Rewiring Oops!

[tjr818]

This winter I replaced the old battery cables back to the Selector switch, bought a new Group 27 house battery and installed battery terminal fuses. Should have been easy right? Well...apparently I did something wrong. When I went to start the engine on Batt 1 I got about a half of a turn out of the battery and then nothing. Guessing that I must have blown one of the battery terminal fuses I switched the Selector switch over to the both position and the engine started. Latter I checked and found that I was getting a reading of 14 volts at battery 2 and 12.7 volts at battery 1, so I think this confirms that I blew the fuse (don't ask why I didn't check the fuse at the time ). What size battery terminal fuse should I be using for the starter on a YSM8? The battery cables are 2-AWG.

[moctrams]

Not long ago I rewired a power boat and installed two new batteries along with proper cabling. I made a mistake in following a well known marine electrician’s advice and installed two fuses from the (+) plus side of each battery to the battery switch; one for battery string one and one for battery string two. When I tried to start the engine with the battery switch in “Both”, the two fuses blew. I bypassed the fuses and tried again with an clamp-on ammeter. The ammeter registered over starting 400 amps. I removed theses fuses and completed the job. I just don’t think this arrangement is a good thing and rely on the per circuit fuses/breakers to protect the system.

Starter current draw specifications vary considerably, depending on engine size, type (gas or diesel), compression ratio, etc. The following auto and light truck current draw information (for 12 volt systems) is a general guideline. It is normal for some starting systems to draw more amperage.

• 4 cylinder gas engine -typically draws up to 160 amps
• 4 cylinder diesel -typically draws up to 350 amps.
• 6 cylinder gas engine -typically draws up to 210 amps.
• 6 cylinder diesel -typically draws up to 450 amps.
• 8 cylinder gas -typically draws up to 250 amps.
• 8 cylinder diesel -typically draws up to 650 amps

[Noel Heslop]

Hi Tim,

Breezy was professionally rewired 2 years ago, for lots of good reasons, mainly age related, and on advice from knowledgeable boaties, so I do now sleep reasonably (boatwise) comfortably.

I now have 100 amp fuses for each of start and house battery, which are both the same battery and age.
But, 1 month ago, Breezy's Yanmar 1GM did not start. So for the first time I linked both batteries, motor started all good.
Returning, later that day, same thing happened, no problem I thought, I linked both, but this time no start.

This time, I had blew both fuses. My friend bypassed starter fuse, started motor for me to motor home, with instructions to leave house battery off until problem was identified. Therefore, this did not overload the electrical system, so I could motor home (no wind).

To cut a long story short, after much testing (which found no issues) and advice, I just replaced fuses, started motor A Ok, and all has been good since. I had a sailing deadline with first mate.
I decided that if push came to shove, and I was by myself, with spare fuses, that is what I would have to do, and cross fingers. I had been re-assured the previous day in our mens and womens boatshed, by an aircraft bloke, that in the airforce they just replaced fuses the first time, and checked only if the second lot of fuses blew....a plane is up in the sky?

To my close looking, the replacement fuses where definitely a much higher quality. Meaning, chrome over brass and thicker, compared to aluminium coloured busted ones? Both are 100 amp rated. I purchased them from a local (well regarded) auto electrician.

Not sure if this will help, but I have related the facts. My next task with a friend is to now, belatedly, draw up a wiring diagram!

Hope this helps, Noel

[Maine_Buzzard]

I believe I have 150A on 4AWG wiring, never a blown fuse. Will check.

You could go much higher with 2AWG.

[tjr818]

...The following auto and light truck current draw information (for 12 volt systems) is a general guideline. It is normal for some starting systems to draw more amperage.
• 4 cylinder gas engine -typically draws up to 160 amps
• 4 cylinder diesel -typically draws up to 350 amps.
• 6 cylinder gas engine -typically draws up to 210 amps.
• 6 cylinder diesel -typically draws up to 450 amps.
• 8 cylinder gas -typically draws up to 250 amps.
• 8 cylinder diesel -typically draws up to 650 amps

Thanks moctrams, Noel, and Maine Buzzard. I have seen that chart before, too bad it doesn't cover 1 & 2 cylinder diesels. Noel, I guess your 25D has pretty much the same motor and starter as our 27. The book says we have a 12 volt 1kw starter. I think that means it will draw 83 amps after the initial draw. I am going to the boat today to reanalyze the situation. It seems like I would want to have a 100 amp fuse and maybe a 150 amp spare.
Out of curiosity, what does it do to the equation when you are drawing from two batteries? Is the amperage shared by each fuse?

[sharkbait]

A lot of things come to play with two batteries in parallel. Internal resistance of the batteries, equalization of the batteries, age of the batteries etc.

[tjr818]

Okay, I've learned something...The starter blew a 100 amp fuse, but it started on a 125 amp fuse. The ampacity of 2 AWG in an engine "room" is 179 amps, so I should be okay with a 175 amp fuse. I know I should be okay with a 150 amp also. Any Thoughts? I can always bypass a blown fuse in an emergency, but I sure could not do it in a hurry.

[Steve Bryant]

Tim,

Here is a really good read on the subject from a very knowledgeable source.

http://www.pbase.com/mainecruising/battery_fusing

[tjr818]

Thanks Steve.
To add to the discussion, the 100 amp fuse says that it is "ignition protected".
"iP, 88 rated" Interrupt rating;
14V @ 10KA
32V @ 5KA"

What does that mean? Surely I didn't draw 10,000 AMPS - did I ?

[Maine_Buzzard]

The IP is about the sealing of the fuse. 88 does not look right, it might be 68...

Still it's pretty much waterproof and would work even if wet.

The 5 and 10 KA numbers mean that it was tested at these high amperages, and still protects the circuit. A smaller fuse might melt into a chunk of metal and cease to be a fuse... I'd be afraid to see that test.

BTW, a fuse will pass 125% of rated current, and you have to draw more to get it to blow. Twice the current should trip it in 2 seconds or so.

150A sounds right to me.