 0.024% — which is why high-voltage DC distribution tolerates long runs that would be impossible at 48 V.
The pattern is the point: voltage drop is a low-voltage problem first. If you are designing a 48 V or 400 V DC rack, size for drop; if you are designing an 800 V traction or charging bus, size for temperature rise and short-circuit withstand.
Send the current profile, voltage class and installation conditions — we will size the busbar and document the calculation.
