DC Voltage Drop Calculator

How this is worked out

How is DC voltage drop calculated?

Resistance is ρ × length ÷ area, using 0.0172 Ωmm2/m for copper at 20°C. Copper rises by roughly 20% at 70°C, which is why the temperature selector is there: a cable that is working hard is not a cable at room temperature, and sizing at 20°C flatters the answer.

Why do you enter the one way cable length?

The current needs to go out and come back, so the cable in the circuit is twice the run length. Enter the one way distance and this doubles it for you. Working off the one way length is the most common mistake on how people end up with cable half of the size.

What does running two or more pairs of cable do?

Running more than one pair side by side is a common way to do a heavy run, and often easier than getting hold of one very thick cable, or they may already have plenty of that cable at hand. Two positives and two negatives is 2 pairs. They share the current, so the total resistance of the set halves, the current through each conductor halves and hence the total voltage drop halves. Set the number of pairs and the whole solution accounts for it, including the unsafe warning, which looks at the current in one conductor rather than the total. Keep the pairs the same length as each other, or the shorter one will take proportionally more of the current, as its resistance will be less.

What is an acceptable voltage drop percentage?

The targets are Waveinverter's, and they go by how voltage sensitive the transmission is, some links are very sensitive, and some links have a lot of leeway. That last part is what nearly everyone misses. Most people spec cable on current alone.

• Battery charging, under 1%. The most sensitive link there is. Here the cost of voltage drop is not the lost watts, it is a battery that never receives its correct charging voltage and wears out early.
• Solar inverter or inverter charger, under 1%. That cable pushes charge back down into charging the battery as well as drawing off it, so the stricter of the two applies.
• Battery to inverter, 2%. An inverter has a low voltage cutoff in its electronics, so an unstable supply, or even a momentary dropout, cuts it out, resets it and stops its AC supply.
• Pump or winch, 3%. A motor is far more tolerant of a dip. Take 12V down to 9V for a second and the pump just slows down, then picks up again when the voltage comes back. It doesn't cut out and stop the way an inverter does, so it gets a bit more room.
• Solar, wind or hydro generation, about 5%. The general rule, and where most runs sit, this one is not a hard rule, and it may be just the case for certain sites with extremely long transmissions. Your voltage drop may be much higher than this, and that could be acceptable, due to the cost of cabling.
• Non-critical lighting, 7%. The most forgiving of the lot, though a big drop will still dim them under load.

What is thermal runaway in a DC cable?

Unsafe is something different from too small. Too small is a voltage drop answer: the cable may work under some circumstances, it just wastes more energy than you want or will be unreliable. Unsafe is about the copper itself. A common reference point the industry works to is 10 amps per mm2 on copper at 20°C, so 100A on a 10 mm2 minimum spec for cable safety for simple maths purposes, and this page halves that to 5 A/mm2 when you tell it the cable is already at 70°C. Above that amp rate per square millimetre it becomes possible for the cable not to be able to conduct the increasing heat left in it fast enough. That heat causes the resistance to rise, higher resistance causes even more heat to be deposited in the cable at a faster rate even though the current is unchanged. This feedback loop is called thermal runaway. This effect can continue all the way up to melting the insulation around the cable and even the copper wires themselves eventually. No voltage drop figure makes it acceptable, which is why the unsafe warning overrides everything else on the page.

How much voltage drop does 6 mm2 cable have at 100A?

Take 100A on a 12V system over a 1 metre run. In 6 mm2 cable you lose 0.57V, which is 4.78% of your 12V, so 11.43V arrives at the load and 57W goes into heating the cable instead of doing anything useful. Go up to 10 mm2 on the same run and the loss falls to 0.34V, 2.87%, with 11.66V arriving. The 6 mm2 answer is also flagged unsafe before the voltage drop is even considered, because 100A through 6 mm2 is about 16.7 A/mm2, well past the 10 A/mm2 reference above. A cable can look acceptable on voltage drop and still be the wrong cable.

How many amps per mm2 is safe for copper cable?

That 10 A/mm2 is a common industry working reference rather than a calculation. Real current capacity depends on insulation, bundling, ambient temperature and how the cable is run, and a proper answer takes all of that into account. Use the flag as a hard stop, not as a licence to sit borderline with it. Note too that some of our cables make reference to a higher rating for duty cycle work, where the load comes on and off and the cable has time to cool between loadings. That is a different question from continuous current and this page does not attempt to model it. If the run matters, tell us what you are wiring and we will size it properly.