Where to put a Fuse

[openseas]

Hi all,
I have a question, if you want to add a load to your boat, whether it is a bilge pump, fan or so on. Do you put the fuse on the positive wire close to the battery or close to the load, and why?
Thanks,
Andy

[Terra Nova]

The fuse protects the entire circuit. But not if you place it near the load. There should be a large enough fuse, close to the battery, for the positive cable which goes to your electrical distribution panel. Then a correctly sized fuse, or preferably a circuit breaker, for each load.

[djmarchand]

Terrra Nova's answer is correct, but let me expand a bit on where to pick up power.

Most add on stuff is connected directly to the battery terminals, with an appropriately sized fuse, nearby (ABYC says 6" I think). Why because it is often easier. But depending on what the load is, it is often better to power it from the DC panel. Either from one of the existing breakers- the shower sump pump is my favorite as the load is small and intermittent or a new dedicated breaker if there is room.

But some loads should not be easily switched off, such as bilge pumps and are supplied from the battery terminals directly or in the case of OEM pumps indirectly through a breaker panel in the engine room with a type of breaker that can't easily be turned off connected directly to the battery and not through a 1, 2, all switch.

David

[Ex-Calif]

Originally Posted by openseas

Hi all,
I have a question, if you want to add a load to your boat, whether it is a bilge pump, fan or so on. Do you put the fuse on the positive wire close to the battery or close to the load, and why?
Thanks,
Andy

You protecting the wire from oversupply - put the fuse near the supply.

Always presume that with any unit you buy with an inline fuse that fuse is to protect the unit not the circuit.

Here is a pretty easy to read primer - DC Circuit Protection - Fuse Circuit Proctection Practices

[sparrowhawk1]

Standard procedure is to run a heavy cable shortest distance possible to a fuse rated for that cable the cable then goes to a battery switch and then to your breaker panel. on the breaker panel the fuses should be once again be sized for the size of wire. For sensitive electronic equipment a smaller inline fuse is installed usually close to the item. this is just for protection of the item. Also sometimes you'll have a heavy gauge wire going to a lighter wire. At that connection you want an inline fuse once again rated for that lighter wire. Bilge pumps are usually on a circuit that doesn't go through the battery switch here connect an inline fuse directly the battery terminal. Or to the battery switch where the power goes in. With this system the only wire not protected his the heavy cable going to the fuse. Most pump switches have fuses in them and its standard procedure to just run the short length of wire from it to the power side of the battery switch. But once again the wire will not be protected

[skipmac]

Originally Posted by djmarchand

Terrra Nova's answer is correct, but let me expand a bit on where to pick up power.

Most add on stuff is connected directly to the battery terminals, with an appropriately sized fuse, nearby (ABYC says 6" I think). Why because it is often easier. But depending on what the load is, it is often better to power it from the DC panel. Either from one of the existing breakers- the shower sump pump is my favorite as the load is small and intermittent or a new dedicated breaker if there is room.

But some loads should not be easily switched off, such as bilge pumps and are supplied from the battery terminals directly or in the case of OEM pumps indirectly through a breaker panel in the engine room with a type of breaker that can't easily be turned off connected directly to the battery and not through a 1, 2, all switch.

David

Excellent advise. Very bad practice to power too many things directly off the batteries. You will end up with a mess of wires and much higher risk of problems and make it much harder to diagnose and work on the system.

I only connect a couple of things directly to my house batteries:

1. The power to the main battery ON/OFF switch. From the switch I run one lead to the DC breaker panel and one cable to the battery switch for the starter battery to use an emergency backup source to crank the engine.
2. An always ON circuit that runs the main bilge pump, alarm system and one small LED light.

These circuits are fused separately right at the battery with a Blue Seas terminal fuse.

Best practice is to connect most things through your battery switch and DC panel except the few things that you want to always stay on.

[Terra Nova]

For protection of an important circuit, such as an automatic bilge pump, I like to use a self-resetting circuit breaker. Sometimes a small piece of foreign matter might cause a bilge pump impeller to stall, when power is first applied. But, re-starting may unstick it and save your boat from sinking. Whereas, if the impeller stalls and blows the fuse, there goes your only chance.

[sparrowhawk1]

I too have replaced the fuse in my bilge pump switch with a circuit breaker. an



emergency is no time to be looking for a spare fuses

[skipmac]

Originally Posted by Terra Nova

For protection of an important circuit, such as an automatic bilge pump, I like to use a self-resetting circuit breaker. Sometimes a small piece of foreign matter might cause a bilge pump impeller to stall, when power is first applied. But, re-starting may unstick it and save your boat from sinking. Whereas, if the impeller stalls and blows the fuse, there goes your only chance.

I tried to design my setup to get the best of both worlds. Ran a fairly large wire, 6-8 if I recall from a 50 amp terminal fuse on the battery to a small bus bar. From the bus bar I have a fused wire going to the smallish primary bilge pump, another fused wire to a high water alarm, and a large jumper to feed two breakers that run two emergency bilge pumps.

If the small bilge pump jams and pops its fuse then I have the high water alarm and two backup pumps on 20 amp breakers. If either of those jam it will pop the breaker still leaving the primary 50 amp fuse intact.

I also keep a spare terminal fuse by the battery which shouldn't take more than a minute to swap out.

[Terra Nova]

The self-resetting circuit breaker is for when the boat is unattended.

[sparrowhawk1]

Skipmac. Are your secondary bilge pumps rated for 20 amps? Because a common cause of boat fires is people putting a bigger fuse in the bilge pump circuit than the wires can handle. When a bilge pump seizes sometimes it's just like a short circuit

[skipmac]

Originally Posted by sparrowhawk1

Skipmac. Are your secondary bilge pumps rated for 20 amps? Because a common cause of boat fires is people putting a bigger fuse in the bilge pump circuit than the wires can handle. When a bilge pump seizes sometimes it's just like a short circuit

Appreciate the concern and that is a very valid point. I did carefully calculate all the wire and fuse sizes and pump draw.

And I was mistaken about the circuit breakers for the pumps. I was just out at the boat and they are actually 25 amps. The pumps are a pair of Rule 4000s.

Specs:

Flow Rate: 4000 gph 15140 lph
Port Type: 2" (51mm) Hose barb
Voltage: 12 Vdc
Non-Automatic
Amp Draw: 15.5 amp
Fuse Size: 25 amp
Height: 7-3/4 inches 196.8 mm
Width: 4-7/8 Inches 124 mm
Weight: 5–8lb. 2.5 kg

One is a bit further from the panel so went up one wire size for the forward pump. If I recall I used 4 and 6 gauge, tinned wire to the pumps. For emergency pumps I want minimum voltage drop.

The Rules I don't think are the best for a regular, every day pump but for emergency backups I think they'll do the job. Add a gallon/stroke manual, a smaller manual and a smaller electric I should be able to move at least 100 gallons/minute. Should give me a bit of extra time to plug any unexpected holes.

Also looked back at the wire from the battery to the always on bus. It's 2 gauge not the 6-8 I recalled.

[skipmac]

Originally Posted by Terra Nova

The self-resetting circuit breaker is for when the boat is unattended.

You have a part number or model for that breaker?

[Terra Nova]

The self-resetting breakers are common at auto parts houses. They are cheap and have a metal housing.

[Colin A]

The auto reset breakers are common in the car and equipment world. You can also find them on digikey etc. Now technically they are a no no on boats. And this actually happens to be one of the few rules that isn't just ABYC but an actual coast guard enforced federal law.

FEDERAL LAW

183.455 - Overcurrent Protection: General

(a) Each ungrounded current-carrying conductor must be protected by a manually reset, tripfree circuit breaker or fuse.