# How to size wire gauge for BLDC motor?

How can I choose a safe wire gauge for a motor? I know the power output is given by power = current * voltage, but in most cases the current will be relatively low due to back-EMF, is that correct?

In that case, how can I determine how much current the motor will draw normally? Is there a rule of thumb of what is the max current draw, based on nominal power and voltage?

I see motors for electric bikes and cars with nominal current (derived from power/voltage) over 100 amps, but the wires don't seem very thick. Judging from AWG tables, it would take some very beefy wires to handle that much current. Is that because in most cases the motor will draw much less than the nominal current?

• Can you provide a spec sheet for the motor in question? I am curious to see the spec sheet for a "100 Amp" motor for an electric bike. Commented Sep 22, 2016 at 10:44
• It can be difficult to wind a motor using large diameter wire. Typically, a bundle of smaller wires is used. Commented Sep 22, 2016 at 13:23
• FiddyOhm, it's not one motor in particular, just an example. But i was looking at this DIY motor, google "masinaeletrica 45kw", that draws a lot of current and the wires don't seem very thick Commented Sep 22, 2016 at 14:01

The current in the motor phase leads is usually greater than the current from the DC supply. You can calculate the RMS phase current if you know the torque constant $K_t$ of the motor and the torque you intend to operate at, and you can calculate $K_t$ from the voltage constant $K_v$ if you know that. If you don't know $K_v$ you can determine it by measuring the back EMF voltage produced at the motor terminals when it's rotated at a known speed while disconnected from the controller.

Many hobby R/C aircraft use quite aggressively thin wire sizes for both DC and motor phases. They get away with it because the wiring is exposed on the frame rather than bundled, and has air moving over it. It's worth it because copper is expensive and heavy!

Take load DC resistance of motor or load and choose wire resistance <2%. Better is <1% for efficiency and starting torque. But wire guage may be selected to reduce peak currents and strain on battery too.

AWG resistance is measure in Ω/m or Ω/mm or Ω/km .

Thus a 100A surge to a 15A motor will lose max torque ( not speed) and Bass woofers in speakers will lose bass response stiffness (muddy drums) measured impedance ratio or dampening factor if wire gauge is not <1% of DCR of speaker.

Purist high power audio listeners expect <0.1% source and wire impedance.

• Voltage drop on the wires means less effective voltage across the motor, which results in a loss of both max torque AND max speed.
– Eric
Commented Sep 22, 2016 at 12:46