battery cable size

[John]

So now I have a second question:

I'm installing an emergency start battery, which would be used for this purpose only. The run from this battery to the switch will be about ten feet. I'm figuring I'll use AWG #1. Do people concur with this?

 

[GuyT]

Seems big

Battery distance requirements to the engine are the total distance of positive and negative wires.
My Yanmar 3gmf has 6 ft. (Negative) and 6 ft. for battery to switch and 6 ft for switch to starter (Positive) - total distance 18 ft and I only have a 4 gauge wire.
I dont have any problems starting the engine although I do think that Hunter should have used 2 gauge for that much distance.

2 Gauge should cover you but you surely cant go wrong with the 1 gauge. Its like extra insurance.
It may be difficult getting that 1 gauge wire into and out of the switch because of the small hole they cut in the plywood and also the bend radius of the wire.

Good luck.

 

[Eddie7045]

In this case bigger really is better

Also go to a marine electrical shop and have marine quality cables made. Marine cables are tinned as well as being solid copper. They are well worth the extra cost.

 

[jviss]

Need a specification first.

1. Decide how much voltage drop you are willing to tolerate on the circuit; usually you select either a 3% drop for running lights, electronics, blowers; 10% for everything else.

2. Measure or plot the maximum length of the source (battery) to load (round trip).

3. Determine load current.

4. Select the wire gauge that satisfies the max. drop at the load current and length.

West Marine has an excellent resource for this, online and in their catalog.

You can purchase Ancor brand marine wire, which is fully tinned, and also marine grade crimp connectors. A low cost ($30) "hit with hammer" crimping tool is available.

Here's West Marine's chart:

http://www.westmarine.com/pdf/MarineWire.pdf

I have rewired a boat. Make sure you have tight, solid crimps on the correctly sized terminals - proper for the wire size, and proper lug size for the connection. If you have to modify a terminal, as enlarging the hole, tin it before installation with solder.

You can enhance this by shrinking some internal-adhesive heat-shrink from the terminal to the insulator of the wire.

 

[jviss]

Why connect it?

Because, Benny, one might need an emergency starting battery in an emergency! Think 7 to 10' seas, pitching and rolling, a real need for engine power, either for propulsion or electrical power - navigation, autopilot, lights - and trying to "swap out" batteries. Or, just flipping a switch.

 

[sailaway78]

Redundancy

Like most, I have a dedicated start battery, as well as a house bank. I also carry one of the rechargable, emergency jump starters. It has other uses besides the jump start feature, so is useful for other purposes. I often use mine to recharge my hand held, or other smaller batteries. I think getting one of these for emergency purposes would be much better, and cheaper than adding another battery. It's certainly lighter than a battery, no hassles or work to install, no cables to add.

 

[John]

connecting it

I agree with jviss; I want the battery ready to go when and if I really need it. Also, suppose I'm indisposed or not even on the boat and my wife or somebody else needs to start the motor and the main bank is dead. In addition, I just think it would take less storage space and look better if it's fastened down under the sink (where I'm planning to put it) and out of the way. From talking with others - Stu in particular - I gather that #1 really is overkill and also harder to work with, so I'm going with #2. And, yes, of course I'll be using marine grade tinned cable.

 

[jviss]

News flash:

"It's certainly lighter than a battery"

It IS a battery.

"no hassles or work to install,"

It has to be connect to something to work - to the starter, the engine battery, something. Might be a "hassle" during an emergency.

"no cables to add"

Will require CONNECTING two cables in order to operate.

That said, I bring one occasionally. Be careful of it flying around while sailing (i.e., needs to be securely stowed).

 

[ozesailor]

Here's an easy way to select your cable size

go to the link below, fill out the fields with voltage, cable run length, amps and wire size and you'll get voltage drop as a percentage.

You should aim for about 3% drop.

Just keep increasing the wire size until you get to 3% drop

 

[John]

start motor

yes, but what can I figure the draw of the start motor is (more or less)? (In case it matters, I have a 35 hp Universal engine.)

 

[Ross]

Re: start motor

John you can get a very rough idea of the maximum starter draw by a look at the lug on the case. For these small marine engines 1/4 inch is not unusual. that would probably limit the draw to about 100 amps. The battery is also a clue in the cold cranking amps rating. You won't go wrong if you go with the size #1 that you chose to begin with if it will fit mechanically.

 

[jviss]

motor current

I am guessing that the motor current while it is turning the crank (not the initial spike, which can be ignored) is less than or equal to 60 Amps. You can know for sure by measuring it (with a DC clamp-on Ammeter), or by reading the part number on the starter motor (probably a Denso starter) and looking up the spec.

Since it is an intermittent load (i.e., not continuous) you can down-rate the cable somewhat, safely; say, on wire gauge.

I think a 10 foot run (20' loop distance) at 65 Amps #4 would be more than enough; #6 would probably be fine.

 

[Nik]

A can of worms...

Just make sure that the ends are made as moisture resistant / proof as possible and the connections are tight. Adhesive lined shrink is a minimum. To use silicone dilectric, non-ox, solder or liquid electrical tape... those are topics that will get you in a world of differing opinions.

Nik