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Old 17-11-2013, 20:12   #1
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Fuse size question

I'm upgrading the wiring in my cabin... I bought the fuse piece (brand is slipping my mind/it's late here) and understand to use terminal rings for the positive and negative to the piece. There's a 5' run of size 10 shielded from a 5 amp breaker on the DC panel.

This will power in my cabin:
1 small hella fan (draws .4 amp hours)
1 LED light and volt meter piggy-backed (light uses .01 amps)
1 dual outlet cigarette lighter (currently my cell phone charger and electric blanket -- blanket draws 35 watts when in use; when cruising my GPS is plugged in too)
and the already fused VHF radio.

So for the I'm wondering if I go with 5 amp fuses for all? These are the automotive type push in fuses. All push on, so my terminal rings will have to be replaced. As this is a distribution center for my cabin I want it correct. Am planning on connecting VHF (already fused) to the same hot post as the incoming -- can't see a reason to double fuse it. (That radio is a secondary - I use it mostly at night and in the morning to check the weather.)

Anyway, what say the experts regarding fuse sizes? I know from ABYC everything needs to be fused so I'm bringing my cabin up to standards. What size fuses for the items listed above, remembering it's all off a 5 amp breaker on the DC panel. And thanks!
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Old 17-11-2013, 21:04   #2
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Re: Fuse size question

The fuses are there to protect your wiring. Size the fuse according to the wire size going to each unit. Size the wire by the max. current draw and the length (return) of the wires.
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Old 17-11-2013, 21:11   #3
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Re: Fuse size question

With all due respect you need help from a qualified person. Your breaker protects the line, the fuses are to protect your equipment. But the way you describe it I would want to see it first before making any recommendations. A few pics would help.
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Old 18-11-2013, 01:07   #4
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10 amp fuses
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Old 18-11-2013, 01:20   #5
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Re: Fuse size question

Quote:
Originally Posted by janice142 View Post
I'm upgrading the wiring in my cabin... I bought the fuse piece (brand is slipping my mind/it's late here) and understand to use terminal rings for the positive and negative to the piece. There's a 5' run of size 10 shielded from a 5 amp breaker on the DC panel......
I realize you are asking about the proper rating for the fuses but I can't help pointing out that there isn't actually a positive and negative side for fuses (or circuit breakers etc ).

The fuse is just in series with the load (usually on the positive side) so there is really only the line (or supply) side and the load side of the fuse.

Forgive me if you already know this - it isn't clear from how it has been written.
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Old 18-11-2013, 07:41   #6
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Re: Fuse size question

Yes you're correct -- my query was ambiguous. I understand that the fuse is on the positive side, that the grounds are all along the top of the unit. The wire runs are ridiculously short -- (6" from the West Marine cigarette lighter to this new fuse unit though the Hella is 24" away)

And yes, I understand a bit about power. ABYC wants fuses, so I'm adding them. What I wondered was the fuse size... Already the breaker will trip at 5 amps so I'm guessing this is simply a double check of sorts so that if the breaker doesn't the fuse will.

As for fuse size, never mind: found it in Calder's... 1.25 to 2 times the breaker. Page 189, Third Edition. Anyway, thanks gentlemen.
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Old 18-11-2013, 08:15   #7
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Re: Fuse size question

Most appliances (I'll use that term loosely for the Hella, the cigarette lighter, etc) do not need individual fuses to protect them. As long as the branch circuit is protected with a reasonable size breaker then the individual appliances don't need fuses.

I think the theory is that the internal wiring in each appliance will trip the branch breaker if there is a fault inside the appliance before starting a fire in the appliance.

But since your loads could possibly exceed 5 amps on that circuit (the VHF and electric blanket can pull more than 5 amps) and the 10 gauge wiring can take a lot more, increase the breaker from 5 to 10 amps and be done with it.

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Old 18-11-2013, 23:34   #8
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Re: Fuse size question

Quote:
Originally Posted by janice142 View Post
I'm upgrading the wiring in my cabin... I bought the fuse piece (brand is slipping my mind/it's late here) and understand to use terminal rings for the positive and negative to the piece. There's a 5' run of size 10 shielded from a 5 amp breaker on the DC panel.

This will power in my cabin:
1 small hella fan (draws .4 amp hours)
1 LED light and volt meter piggy-backed (light uses .01 amps)
1 dual outlet cigarette lighter (currently my cell phone charger and electric blanket -- blanket draws 35 watts when in use; when cruising my GPS is plugged in too)
and the already fused VHF radio.

1a fuse
1a fuse
15a fuse

change the breaker to 20a.

vhf on it's own 10a fuse before the 20a breaker. or if after the breaker change to a 30a. still fine with the 10awg.

vhf fuse needs to be at the panel, not behind the radio as normally found.
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Old 18-11-2013, 23:50   #9
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Re: Fuse size question

Quote:
Originally Posted by Tellie View Post
With all due respect you need help from a qualified person.
I thoroughly agree. DeepFrz got it right:
Quote:
Originally Posted by DeepFrz View Post
The fuses are there to protect your wiring. Size the fuse according to the wire size going to each unit. Size the wire by the max. current draw and the length (return) of the wires.
Although when sizing your fuses I believe you must also take into account the maximum amperage that the device is rated to handle. For example, my solar controller requires I protect it with a 30 amp fuse, regardless of the wire size I use to connect it to the battery. Another example would be my panel, which is rated for 100 amps. If I use a 6 gage wire I still need to size the fuse for 100 amps even though the wire could handle quite a bit more.

MaineSail has written quite a bit on this subject. You can find his articles here: Compass Marine "How To" Articles Photo Gallery by Compass Marine at pbase.com
It's where I've learned pretty much everything I think I know. Thanks MaineSail!
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Old 18-11-2013, 23:55   #10
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Re: Fuse size question

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Originally Posted by janice142 View Post
Anyway, what say the experts regarding fuse sizes? I know from ABYC everything needs to be fused so I'm bringing my cabin up to standards. What size fuses for the items listed above, remembering it's all off a 5 amp breaker on the DC panel. And thanks!
I'm not clear on what you have here, but you should have a fuse within 7 inches of the battery. That fuse protects the wire running to the panel and the panel itself.
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