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Old 09-16-2006, 01:53 PM
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Electric fan amperage/wiring

I just purchased a 3000cfm electric fan to cool my big block chevy. When I hooked it up using a relay and 30 amp fuse it blew the fuse after my voltage gage went from 14 volts to barely above 12. I replaced the fuse and it did not blow but it seemed to pull alot of current. I hooked up the amp meter and it reads 28 amps with voltage gage reading 12 volts this will drain my battery if I run it constantly. Question: Is this too much current for an electric fan? I am running a 100 amp altenator and that should be plenty, but is there something im overlooking?

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Old 09-16-2006, 03:26 PM
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electric fans normally runs on demand, turn off and on for stop and go traffic
so will not be drawing current all the time. steady cruising supposedly will have enough air flow through the radiator so not need fan

consider using
40 amp maxi fuse
relay - 70 amp (P&B VF7) power connector is 3/8" compared to 1/4" for 40 amp VF4 relays both look the same
visit http://relays.tycoelectronics.com/auto.asp for relay descriptions

Last edited by naka; 09-16-2006 at 04:01 PM.
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Old 09-16-2006, 04:47 PM
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Doc here,

Fans are CURRENT hogs..

If the fan is drawing 28 amps (a bit high..but not too much) then the start spike is a Multiple of that..

The Spike time is that time Which The fan must overcome gravity and outside resistance force..It must draw more current to start the rotor turning from zero RPM'S until it reaches about 1725 RPMS..as the RPM curve inclines, the Current Curve Declines. This is why a PROPER Fuse link and relay are necessary, as well as properly gauged wire.

If the Fan , for instance Sucks up a plastic bag, it can rotor lock and draw as much as 100 amps before something lets loose..you want to be sure the Harness, relay and Protection devices are up to the task.

To Install a proper relay:


  • #85 to the temp sender.
  • #86 to a switched , fused 12 volt source. about 2 amps will do.
  • #87 To the Battery, Via a proper Fuse link. This wire should be about 10 Ga.
  • #30 To the Fan Motor. The wire also should be about 10 ga.
  • #87a will not be used.

This Configuration Should enable you to deliver the maximum amount of current from the source (Battery) to the load (fan) without Heat loss through the wire or devices..providing more available current to do more useful work, and less danger of meltdown and fire.

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Old 09-16-2006, 06:09 PM
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AHHH YES, That is exactly right doc, when the fan came on it would spike the ammeter to 60 amps. I had used 12 gauge wire initially and noticed it was warmer than it should have been, that explains everything. Do you know of a device that will limit power initially then apply full power after it gets moving to help reduce amp draw at turn on. I really want to use this fan because it moves air like you wouldnt believe, more than enough to cool my BBC.
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Old 09-16-2006, 06:33 PM
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Doc here,

Just by installing the larger wire and relay, will help cut the draw a bunch..

other than that, there is the EXPENSE of an automatic Variable speed DC motor control..but that is Wayyyy overkill in my opinion.

ANY motor started at Full potential will spike on startup without a speed controller, (Slow to start and steps to full speed.)

It's the protection device , wire gauge and relay switching device that must be made to adapt (within $$ Reason)..

Why we Advocate the Fuse link of high amperage, it won't open on a start spike (as any standard fuse would, it takes time to heat and melt a link) and The wire gauge ALWAYS is installed HIGHER than the ampacity of the load worst case, but slightly Less on the fuse link.. (your protecting the harness NOT the fan..Common misconception.)

Of course you want to use a relay whose Contact rating is up to the task as well , so the contacts won't pit, burn or weld.

Go with the big relay, and the upgraded wire (10 ga at least) and I think the numbers your getting will come way down..Probably, Half that current was wasted generating HEAT in the under rated components / Wire, and doing NO useful work.

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