Size Fuse to Use

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Size Fuse to Use

Postby Capebuild » Sat Aug 21, 2021 5:27 am

I'm running all my loads through a Blue Seas Fuse block. That BS fuse block is fed though a single fused circuit from a Blue Seas safety hub. My total load draw, if everything were on at one time (highly unlikely), is around 10 amps.

My question is since that total amp draw is 10 amps, would I use a 10 amp fuse in the single fused circuit in the safety hub, or would a 15 amp fuse be a better choice?

Thanks very much.

John
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Re: Size Fuse to Use

Postby H.A. » Sat Aug 21, 2021 6:19 am

Ultimately fuses are to protect wiring. Select a fuse appropriate to the wire served.
But there is nothing wrong undersizing a fuse to closer match the “worst case” expected load.
To directly answer your question and assuming the wire is sized correctly. Use a 15 amp.
Albeit insanely small value, voltage drop across the 15 amp will be less than 10 amp.
Capebuild wrote:I'm running all my loads through a Blue Seas Fuse block. That BS fuse block is fed though a single fused circuit from a Blue Seas safety hub. My total load draw, if everything were on at one time (highly unlikely), is around 10 amps.

My question is since that total amp draw is 10 amps, would I use a 10 amp fuse in the single fused circuit in the safety hub, or would a 15 amp fuse be a better choice?

Thanks very much.

John
Last edited by H.A. on Sun Aug 22, 2021 3:43 am, edited 1 time in total.
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Re: Size Fuse to Use

Postby DrewsBrews » Sat Aug 21, 2021 7:53 am

There are multiple goals. One is safety, another is reliability. Making sure you use appropriate gauge wiring and connections so you can use appropriately sized fuses provides safety while not having to deal with blowing fuses at every trip.

Best practice to keep fuses above your expected loads so if you think you might draw up to 10A then a 10A fuse is too small. All wiring and connections should be appropriately sized to handle it... Generally at least sizing wiring for double the expected max load prevents heating up wiring and excessive voltage loss. Also want to keep fuses below the capacity of the power source max draw rating. So going too high value is not prudent either even if the wiring itself can handle it.

The other fuses in the block should be protecting the load wiring coming out of the block. So any "upstream" fuse should be sized to protect components/wiring in between the fuses. Fuse blocks can probably handle well over 15A before being in danger of melting. So as long as any wiring between can handle up to 15A safely then a 15A fuse would be a good value IMO (keeping power source capacity in mind too).
Last edited by DrewsBrews on Sat Aug 21, 2021 8:53 am, edited 10 times in total.
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Re: Size Fuse to Use

Postby tony.latham » Sat Aug 21, 2021 8:18 am

Just like the circuits in your home, it's not about the potential draw but what the wires can handle. Plug a microwave and a toaster into the same circuit and the breaker will do its job and pop.

So... it depends on your wire size. Me? I step the fuse down a bit from there but I have never had a fuse blow.

Tony
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Re: Size Fuse to Use

Postby Capebuild » Sat Aug 21, 2021 8:47 am

thanks all for the replies and help. I should have mentioned the wire size I'm using in original post.....I'm using 14AGW. So I think the 15 amp fuse will probably be the way to go.

Thanks again.

John
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Re: Size Fuse to Use

Postby TimC » Sat Aug 21, 2021 9:30 am

Like the others have noted. Protect the wire AND the surfaces surrounding it in case it heats up and causes structural damage. A wire continuously running hot may not ever cause a fire but could melt or degrade surfaces it is run through. It is a very small expense to over gauge your circuits.

When you are designing an electrical circuit in a teardrop (and just about anywhere really) go through this thought process...

#1 What loads (amps) do I want on this single circuit.
#2 What size wire will handle that load. Size the wire at least one gauge larger than normal draw (running and startup)
#3 What size fuse will protect that wire. Size the fuse at that wire's capacity or below.

You mentioned in a previous thread you would like to simplify your wiring by using 14awg everywhere (at least most everywhere). Use that in all the branch circuits downstream of the fuse panel and fuse for 15a or below and you are good to go.

Battery to fuse panel, solar charge controller to battery and solar panel to charge controller are another issue entirely.
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