wire gage questions

N

nimrod

New member
Previous to my ownership, the batts in my boat were moved to under the aft galley seat. In doing so, two SAE 8 gage wires were run from the "common" battery switch post to the (+) side of a fuse block in the lazarette. The length of the cable run is about 12 feet.

I'd like to replace those two wires with a single marine-grade cable. What would the appropriate wire gage be?

Also, the charging cables from the main outboard (115 Yamaha with 25 AMP alternator output) were spliced as well. The (+) cable was spliced to an SAE gage 2 cable, and the (-) cable was spliced to an SAE gage 6 cable. The length of this run is about 18 feet.

Is this the appropriate wire gage for this length of run and should the gage of the cables be the same?

jd
 
Do a Google search. Lots of ways to calculate wire gage if you know how much current it will carry and the voltage. It's pretty simple. Based on your info, I'd think a #6 for each would be sufficient.

Charlie
 
nimrod":iusugrma said:
Previous to my ownership, the batts in my boat were moved to under the aft galley seat. In doing so, two SAE 8 gage wires were run from the "common" battery switch post to the (+) side of a fuse block in the lazarette. The length of the cable run is about 12 feet.

I'd like to replace those two wires with a single marine-grade cable. What would the appropriate wire gage be?

Also, the charging cables from the main outboard (115 Yamaha with 25 AMP alternator output) were spliced as well. The (+) cable was spliced to an SAE gage 2 cable, and the (-) cable was spliced to an SAE gage 6 cable. The length of this run is about 18 feet.

Is this the appropriate wire gage for this length of run and should the gage of the cables be the same?

jd
Click to expand...

Hi JD,

You have a 24' run for your battery cable (it's a round trip). In order to handle 35 amps (that's the typical main breaker) you should use #6 if you want to keep the circuit down to about a 3% voltage drop. You can go as small as #12 from a pure ampacity standpoint but you'll have up to a 10% voltage drop.

The cables don't need to be different sizes on the engine harness, someone didn't understand the nature of a "complete circuit" and obviously made the positive wire larger to "handle the load".

The main load from the engine will not be the alternator but the starter, you want to make sure you'll have enough power (voltage) at the starter to get the starter spinning at the right speed (and not to draw too much current).

I suggest finding out what the starter draws or at least what size fuse is inside the engine to protect it. I know on the Honda BF90 it's a 90-amp fusible link type. Based on that for the 36' round trip I'd suggest at least #6 (this is allowing for a 10% voltage drop, you don't need to stay at the 3% level for the starter) though #4 wouldn't be overkill (at least for starting, it's way more than necessary for charging).

Les
 
Les, that really helps, thanks for the response.

The 18' of run is the total length (one way) from the batt switch to the motor. The length of the spliced-in cable is about 20' round trip.

I was able to get hold of a 2011 Yamaha rigging guide that specifies 2AWG for 13-22' of total round trip run for a 115HP. I'm wondering if the specs in the 2011 rigging guide might also be appropriate for my 2006 motor.

jd
 
nimrod":3j2qdcs7 said:
Les, that really helps, thanks for the response.

The 18' of run is the total length (one way) from the batt switch to the motor. The length of the spliced-in cable is about 20' round trip.

I was able to get hold of a 2011 Yamaha rigging guide that specifies 2AWG for 13-22' of total round trip run for a 115HP. I'm wondering if the specs in the 2011 rigging guide might also be appropriate for my 2006 motor.

jd
Click to expand...

Probably identical, unless the standards have changed, as the starter motors for the different years probably take the same amperage, and may, in fact, be identical parts with identical part numbers and specifications, including the amperage draw.

Joe. :teeth :thup
 
nimrod":3cm3gqh9 said:
Les, that really helps, thanks for the response.

The 18' of run is the total length (one way) from the batt switch to the motor. The length of the spliced-in cable is about 20' round trip.

I was able to get hold of a 2011 Yamaha rigging guide that specifies 2AWG for 13-22' of total round trip run for a 115HP. I'm wondering if the specs in the 2011 rigging guide might also be appropriate for my 2006 motor.

jd
Click to expand...

JD,

Sorry if the one-way and round-trip stuff is confusing (it is to me!). I find that a lot of folks are not aware that you need the total length of the circuit (the round trip length) and not just the distance from one object to the other (like from the battery to the engine). So if actual physical path distance is 18' then the distance of the wire run when you look at a chart will be 36' .

I'd go with the information you have from the 2011 rigging guide since that's the best thing you've got right now.

The maximum current on #2 AWG wire (in most circumstances) is 180 amps and that's good out to 40 feet (round trip) with a 10% voltage drop.

I don't know really what your starter draws but I'd say #2 wire is very conservative and you'll certainly have a "no issue" run that way.

Les
 
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