Battery Cable Length - Montauk

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
Posts: 55
Joined: Sun Sep 25, 2016 1:57 pm

Battery Cable Length - Montauk

Postby Mattwarner » Wed May 02, 2018 12:14 pm

I am relocating [the engine cranking] battery [from the transom] to the console of my classic Montauk which is currently stripped and being painted so I cannot easily measure [the length of the needed cables] myself.

Would 5-meter-length be sufficient for each cable to run from the console to the engine?

I would have thought so, however I am sure i have read on here (or Whaler Central) that 17-feet is required. I do not want to order too little short and have to [make a splice].

The place I am ordering from does 2 AWG in 10-meter or 15-meter lengths.

I am planning on using black insulation, cutting the length in half, [and marking the positive conductor] with red heat shrink on either end.

Also, is 2-AWG okay?

Or is it excessive?

In the past I have used three-gauge for a 50-HP with no problem, but I figured the longer run and 90-HP two-stroke-power-cycle engine would require more Amperes. It is expensive stuff!

User avatar
Phil T
Posts: 1160
Joined: Thu Oct 22, 2015 6:08 pm
Location: Was Maine, now Kentucky

Re: Battery Cable Length - Montauk

Postby Phil T » Wed May 02, 2018 12:26 pm

Matt--I used and recommend 2-AWG marine tinned battery wire for my relocation.

(FYI - Matt is in the UK)
Member since 2003
1992 Outrage 17, 1992 Evinrude 115

Posts: 7341
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

Re: Battery Cable Length - Montauk

Postby jimh » Wed May 02, 2018 2:52 pm

Regarding the length of electrical cables, there is a corollary to Murphy's Law that states:

    Any cable cut-to-length will be too short.

I cannot help you with a figure for the length, but I recommend buying more than you figure will be needed.

The necessary wire gauge must be chosen to minimize voltage drop. This is an electrical topic, which is discussed at some length in the SMALL BOAT ELECTRICAL forum, both past and present versions of the forum. Here is a good article to read:

Recommended Battery Cable Size

As general advice:

Extending Battery Cables

Don't Make Splices, Especially in the Tunnel If you want to move the battery to the console from the transom, do not splice the cables so the splice is in the rigging tunnel and subject to chronic immersion in water. A splice in the rigging tunnel is the worse possible place to make a splice. You can't see it. You can't easily repair it. It is buried in a tunnel.

You Must Increase Conductor Size If you want to extend a high-current circuit like the engine starting circuit, you have to increase the size of the conductors so that the total voltage drop is not excessive. Typically the original cables were just large enough to work for the original length. They will be too small for the longer length. You cannot extend them using the same gauge of cable.

The Size of the New Cables If you want to extend the cables, you have two choices. You can add to the original cables with cable that is very much larger than necessary so that there is almost no voltage drop in the new cable segment. Or you can discard the original cables and replace them with a new length of the appropriate size, which will be larger than the old cables but not as large as if you only replaced a portion of the cables.

Let me give an example. Let's say the starter motor can tolerate a voltage drop in the cables of 2-volts maximum. The original cables were probably sized so that they provided no more than 1-volt drop. In the new installation we want to maintain that same margin.

If we keep the original cables and extend them, the total voltage drop in the system will be too high. Even if we use monster cable to extend, there will be some drop in the new cable. There will be drop in the splice. The original drop of 1-volt was all we were willing to tolerate. So any addition adds more voltage drop.