How much current draw.....?

Does it take to run an Rv8? I'm looking at installing a thermo fan (10amp max) and remote coolant pump (20amp max) and I want to make sure I have an alternator up to the job. I have a brand new 70amp sitting 'in stock' and I would rather use it than buy another one. What current draw would the ignition system, fuel pump, headlights and small stereo pull?
Handy little online calculator says 75 watt bulb will draw just over 6 amps. I assume that's 6 amps each as they're in parallel, so lets say 12 amps for the highbeam.
So we're up to 42 amps. I imagine the instrument lights wouldn't draw much and about .5 amp per 5w bulb....
two (at a time) indicators at 12w will be an additional 3.5 amps.....
Brake lights also 21w x 2 is another 3.5....
A typical stereo draws about 5 amps.
Fuel pump about 5 amp...

So if the fan/water/fuel pump are running, at night with the high-beam, indicators and brake lights are on with the stereo at full tilt I'm looking at between 60-65 amps.... can that be right?

I'm assuming the ignition will draw more than 5 amps so maybe I do need a bigger alternator after all.......??


Active Member
I doubt the normal ignition will draw 5A. A 10A thermo fan isnt going to push much air, but depends on fan size. I would find an example of an electric coolant pump doing the job before going there. Around 2005-6 did a test on the Davies-Craig EWP, and DC slapped an injunction on them to stop publication - the relevant page has now been removed from the site. I make the high beam draw 21A (2 x75W + 2 x 50W=250A/12), not 42A. Your 70A alternator should be fine, as long as all your connections are good, especially the earths, and you put relays close to the loads to unload the std fuse block.
For current checking, I use a meter like this on cars with blade fuses.
Auto Current Tester | Jaycar Electronics
For cars without blade fuses (ours!) get a fuse adapter like this - type to suit the meter ...
MUYI 2 Sets Inline Fuse Holder 10 Gauge Waterproof Pigtail Blade Fuse Holder with 40AMP ATC Fuses : Automotive
Bare the ends of the fly leads to connect to the Lucas fuse holder ends, with fuse removed. Finding high currents where they shouldnt can be very useful.
Awesome, thanks for the advice.

I had a long conversation with the dude from Davies Craig and I've already bought the pump, so I sure hope it does work lol. For a fan, I'm not 100% sure yet - it depends on space & placement, probably going a single 16" or two smaller - not sure which way is best.

oh - I said up to 42 amps as I was adding 30a draw for the fan+pump as well. So it would be 51 amps by your maths?
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Active Member
Single 16" fan would suit the rad shape best IMHO, but its going to be tight getting one in there.. Yes, 51A, but if I understand the EWP logic, it wont run all the time, nor will the fan.


Well-Known Member
A single large fan every time. 2 fans are inefficient. Work out the area swept (i.e. minus the voids for the motor and the surround) and you'll soon see why. Two fans will barely sweep half the area. And whatever you do, do use a 2 speed thermoswitch, you simply don't need full power in most cases (and for me in every case so far up to 37 degrees) and IT WILL BE VERY LOUD IF YOU DON'T. And as stated up front in a car, you need quality with sealed bearings because it will not live long if you don't. I used a SPAL one which AFIAK is the largest that'll fit without cutting or bending anything.

Like what's already been said, these are not constant loads, your battery is the buffer - so forget brake lights and indicators. Remember people run electric PAS which draws more than any of these - again this may look scary (40 amps) but only draws under load i.e. when you make steering inputs. Which is why modern cars all use it - it's more efficient than having a mechanical pump running all the time as well as easier to package.

I think 70-80amps for the alternator is about as much as you can run without needing to look at some of the main wiring also. I've seen compatible types up to 150amps but I think you'd be looking at a major rewiring job.

TBH your only challenge is the water pump, I'd wire that to the terminal block on the floor directly with whatever control and relay required. I'd also put in a warning lamp!

Is this all really necessary? It seem a bit OTT in the cooling department. 3 core radiators and big, front mounted fans are known to work well and the obvious easy gain of an expansion tank. Some also like waterless coolant. I only have the fan and tank, not even close to having temperature problems.
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Active Member
I have fitted a hot power point up near the power brake, copied from a Porsche example.

The brown lead on the right goes to the main battery connector on the starter, the two red leads come from the 90A alternator - pretty robust unit. Since the Porsche battery (928) is under the spare in the boot, this post is the official jump start connection, so its current capacity is high.
Advantage is the bakelite base is mounted to the inner wing, the metal post threads into a captive M6 nut inside the bakelite, and there is a screw on cover that protects against accidental shorts. Its tapped off the big feed to the light switch from the shunt, protected by a 40A resettable breaker. I am feeding directly to the off side headlight relays , and another lead goes across to the nearside headlight relay set. Easy to undo, and add a new tap with a ring terminal.
Anyway, the point of my rant/ramble is that hot points (that are safe) up near the front of the engine bay are VERY useful!
Also, re radiator size , this link may be of interest.
What size radiator do I need for my car? - Super 7th Heaven
I have also seen some tests (Sunbeam Alpine US club) indicating the importance of closing the gap between the bottom of the radiator and the front crossmember to prevent air recirculating through the gap and around to the front again in slow traffic.
Is this all really necessary? It seem a bit OTT in the cooling department. 3 core radiators and big, front mounted fans are known to work well and the obvious easy gain of an expansion tank. Some also like waterless coolant. I only have the fan and tank, not even close to having temperature problems.
It would be a bit OTT in a P6....but this engine is going in a Mk2 Cortina, so less rad area & space in general...I MAY be able to make room for a larger radiator, but I'm not up to that yet.