Someone on here posted once that they measured their amp draw on high at startup and it spiked all the way to 70. I think it may depend on how healthy / old your fan is though.
I run 40-amp relays and my ECM controls them. It always cycles through low before going to high which means it is already spinning when high kicks on. It stands to reason that it shouldn't draw as many amps that way as it would starting from a stop and going straight to high.
BUT, I have also tried manually switching high on on many occasions and it has never blown a fuse or failed. For temporary reasons right now I only have the high side wired and my 40-amp relay seems fine with it. I bought my fan new though.
The more resistance there is the more amps it will draw. An old fan is going to have dirt, grime, and wear that will increase resistance and draw more amps.
I only recently bought a clamp meter, I will try to get a measurement on my fan hi and low soon ...
