Wire harness

[GregH]

Can someone point me to the thread about wireharness current loads? Been reading all day and getting glimpses of answers...but I must be missing something

I've been trying to understand why the red wire from the solenoid to batt on the switch has to be 8awg. I can understand if one is still running an ammeter, but if one isn't, couldn't it be 12awg to match the next smallest wire size being used?

Also, once wiring drops to 14awg shouldn't that new sized run also be fused(ex. Purple wire)?

[ndutton]

Conductor size is based on two factors: load and length. The loads on the red wire (batt to control panel) include

• Engine ignition system ~ 4 amps
• Often the exhaust blower ~ 5 amps
• Gauge power ~ nominal
• Gauge lighting ~ 1 amp
• Starter solenoid coil ~ 10 amps

Total = 20 amps, assumes a panel voltmeter or a shunt ammeter, mechanical fuel pump and engine driven water pump(s).

The length factor has to do with the inherent resistance of copper wire which reduces the voltage for every inch of length, factors being wire gauge, length and amperage. Voltage drop for essential circuits should be held to 3% and there is a wire capacity calculator to that end in the USCG Standards category on the forum. If your engine wiring harness is 5 feet, according to the calculator you'll need a minimum of 10 gauge wire to achieve the 3% standard. The longer the harness, the bigger the wire needs to be. At 10 feet in length, even 8 gauge is marginal (3.73% drop) but considering the intermittent nature of the starter load, manageable.

Note that different calculators provide slightly different results but they're all pretty close. As for fusing, the fuse protects the wire. A fuse can be smaller than the wire ampacity but not larger. According to the ampacity chart provided by Blue Seas, 14 gauge wire with 105°C insulation rating is good for 21 amps outside the engine space, 17 amps inside. If fusing each conductor to it's maximum rating you would fuse the 14 gauge purple ignition (or larger depending on voltage drop considerations) wire separately from the 8 gauge red batt wire. However, nothing says the #8 red wire must be fused exactly at its maximum.

edit:
Using my installation as an example of seemingly unnecessarily large wire, my engine wiring has all the above plus an electric fuel pump and two electric FWC circulation pumps. With a single circ pump and the panel located in the aft cockpit (long run) my purple ign wire was 12 gauge, voltage drop was barely under 3% by measurement. The addition of a second circ pump pushed it over the 3% threshold so the purple wire was increased to 10 gauge bringing the drop back within spec.

[joe_db]

The starter itself is 120-130 amps.

[ndutton]

Joe, my understanding was Greg was asking about the engine harness, not the high amperage battery and starter cabling. My comments included the keyswitch/starter button to solenoid coil amperage.

[GregH]

Originally Posted by ndutton

Conductor size is based on two factors: load and length. The loads on the red wire (batt to control panel) include

• Engine ignition system ~ 4 amps
• Often the exhaust blower ~ 5 amps
• Gauge power ~ nominal
• Gauge lighting ~ 1 amp
• Starter solenoid coil ~ 10 amps

Total = 20 amps, assumes a panel voltmeter or a shunt ammeter, mechanical fuel pump and engine driven water pump(s).

These were the numbers I was trying to find/figure out thanks! And along with https://www.bluesea.com/support/refe...e_Sizing_Chart i figured out where I was having my mental block