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How to calculate fuse sizes

Crinan12

Member
Joined
6 Mar 2019
Messages
335
Hi
I have the wee glass fuses on my boat
How do you know what is the correct amp fuse for each thing? For example cabin lights, deck light, stern light, masthead light, the USB sockets etc

Thanks
 

Paulg25

Well-known member
Joined
20 Oct 2019
Messages
1,343
Location
Delta Quadrant
As far as I am aware, the fuse should be there to protect the wiring (assuming the fuse is at the battery end), so it should be sized according to the wire gauge and length.
 

VicS

Well-known member
Joined
13 Jul 2002
Messages
45,942
Hi
I have the wee glass fuses on my boat
How do you know what is the correct amp fuse for each thing? For example cabin lights, deck light, stern light, masthead light, the USB sockets etc

Thanks
The fuse must be larger than the maximum current drawn by the item(s) supplied........ some items may draw a high current at at start-up which has the catered for .... best to see the equipment manual where possible ( a slow blow fuse may be specified for some items)

It must be less than the safe working current of the lightest wiring it supplies ( length is not relevant) but best to keep it as low / close to the equipment current draw / recommended size as possible .
In fact it should not be greater than any recommended size in order to ensure adequate protection to the equipment itself.

Some rationalisation is sensible to avoid having to keep a wide range of sizes as spares
 
Last edited:

lustyd

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Joined
27 Jul 2010
Messages
6,121
Location
.
As far as I am aware, the fuse should be there to protect the wiring
Strictly speaking, the fuse is there to protect the boat. If you fitted 70mm cable for everything you'd still want smaller fuses than needed for the wire to be protected because equipment would still be capable of catching fire. As such, a fuse rated to the workload would be useful, and you should ensure the wire is capable of greater load than that rating.
 

Bristolfashion

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Joined
19 May 2018
Messages
2,681
Location
Southampton
The size should be stamped on each fuse - they are quite hard to see / read.

Assuming that the wiring is correct, each item should say what kind of fuse is required. Using the fuse size / type indicated for a similarly powered device should be fine.

The only thing I can add is that wiring can be over size - you can run a radio off 110A wire, but the device probably needs a 1 / 3 / 5 A fuse, not 20A or above. There's nothing to stop you using a 20A fuse next to the battery +ve with a 3A fuse next to one device, 1A next to another & so on.

In practice, the fuse rating should lie below the max current rating of the cable & above the max current draw with all devices operating. So a 25 A cable feeding 5 devices that each draw 2A (=10A) might have a 15A fuse.
 

Stemar

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Joined
12 Sep 2001
Messages
15,725
Location
Home - Southampton, Boat - Gosport
Looking back at my experience with that kind of fuse, if I were going to review fusing arrangements, I'd chuck 'em out and fit blade type fuses. The glass ones have given me grief on every vehicle, wheeled or floating, that's had them. The blade fuse holders I have are fitted with an LED that comes on if the fuse blows, which is useful, thought the only times I've had a fuse blow it's been self-inflicted. Do as I say, not as I do - turn the effing thing off before faffing around with the wiring!
 

Crinan12

Member
Joined
6 Mar 2019
Messages
335
Thanks all
To fit blade fuses I would need to change the whole electrical panel I think. Maybe a job for another day though!
 

William_H

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Joined
28 Jul 2003
Messages
12,310
Location
West Australia
To sum up essentially you want 5 amp fuses for those services listed. All the above theory is correct hiowever it is unlikely you will have wiring so light as not capable of carrying 5 amps without glowing red. All assuming you have LED lights. Incandescent lights might draw more than 5 amps so if that is the case you need to determine wattage and current drain. Might need 10 amp fuse. VHF radio probably OK with 5 amp but check the makers recommendation. As said glass fuses often give bad contacts especially if only contacted by end pressure so be aware. ol'will
 
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