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post #11 of 37 Old 08-26-2014
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Re: fuse popped, ARRRG

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Originally Posted by eherlihy View Post
Hi Denise,

You should size the MRBF to the location, wire gauge and insulation temperature rating. I have AWG 2 leads with 105șC insulation going to my batteries, and this is inside the engine room, so for me the proper size fuse for me is 178.5A - call it 175A.

Look at the table below, and find your wire gauge, then find the insulation, and read what should be your fuse rating.
(Posting this image from Maine Sail's site);


[EDIT] If sized too small, you WILL blow fuses as your battery voltage drops. You know that the voltage will drop as you spend a night at anchor, and use the batteries. Voltage down means current up to produce the same power to the starter motor.
Yep and voltage drop across the wire only compounds battery sag.

If need be you can go to 150% of that table which on 4GA 105C wire is about 240A..

On another note ANY FUSE is ALWAYS SAFER than NO FUSE !!!!

I have blown a 300A MRBF fuse on 15' of 8GA wire and the wire never even got a chance to get warm to the touch.. Way out of ABYC specs and I am not recommending it but fuse so you don't have nuisance trips. If this requires upsizing the piddly 70's wire builders used so be it. Catalina now uses 1/0 or 2/0 on the same size engines they did in the 70's & 80's when they were using 4GA.......

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Last edited by Maine Sail; 08-26-2014 at 09:46 AM.
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post #12 of 37 Old 08-26-2014
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Re: fuse popped, ARRRG

These charts are also relevant: they show that MRBF fuses WILL support higher amperages than their nominal rating for short (< 10 sec) periods of time;

Recommended Fuse Range; (200A for ~8 sec)


Fuses that Denise used; (not so good, 200A for ~1.1 sec)


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Last edited by eherlihy; 08-26-2014 at 09:41 AM. Reason: added second chart
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post #13 of 37 Old 08-26-2014
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Re: fuse popped, ARRRG

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Originally Posted by eherlihy View Post
This chart is also relevant: it shows that the fuses WILL support higher amperages than their nominal rating for short (< 10 sec) periods of time;
ANL, Class T and MRBF fuses are intended for motor starting loads and to get you well past the in-rush but they still need to be sized larger than the continuous average in order to not "nuisance trip"...

If you look at the 125A and the 300A MRBF you will note that the 300A can handle 200A for approx 10 full seconds but the 125A fuse can barely get beyond 1 second at 200A.. Even the 200A fuse can handle about 8 seconds at 200A. This window on the 125A fuse is simply too short for a starter motor....

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post #14 of 37 Old 08-26-2014 Thread Starter
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Re: fuse popped, ARRRG

thanks gents, no need to beat me up! The 125 handled quite a few starts without a problem. I know, it's not will it fail, just when, I'd love to see the real time amps on my starter. It turns the engine over quite easily. And if the engine bat gets low, the house bank comes in thanks to the ACR. Love how it all works. just the starter is the bugaboo.

As i said it's 2 cylinder and all the #4 wire runs are short. (less then 6ft) For now, I'lll get a 175 cube and keep a spare. Maybe change the starter cable to #2. AGAIN. the terminal was very very loose. Like a 1/16" play. which was the 1st real cause.

a regular amprobe type clamp on meter will not give me DC amp reading right?

I may change all the cables to #2. but it's all $ related of course.

Denise, Bristol PA, Oday 30. On Tidal Delaware River, Anchor Yacht Club.
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Last edited by deniseO30; 08-26-2014 at 02:17 PM.
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post #15 of 37 Old 08-26-2014
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Re: fuse popped, ARRRG

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Originally Posted by deniseO30 View Post
a regular amprobe type clamp on meter will not give me DC amp reading right?
If its rated for DC amps it should

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post #16 of 37 Old 08-26-2014
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Re: fuse popped, ARRRG

A 5 ft length of 1/0 cable, terminated at both ends, is $31 at Genuinedealz.com, with free shipping. Just saying.

No, I don't own the company!

But I did use them to make new cables between my batteries, switch, and starter. They even use the proper 4-jawed crimper.

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post #17 of 37 Old 08-26-2014 Thread Starter
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Re: fuse popped, ARRRG

Yes Mark thank you, I've been using them for most of my wiring and related purchases. Great outfit too!

Denise, Bristol PA, Oday 30. On Tidal Delaware River, Anchor Yacht Club.
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Re: fuse popped, ARRRG

I think my problem was not so much with the wire guage, but the quality. The original Bristol cabling was fine. But someone added a second battery bank, and did a crap job. The cabling wasn't tinned, and the crimps on the terminals were badly done. The terminals weren't tinned, either. Finally, no heat shrink at the terminals.

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Re: fuse popped, ARRRG

Mine had a mix of re purposed jumper cables and non marine cable.

Denise, Bristol PA, Oday 30. On Tidal Delaware River, Anchor Yacht Club.
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Re: fuse popped, ARRRG

I'm going to borrow a DC clamp on amp meter! I want to see this "mega amp" draw my little starter is claimed to be drawing on startup!

Denise, Bristol PA, Oday 30. On Tidal Delaware River, Anchor Yacht Club.
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