Correct fusing?

Davidasailor26 · Jul 4, 2018

A 3 amp fuse on the whole circuit should work fine, as long as there are no other loads sharing that circuit. The important thing is that the circuit be fused to the lowest amperage of any of its wires. If the circuit is shared you may need to fuse at the fuse box to the amperage of the 14 gauge then put the 3A fuse at the step down.

Stu Jackson · Jul 4, 2018

Fuse for the load and then fuse for the wire size, too, since it's bigger.

Wire Sizing Charts from West Marine http://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity www.bluesea.com also has good material on this and fusing, but it's a PDF download and doesn't link. Here's the link to the page, scroll down for the PDF download of the Fuse & Wire Sizing Tables: https://www.bluesea.com/support/reference

jviss · Jul 4, 2018

dave_and_dd said:
Do I need to put the fuse holder back on?

Yes! The fuse at the panel protects the wire. The one in the fuse holder protects the device. It may be an important part of reverse-polarity protection, for example. Keep it in the circuit.

Davidasailor26 · Jul 4, 2018

jviss said:
Yes! The fuse at the panel protects the wire. The one in the fuse holder protects the device. It may be an important part of reverse-polarity protection, for example. Keep it in the circuit.

If the fuse is integrated with the device then I agree, best to keep it in case the device somehow depends on it. The way I read the OP though is that the fuse is just inline in the supply wire going toward the device. In that case there's no need to keep the fuse there versus further up what is essentially the same wire.

Davidasailor26 · Jul 4, 2018

Davidasailor26 said:
If the fuse is integrated with the device then I agree, best to keep it in case the device somehow depends on it. The way I read the OP though is that the fuse is just inline in the supply wire going toward the device. In that case there's no need to keep the fuse there versus further up what is essentially the same wire.

Actually, I'll correct myself a bit upon further consideration. If the fuse is designed as part of the reverse polarity protection then you need to keep a fuse of that capacity somewhere between the device and the nearest place that you could possibly have an accidental connection from the red wire to ground. If you're permanently connecting the device's red wire to the 12V from the boat then I'd argue it's still adequate to move the fuse further up the line. If you have quick connect plugs that could be accidentally swapped then you should keep the fuse downstream of that.

jssailem · Jul 4, 2018

I see that the fuse is protecting the 22 gauge wire. Since the power is coming from the 14 awg wire you want the fuse within 7" of the connection. I suspect that is were the company placed the fuse before you removed it "In order to run the cable down from the nav-pod into pedestal guard and down to below the cockpit sole for hookup"
Now you can change the type of fuse, say a 3 amp in once of these...
https://www.mouser.com/Circuit-Protection/Fuse-Holders/Fuse-Holder/_/N-axfrp?P=1z0z7wd

jviss · Jul 4, 2018

It's generally good practice to keep the fuse that's supplied with the equipment.

LeslieTroyer · Jul 4, 2018

If you put a 3A at the panel it will protect both the wire and the device. Plus you can find it when/if it blows at an inoppertune time.

A fuse will never protect from reverse polarity

jviss · Jul 4, 2018

LeslieTroyer said:
A fuse will never protect from reverse polarity

A fuse is often an integral part of a reverse polarity protection scheme. Icom uses this, for example. A reverse polarity protection diode inside conducts when reverse polarity is applied, but relies on the external fuse to open in this case. Without the fuse, the radio is dead.

So, no, you are wrong saying "a fuse will never protect from reverse polarity."

rgranger · Jul 4, 2018

LeslieTroyer said:
If you put a 3A at the panel it will protect both the wire and the device. Plus you can find it when/if it blows at an inoppertune time.

A fuse will never protect from reverse polarity

@LeslieTroyer beat me to both of my points .

An in line fuse is fine ... unless you blow a fuse and need to find it later. And you would need a diode to protect from reverse polarity

jviss · Jul 4, 2018

rgranger said:
And you would need a diode to protect from reverse polarity

The diode is usually inside the device, the fuse outside.

jviss · Jul 4, 2018

So, here's the scenario. You have some devices that are on a dedicated branch circuit, with a 15A breaker at the panel, to protect the wire, and an inline fuse per the device manufacturer. So, no problem, move the fuse to panel, near the breaker, if you like. But if you have a circuit like "electronics" that's a branch with several devices on it, like AIS, antenna mux, protocol converter, etc., you can't put the fuses at the panel, you have to leave them at the devices - which makes sense. If a device doesn't work, check the fuse where the device is located. Many device reverse polarity protection schemes rely on an external fuse.

Davidasailor26 · Jul 4, 2018

LeslieTroyer said:
A fuse will never protect from reverse polarity

Not by itself, but combined with a diode it can. See, for example, method 5 at https://www.edn.com/design/power-ma...st-reverse-polarity--Methods-examined--Part-1

jviss · Jul 4, 2018

Dave, I get what you are saying, and it makes sense. But as a one time circuit design engineer I can tell you that many people don't know the designer's intention, and get burnt redesigning things in the field.

Davidasailor26 · Jul 4, 2018

dave_and_dd said:
Interesting comments folks. To clarify a couple items. Yes, I'm permanently installing this new chart plotter. No quick connects, nothing to chance unplugging and replacing it with accidentally. Absolutely nothing else is on this circuit.

I am replacing a previous device which did have an in-line fuse where the #22 wire connected to #14. The circuit had a 10 amp fuse at the panel. So...the First half run of #14 was protected by the 10amp fuse and the #22 second half was protected by 3amp.

Les is hitting on my concern, that 3amp fuse was buried inside corrugated shielding in the ceiling of the rear cabin above the steering cables. I would have never known it was there. Had it blown, the 10amp fuse at the panel would have been fine, but there would have been no power due to the interruption half way up to the pedestal.

Now if on the other hand, I was hooking it up to a 10amp circuit that needed to stay 10amps due to other items, then it would have to stay in line at the juncture...but that's not my situation.

Dave in Tampa

Given that set of assumptions and constraints (particularly limited chance of reverse polarity at the device and limited access) if it were my boat I'd just go with the 3A fuse at the panel. Of course, as they say, your boat your choice.

Davidasailor26 · Jul 4, 2018

jviss said:
Dave, I get what you are saying, and it makes sense. But as a one time circuit design engineer I can tell you that many people don't know the designer's intention, and get burnt redesigning things in the field.

Yeah, I see what you're saying. I can imagine that's a risk to be aware of.

jviss · Jul 4, 2018

Davidasailor26 said:
I'd just go with the 3A fuse at the panel.

How would you do that? Are you saying you'd use a 3A circuit breaker on the panel, and have nothing but this device powered from that circuit? I don't think I've ever seen a 3A CB (I've seen 25A, then 10A, 15A, 20A, and so on). Plus, how do you know it will have the time-blow characteristics of the fuse?

Correct fusing?

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SBO Weather and Forecasting Forum Jim & John

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