Bow Thruster cable/fuse Issue

I am having a company in Rhode Island install a new battery system on my 2003 Mainship 30m Pilot II and as they looked over the present setup they came across an issue with the bow thruster positive cable/fuse. I will quote the email instead of trying to describe the issue myself.

"We've started to dig into the battery system upgrade and we've made a note-worthy discovery in the existing cabling that feeds the bow thruster. Close to the battery switch, there's a high-amperage fuse block that's connected in line with the bow thruster positive cable, which partially melted at some point due to excessive heat.........The jacket on the positive cable that feeds this fuse also started to melt, so it's apparent that either a) the existing bow thruster cable is undersized, or b) the cable is over-fused (currently a 350 amp fuse)."

Before I talk to the company doing the work I would like to have some knowledge from those that have more electrical experience than I about what this might mean and what solutions might be needed. It is a new to me boat and I am just learning about the different mechanical and electrical systems.

Thank you

My take on it is that a fuse is meant to protect the electrical components from passing so much power that they overheat and become a danger to other things. If your cable can heat up from using the bow thruster enough to melt the insulation on the cable that means that several things:

First, check the connection where it heated up.

Second, check the power draw on the bow thruster and see if the cable is undersized.

Third, check the actual amperage draw using the thruster. Make sure it is within mfg's ratings.

you don't want a fuse to blow when you use your thruster, since they often blow when you REALLY need the thruster, which is why they blow, usually over-use.

If the cable on one side of the fuse block is partially melted it is likely that the crimp on that end of the cable is poor or the cable was not tightly connected to the fuse block. If the entire cable was too small the entire cable would get warm, not melt one spot. Its easy enough to check the power requirements of the bow thruster and check the size of the cable and fuse to see if everything is sized appropriately. One thing when sizing cable is the length of the run must be taken into account. For example my 1000w watt windlass would be fine with a short run of 1/0 cable, but because of the length of the run it needs 3/0.

Ken

I agree with stubones, in that it is possible that you have a poor connection at the fuse that created the heat. If that is the case, your cable and fuse may be adequate but the connection was bad.

I would also do as he suggested and check the power draw from the bow thruster to ensure that you have a large enough cable. The size of the cable will be determined by the max sustained amperage of the thruster, the length of the circuit (not just length of the cable from the battery to the thruster the entire circuit), whether the cable runs through an engine space, and the acceptable voltage drop (3% I would imagine). For a thruster, on a largish boat, that can be one big cable.

Finally, regardless of what you find out in 1 and 2, you should have the company install a fuse within 7" of the battery driving the thruster that is sized to protect the wire not the thruster. There are very specific tables to protect the wire based on its size, and again whether it runs through an engine space or not.

Determine the needed wire size here:
Circuit Wizard - Blue Sea Systems
You will need to know the full load amps for the thruster and the circuit length. Circuit length is the length of the positive and negative cables to and from the thruster to battery added together.

I had a similar occurrence when. unknown to me, my 24 volt charger went out, letting the voltage drop to almost 15 volts. This apparently pushed the amps over the cable rating and it melted. The slow blow fuse obviously never blew.

dimer2 said:

I had a similar occurrence when. unknown to me, my 24 volt charger went out, letting the voltage drop to almost 15 volts. This apparently pushed the amps over the cable rating and it melted. The slow blow fuse obviously never blew.

Click to expand...

Cables are sized to allow for 3-5% voltage drop at full load. Fuses are "sized" to protect the wire (from melting).

You might want to double check the fuse and/or the cable so they follow that mantra...

Many folks feel only a Class T fuse works well with real high amperage.

The change over might save a fire.

Thanks What Barnacles, the diagnostics and subsequent repair was performed by the yard (Michelle insisted as this was all under the bow bunk in which we sleep). Incidentally, the original install was also done professionally and the invoice shows all parts, including the slow blow fuse, was supplied by the thruster manufacturer. Wire run was only 3 feet. No problems since so hopefully they got it right.

FF, you would think but Vetus recommend 'slow blow'.

Vetus Bow Thruster Fuse C20 - 250 Amp

Home : Thrusters : Thruster Fuses : ZE250

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“Slow blow” type 250 Amp fuse, suitable for Thrusters, Windlasses etc. Encapsulated in glass to prevent fire.

2 things - one, many recommended wire sizing guides are either 3% or so drop and the others are 10%. When I worked for a marine electronics place they never referred to either...just used what they were 'accustomed to". When we got into installing things like inverters and windlasses (they wanted me to start selling thrusters also)...I suggested they start using those tables.


So I wouldn't assume installed wiring meets any standard...It could be just the right length leftover from "another job".


I also tend to see CBs installed on things like windlasses and thrusters...no one wants to be fiddling with fuses when resetting a breaker for something needed quickly is easier. Sure check manufacturers preference, but I really have only seen CBs for these motors but I do see fuses offered in many of the sales packages or accessories. I guess boat manufacturers or installers pick their preference.

The fuse in line with the positive cable should be sized for the cable. based on length calculations with a 10% voltage drop, it appears that your cable is undersized, or as mentioned earlier, a bad connection on that side of the fuse. excessive heat can cause a copper cable to become brittle. This has apparently been an unseen issue for quite some time. My recommendation would be to replace the cable. The cable can be cut back from the damaged spot and saved for shorter runs later, so not a total loss. I prefer a circuit breaker rather than a fuse, but if you choose to use a fuse in the circuit, Spares are always a good thing to have.

My $.03

seadogmike said:

The fuse in line with the positive cable should be sized for the cable. based on length calculations with a 10% voltage drop, it appears that your cable is undersized, or as mentioned earlier, a bad connection on that side of the fuse. excessive heat can cause a copper cable to become brittle. This has apparently been an unseen issue for quite some time. My recommendation would be to replace the cable. The cable can be cut back from the damaged spot and saved for shorter runs later, so not a total loss. I prefer a circuit breaker rather than a fuse, but if you choose to use a fuse in the circuit, Spares are always a good thing to have.

My $.03

Click to expand...

What type/brand circuit breakers handle 200+ amps DC? Normal searches turn up zip. My boat has a 300A fuse for the thruster.

True ...not many but have seen them...some of the thruster charts go up to 150 amps...then jump to 24V thrusters....

Eaton makes a line of DC circuit breakers (with some stand-alone breakers too) that go up to about 300 amps. Here is a vendor with some of them.

EATON Hi-Amp Marine Circuit Breakers - Panel Mount

High Wire said:

What type/brand circuit breakers handle 200+ amps DC? Normal searches turn up zip. My boat has a 300A fuse for the thruster.

Click to expand...

A circuit breaker is just that. It does not know the difference between AC and DC.

seadogmike said:

The fuse in line with the positive cable should be sized for the cable. based on length calculations with a 10% voltage drop, it appears that your cable is undersized, or as mentioned earlier, a bad connection on that side of the fuse. excessive heat can cause a copper cable to become brittle. This has apparently been an unseen issue for quite some time. My recommendation would be to replace the cable. The cable can be cut back from the damaged spot and saved for shorter runs later, so not a total loss. I prefer a circuit breaker rather than a fuse, but if you choose to use a fuse in the circuit, Spares are always a good thing to have.

Click to expand...

For a critical system such as a thruster, I would go with a cable sized to allow no more than a 3% voltage drop. 10% is fine for non-critical systems such as cabin lighting etc...

As you pointed out, the fuse in the cable is designed to protect the wire, not the thruster. The fuse simply is there to keep the cable from burning if there is a catastrophic failure such as a dead short. The fuse should never blow, but if it does that circuit needs to be cut and the underlying problem fixed before it is re-energized. For this reason, a circuit breaker is the wrong solution.

It may be that a circuit breaker should be included in the circuit to protect the thruster, but this would be in addition to the fuse in the positive cable within 7" of the battery.

Again, a CB would be there to protect the device, and could reasonably be reset, a fuse is there to protect the wiring and should never be replaced until the fault is found and fixed or you risk fire.

boatpoker said:

A circuit breaker is just that. It does not know the difference between AC and DC.

Click to expand...

Are you sure? DC has no zero crossing point so is more liable to arc.

dhays said:

For a critical system such as a thruster, I would go with a cable sized to allow no more than a 3% voltage drop. 10% is fine for non-critical systems such as cabin lighting etc...

As you pointed out, the fuse in the cable is designed to protect the wire, not the thruster. The fuse simply is there to keep the cable from burning if there is a catastrophic failure such as a dead short.

Click to expand...

Many electric thrusters have a thermal protection button to protect the motor from overloads. If you over-use it, the motor gets too hot and the thermal breaker opens, and no thruster until it cools down...

dhays said:

For a critical system such as a thruster, I would go with a cable sized to allow no more than a 3% voltage drop. 10% is fine for non-critical systems such as cabin lighting etc...

As you pointed out, the fuse in the cable is designed to protect the wire, not the thruster. The fuse simply is there to keep the cable from burning if there is a catastrophic failure such as a dead short. The fuse should never blow, but if it does that circuit needs to be cut and the underlying problem fixed before it is re-energized. For this reason, a circuit breaker is the wrong solution.

It may be that a circuit breaker should be included in the circuit to protect the thruster, but this would be in addition to the fuse in the positive cable within 7" of the battery.

Again, a CB would be there to protect the device, and could reasonably be reset, a fuse is there to protect the wiring and should never be replaced until the fault is found and fixed or you risk fire.

Click to expand...

At some point the 3% drop for individual items is often overlooked because the wiring size would be ridiculous. The 10% is much more common for everything but panel boards, running lights, electronics, maybe bilge pumps. Thrusters while likeable, and some would swear are needed, they aren't really critical...and if they are a CB is a better answer.

There is usually a pretty good difference in sizing correctly and ampacity where the wire overheats.

When did fuses become the only suitable circuit interruption for wiring?

If so most houses are in BIG trouble.

psneeld said:

When did fuses become the only suitable circuit interruption for wiring?

If so most houses are in BIG trouble.

Click to expand...

On a thruster, the last thing you need to worry about is finding a replacement fuse and wrenches to swap it out, since most likely, it failed when you are bow to a 2 million megayacht and were using the thruster to navigate around the big boat. So, a breaker, that can be reset fairly quickly is not a terrible idea.

stubones99 said:

Eaton makes a line of DC circuit breakers (with some stand-alone breakers too) that go up to about 300 amps. Here is a vendor with some of them.

EATON Hi-Amp Marine Circuit Breakers - Panel Mount

Click to expand...

I do see a 200 amp but nothing higher, even on the Eaton website. There are cheap Chinese 300A for car stereos. No way I would trust one.

It's interesting to speculate as to why this happened, but time is better spent making sure it won't happen again. Check to see that the cable is the correct size for the load and check to see that the overcurrent protection is the correct value for the cable. Check to see that all connections are clean and tight and that cable ends are terminated properly.

"So, a breaker, that can be reset fairly quickly is not a terrible idea."

A breaker can ONLY be reset after it cools down.

Its sked , not yours!

High Wire said:

What type/brand circuit breakers handle 200+ amps DC? Normal searches turn up zip. My boat has a 300A fuse for the thruster.

Click to expand...

Marine DC Breaker 200 amp

see post #22.... Archie made it pretty clear on druthers....