Electric Fan

Reply #15 – May 18, 2007, 12:27:01 AM

Quote from: rbohm;147491

these number are for current draw while running. these fans pull between 80 and 100 amps at start up, thus you need a relay rated at 100 amps at least when using these fans. either that of get a fan controller that has a "soft start" feature. even then these fans will pull around 40amps on start up.

[COLOR="Red"]WRONG[/COLOR]
you got that backwards.,, and very illegal. YOU rate fuses/fuse links/ relays ect at 80 to 85 % expected max duty cycle amperage draw just as you would if you were to add a breaker in your home electrical box. We "under fuse" for a reason, we never over fuse for obvious reasons.
in your case you would have a relay in place that would survive wiring melting and a fan catching on fire before the actual "FUSE"(ie-relay) ever failed.

the relay is suppose to be the weakest link in the circuit aside from a possilble fuse in line with the coil.

The "instantanious" current draw you describe does not really count in this cirspoogestance because it only peaks from the point of instant power then quickly declines in draw. If a circuit breaker were used instead of a relay, perhaps the breaker would thermal or trip but relays deal with this momentary fluxuation with a little more patience than a fuse or a breaker would.
Over time, the relay is suppose to be the failing part. I dont know off hand what the relay amperage is on the M7 but i would not be the least bit surprised if it were at or about 35a. Thats a fairly high rating in general for vehicle applications.

then again, since i dont know the m7 relay size, i could be wrong.

Your "soft start" feature you mention is probably just an RC time constant combo that delays the fan coming up to speed quickly. IMHO,, this this would increase the time it takes for the fan to overcome the spike and increasing current draw for the first 1/2 to 1 second thus keeping you a max current draw for a longer period of time. Im not sure i like that idea,(unless its wired so the capacitor takes the beating).

there aint no way in hell a m7 fan relay is an oem 100 amp relay. If that were the case, you would have 6/awg wiring comign out of the relay.(depending on loop length of the power and ground distance)