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Aftermarket G car cooling fans

11K views 44 replies 15 participants last post by  snowrx 
Szing Wire For The Load

Once you get over about 30 MPH the fans are no longer important.
I can't help myself . . . once you get under about 30 mph, fans are pretty important :grin2:

The fan circuit in the car originally is what, 40 amps? (The rating of the relay) So I'd think you would be OK.
Not necessarily (and very unlikely if you're inferring the circuit can handle 40 A). The amperage rating of a relay does not indicate the current carrying capacity of a circuit. Relays are typically sized in excess of the load they are expected to switch to provide a measure of reliability and safety. For example, a typical pair of 55 W headlights might only draw about 8 A but are commonly switched by a 40 A relay.

@Jenna'sEspirtTurbo and @mcfly007 - What follows is an explanation of how you figure out whether or not existing wire will safely and efficiently carry a specified load - or - whether you need to refit a larger wire gage.

Sizing Wire
You can determine an appropriate wire size for a circuit if you know the system voltage, the load and the length of the wire. Conversely, you can also look at existing wiring and determine the maximum load it can carry. A little math is involved . . . you'll need Ohm's Law. It defines the relationship between volts, amps and resistance where

I = amperes
V = volts
R = resistance in Ohms (Ω)

I = V / R or V = I x R or R = V / I (these are all algebraically equivalent)

Also useful to know is that a Watt (W) is a unit of power where

V x I = W or I = W / V

Here is an example of calculating an appropriate wire size for a 200 W set of driving lights that uses 20' of wire. All wire specifications are approximate (more about that later). I use a nominal system voltage of 13.8 V and want no more than a 0.5 V drop for a continuous load. The goal is to find a wire size that provides an acceptable voltage drop and can also safely carry the amperage.

The 200 W lights draw this many amps
200 W / 13.8 V = 14.5 A

Let's evaluate 12 AWG wire for this circuit. It has a resistance of 1.88 Ω per 1000'
1.88Ω x 20'/1000' = 0.0376 Ω for 20' of wire.

The voltage drop across the 20' of 12 AWG wire is
14.5 A x 0.0376 Ω = 0.5452 V
So the voltage drop is over 0.5 V and not acceptable.


Now let's evaluate 10 AWG wire for this circuit. It has a resistance of 1.10 Ω per 1000'
1.10 Ω x 20'/1000' = 0.022 Ω for 20' of wire.

The voltage drop across the 20' of 10 AWG wire is
14.5 A x 0.022 Ω = 0.319 V
So the voltage drop is under 0.5 V and acceptable. Additionally, 10 AWG wire is rated for a maximum of 33 A when it is bundled with other wires: the lights only pull 14.5 A, so the wire size is also okay for the expected load.

Wire Size Calculator
If you don't want to do the math, there are on line calculators that will do the work of sizing wires for you. Here is one example I stumbled on via Google. When we plug in 13.8 V, a 14.5 A load, 20' of wire and a 2% voltage drop (0.28 V), it recommends 8 AWG. For a 5% drop (0.69 V), it recommends 12 AWG. So it is producing very similar results to the values I used in the examples above (where a 0.5 V drop is about 3.5%).

Use The Right Wire
I use GXL-grade wire when I wire things in and around engine compartments. This is not the cheap vinyl insulated wire that is the only thing most automotive parts stores sell - you'll need to get GXL wire from an industrial supplier. GXL meets an SAE standard (J-1128) aimed at applications such as automotive use. The cross-linked polyethylene insulation is oil resistant and designed to withstand the high temperatures in engine compartments (up to 125°C / 257° F). You can also use SXL-grade wire: it has a heavy insulation thickness making it a little more difficult to route when compared to the medium wall of GXL.

Good wire manufacturers publish data for their products, including Ω resistance per 1000' of length. A quick search reveals, however, that not every manufacturer makes it easy to find. Luckily, there are lots of generic sources out there that will get you very close.

Finally, I don't know if an Esprit uses AWG or metric wire gages. You'll need to convert back and forth if you're evaluating in-place metric wire. There are charts available that show a conversion between AWG and metric, but most typically show the actual wire cross-section area in square millimeters rather than the next larger standard size metric wire (such as 10 AWG ≈ 6.0 mm, 12 AWG ≈ 4.0 mm and so on). See IEC 60228 for standard metric wire sizes.

Glen
 
Spal Specs

Spal doesn't list draw or wire requirements for their fans, just voltage requirement.
Spal wouldn't list an appropriate wire size because they don't know the system voltage or the wire length - both these things must be taken into account to correctly size the wire. Spal does list the amperage for some of their fans, it's just a little hard to find. Here is an example:

Spal USA site

Spal Specifications Page (click on 'Fan Technical Documents)

Spal VA11-AP7/C-57A specification

Their amperage specs are at 13 V, so the amperage would drop in a 13.8 V system. Also note that they use a comma for the decimal point instead of a period.

Glen
 
Tech

It doesn't really matter to me how any particular person goes about maintaining their car. Do what you will to the best of your ability. Hopefully you'll end up with a satisfactory result and enjoy knowing that you've done a good job.

But it does matter to me when I see the pursuit of maintaining gorgeous and significant vehicles - like the Esprit - being dumbed-down.

I wouldn't worry about the math. If the circuit was able to handle the OEM fan motors it will be more than capable of running the Spals. They draw less current and move more air. Of course this assumes the circuit (the wiring, fuses, relays, and connectors) are all in good condition. If modifying any wiring use the same gauge wire as is already there.
The math really isn't that hard. The "algebra" is nothing more than simple multiplication and division. David - you had to do some math just to be able to say with any authority that the Spals pull less amps (since the Spal spec uses a non-standard 13 V benchmark). Replacing old wire with the same size without evaluating its current carrying capability would only serve to perpetuate the penny-pinching decisions Lotus made decades ago. Why not do the math so you know how close Lotus was running the wire size to the margins?

. . I get the idea of going cheaper to stay financially solvent . . .
All automotive manufacturers are concerned about cost, and reducing the size of copper wire to the absolute minimum is a very common way to cut costs. I'm sure you can imagine Lotus doing this very thing to save money.

I just replaced the factory headlight wiring on my 1990 VW Corrado - they were far too dim to drive at 75 mph on the interstate at night. In their day, these cars sold for the same as an Audi A4 in today's dollars. VW was selling them at a loss while trying to establish a market: they cost twice as much as the competing Golf. The headlight circuit was over 12' of 0.5mm/20AWG wire that was controlled solely by the switch on the dash. The wiring alone, in its pristine factory condition, caused a 1 V drop (≈30% loss of lumens from the bulb) and consumed 8 W of power. Numerous oxidized spade connectors as well as the dash-mounted switch added even more losses. I didn't need to replace bad headlights to see where I was going, I simply needed to replace the bad choices VW made in wiring.

So David, I have no doubt you've seen lots of examples of this particular car and can infer that the wire size is adequate. But do the OEM fans die or end up being inadequate because they were cheap or simply because they were pulling too many amps (and running hot) at the low supplied voltage? If you don't do the math, you'll just be guessing about what's going on or maybe just hoping this was the one circuit where Lotus didn't go "cheaper to stay financially solvent."

For anyone else reading this thread years after it was posted, I'd encourage you to learn all you can about doing things the right way. Suffering an electrical fire or sitting on the side of the road waiting for a tow with all your coolant on the ground: that's a false economy just to save a little money on wire or the time to multiply a few numbers. In this age of the internet and instant gratification, it's all too easy to take what appears to be a shortcut and go directly to some (any) answer. Learn how to derive the answer yourself and take pride in technical know-how instead of denigrating it.

Glen
 
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