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What fuse

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URL: https://www.fishing.net.nz/forum/forum_posts.asp?TID=133126
Printed Date: 04 Feb 2026 at 2:25am


Topic: What fuse
Posted By: AdAstra
Subject: What fuse
Date Posted: 19 Mar 2020 at 5:38pm
To anyone with an understanding of the wiring for the Blue Seas M-ACR system.
What size wire and fuse would you recommend for wires running between batteries and ACR.
On their instructions they start with #10 AWG and 30A charging circuit with 40A-50A fuses (cannot understand why such heavy weight fuses).
However, my own alternator only puts out 20A.
Any pointers would be much appreciated

Cheers

David


Replies:
Posted By: OuttaHere
Date Posted: 20 Mar 2020 at 11:16am
It's probably to deal with surge current if you get a charge imbalance. Say you're sitting there with motor off and running from house battery for a few hours, drain the house battery to, say, 11.5v; when you start the motor and the charging relay opens there will be a significant flow of current from the start battery to the house battery as the system tries to hit a happy equilibrium of voltage. Fuses are funny things in terms of the rating; their momentary rating is usually way, way over what's stamped on the side, likewise cable.

10AWG is likely fine for 20-30A sustained, especially if it's a very short run of cable. I'd consider going up to maybe 8AWG if it's a bit longer.
The fuse is mostly there to prevent a fire if you short a cable out. If you get a dead short the battery will happily dump about 500A into that cable and pop a 50A fuse before you can say "oops".

Posted By: AdAstra
Date Posted: 20 Mar 2020 at 11:27am
Thanks Rozboon. Just the info I was after.
To clarify things, I'll leave the #10 wire (No run is more than .5meter) but pop in a 40A fuse (can't go higher with having to change fuse holder)?
This is more in line with what Blue Seas is indicating without actually saying it! .
Cheers

Posted By: smudge
Date Posted: 20 Mar 2020 at 5:49pm
Yeah I am too lazy to look up the current ratings for #8awg & #10awg but as a sparky (an old ex sparky) I think Rozboon is on the money. Fuses are really there to stop fires. Technically it should protect the alternator too so you could try a 20amp fuse  but keep some spares - along with some a little bigger. Wire fuses have a pretty coarse rating so I'd try a 25 or 30 before the 50 personally

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Best gurnard fisherman in my street

Posted By: AdAstra
Date Posted: 20 Mar 2020 at 10:42pm
Thanks Sumdge. 20 it is

Posted By: Ad Astra
Date Posted: 14 May 2020 at 10:42am
On same subject, what size wire should run from house battery to battery switch.
For some reason I have started to worry that my one is not correct.
I am using 10AWG marine wire.
I need to know what happens in the event of start battery going flat and I move the battery switch to combine. Does then the house become the start battery sending 300+ amps up the 10guage wire with an obvious outcome or, does the system bleed off voltage from house to start battery?

Posted By: pjc
Date Posted: 14 May 2020 at 12:05pm
Start/house batteries should independant of each other,should be able to switch off start battery ,is there a fitted isolator,vsr thing

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Sex at 58.Lucky I live at 56

Posted By: Ad Astra
Date Posted: 17 May 2020 at 5:46am
Yes a Blue Seas ACR


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