Moving dual battery setup to stern WIRE GAUGE? 1977 Johnson 70Hp 70EL77S

[airbus320]

1977 Johnson 70Hp 70EL77S

I've done some pretty extensive searching online and on this site without any conclusive results.

I am moving the batteries on my boat to the bow for weight and balance. I have a dual battery setup with a [FONT=Arial, sans-serif]Attwood 4-Way Battery Selector [/FONT]Switch.

This is a fiberglass boat. The round trip length is approximately 30 ft. from where the old battery location was, the ends of the cables coming from the engine are located in the stern. There is approximately 4-5 feet (8-10 round trip) of cable from where the batteries used to be in the stern. Those cables go through a harness and directly into the engine.

I have looked at many different charts and guides to help me determine wire gauge, but have come up with mixed results.

1. What would the recommended wire gauge be for this run?
2. Can I splice the new wire to the existing cables that go to the engine or is it recommended to replace the entire run from the battery, through the harness to the respective cable connections in the motor? (that will add another ten feet R/T to the run approx).
3. What is the recommended circuit protection for the battery to engine run? I have a maxi fuse holder Blue Sea Systems MAXI Fuse Block - 30 - 80A that I planned to install at the battery end of the run, for exclusive protection of the battery to engine circuit.

I am an aircraft mechanic of 13 years with experience in wiring, circuit protection and troubleshooting....but somehow, this simple problem has me stumped.

Thanks for your help!

Joe

 

[F_R]

1. I would hate to hazard a guess, other than to say probably 3X the original. Just my 2C.
2. You can splice them. But remember, total resistance is the resistance of the old cables plus the resistance of the new ones. Even so, I'd use the old ones just for convenience sake, along with generous gauge new ones.
3. Do airplanes fuse the battery cables? Cars & boats generally do not. In fact, a blown fuse would cause damage to engine electrical parts. Yeah, I know, something was damaged anyway.

 

[F_R]

BTW, putting the batteries in the bow section is a poor choice. They get a rough ride up there, I wouldn't go further forward than amidship.

 

[airbus320]

Thanks for the tips! I opted to spread the batteries out in the stern, to better balance out the weight. This proved to be a satisfactory solution.

 

[airbus320]

1. I would hate to hazard a guess, other than to say probably 3X the original. Just my 2C.
2. You can splice them. But remember, total resistance is the resistance of the old cables plus the resistance of the new ones. Even so, I'd use the old ones just for convenience sake, along with generous gauge new ones.
3. Do airplanes fuse the battery cables? Cars & boats generally do not. In fact, a blown fuse would cause damage to engine electrical parts. Yeah, I know, something was damaged anyway.

In aviation, everything is about fire protection. Fuses and circuit breakers are installed as close to the source of power as possible, to prevent the wire from overheating in the case of a short and causing a fire in flight. Component damage is a secondary concern *only*. Component and system redundancy is ensured, to compensate for fire protection priority. All aircraft electrical circuits are protected as close to the power source as possible, to protect the run from any possibility of overheat and fire. Additionally, the CB's used on aircraft are "trip free", meaning that the re-setting button cannot be held by the pilot, in an airborne attempt to re-establish the circuit by the flight crew. Once they open, they cannot be reset until the breaker cools down or the condition that caused the short has been found and rectified. Aicraft battery cables are very carefully routed and extremely well protected, up until they reach the first contactors, hall effect and automatic/manual overcurrent disconnect devices. My failure to realize that the absence of conductive material in my FIBERGLASS boat, would make the chances of a short to ground and extremely unlikely circumstance :lol:. I was extra careful with my circuit and battery cable routing and gauge selection.

Everything worked out well in the end.

Thanks again for the tips!

JD