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.
- 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.
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.