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Help: Wire sizing in regards to inductive loads...

Practical Motorsport Wiring - Club Level

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I need assistance in regards of sizing for my wiring.

i am trying to size the wiring for the generator and the coils. Both are inductive loads which might change the idea of the maths..

What we are talking about?

750 ccm 4 cylinder engine with nearly 12000 rpm max

Ignition is wasted spark.

Wich fakts do we know for shure?


primary Resistance: 1,8 - 2,8 Ohm

secondary Resistance 10 - 16 kOhm


Anker 1,0 Ohhm

Rotor 2,3 - 3,5 Ohm

Max Current built by the Generator: 30,7 A; 14V at 6000 rpm


Did the maths with the smallest Resistanance to get the biggest current:


primary 12V/1,8 OHM=6,6...A

secondary 12V/10 Ohm=1,2 A


12V/1 Ohm= 12A

12V/2,3 Ohm=5,2A

Is this also true for inductive loads?

Next question: I`ve been told that the max current should`nt reach more then 50 percent of what the wire could do. Would you suggest to do so?

Which awg would you suggest for the wiring from the cdi to the coil? Wich awg would you suggest for the generator?

Thanks in advance!

Pretty sure because it's DC, the wire sizing wont change depending on the amperage for inductive loads.

With your coilpacks, don't forget to consider their duty cycle too ;)

I always get stressed when sizing wires for generators and alternators, especially over long distances and will end up just copying the OEM

With regards to the generator / alternator, I don't think you need to consider it as a load, or inductive load. Think of it as a black box that contains the mystery AC bits, the recifyer and the regulator. Out of this box we get a magic DC supply that is already regulated to 14V and can supply up to 30A when needed. Put a good wire on it straight to battery + and probably a 30A or 50A fuse.

You can use first principles to calculate an inductive load, but I think there are too many dynamic variables for normal people to calculate this. You would need to know the number of windings inside the coil for example, and then consider how fast it is switched on and off. Rather than calculate, it's probably better to just do a real test and measure the current draw.

OEM wires from your motorbike are probably 18AWG for most things, maybe some 16AWG and 14AWG wires for power.

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