New to Wiring, Looking for Feedback on Improving Circuit Plan for a Mirage 5.5

[Davidasailor26]

Would the ampacity be for the wires leading from the battery until the next fuse/breaker, I assume? The ampacity of the wires leading from the battery to the panel (where there are circuit breakers in the panel) is 35... so based on what I've been told here, I should get a DC-rated T fuse of 35 (or less, but more than my maximum load) amps capacity, placed close to the battery. The wires from my panel to individual loads are 20 amps, and my circuit breakers on my panel are from 15 down to 5 amps... so in both cases, the fuses/circuit breakers are less than the wire's capacity, so the fuse/breaker would go off before the wires failed, and that is what is desired, if I'm understanding it correctly?

Yes, you’ve got that all correct. But you probably won’t find a 35 amp class T, so the safest thing there would be to use the smallest class T you can get near the battery, and a 35 amp fuse just beyond that.

Sounds like you’ve got relatively small gauge wires though. Depending on the length of your wire runs you should make sure the wires are large enough to handle your loads with minimal voltage drop. Often the voltage drop will be the restricting factor in wire size rather than overall ampacity. It sounds like your primary wires are 14 AWG? If so consider that to keep the voltage drop minimized that’s only good for up to 10 amps if the circuit length is 10 feet. The 10 feet is the total round trip length of the circuit. Also keep in mind that voltage drop is cumulative, so there’ll be some drop from battery to panel over the 14 AWG, and some drop from panel to each load over the smaller gauge wires. Ideally the cumulative drop for loads like nav lights should be kept under 3%, or 0.36V.

 

[Tally Ho]

I used a fuse like this when I replaced my batteries a few years ago (i didnt have any fusing at the time).

Samlex America Samlex Solar DC-FA-100 Fuse Assembly https://a.co/d/4Xxlo9n

I used a 300 amp fuse after I blew the 100 amp when starting the diesel engine the first time.

but you don’t have an in-board, so the smallest fuse should be fine.


Greg

 

[JBP-PA]

Many people suggest that a MRBF type fuse is OK on LFP batteries as long as it's only one battery and 100Ah or less. These can be found down to 30A and have a 10,000 AIC which is not quite the 20,000 AIC of class T, but "probably " good enough for a single 12V 100Ah battery.

 

[dlochner]

I used a fuse like this when I replaced my batteries a few years ago (i didnt have any fusing at the time).

Samlex America Samlex Solar DC-FA-100 Fuse Assembly https://a.co/d/4Xxlo9n

I used a 300 amp fuse after I blew the 100 amp when starting the diesel engine the first time.

but you don’t have an in-board, so the smallest fuse should be fine.


Greg

A Marine Rated Battery Fuse like the one Greg mentioned works well with LA batteries. The problem is with the LFP batteries, if there is a short the power of the battery will blow the fuse and then arc over the blown fuse, that's why a Class T fuse is necessary. The fuse is filled with silica which prevents the arcing. In the OPs case he is using fairly small gauge wire, in the case of the a direct short from the LFP battery the wire will turn red hot in seconds, presenting a fire hazard.

McMaster.com does have small amperage Class T fuses in a cartridge format. A quick look at McMaster did not reveal any cartridge fuse holders, but I didn't spend much time looking.

One workaround would be using a larger marine Class T fuse and holder. Take a short large cable from the battery to the fuse holder and then to a battery switch (which would be a good idea regardless) and then to a fuse for the run to the panel.

If I was to do this, I would use BEP Marinco Pro switch and fuse holders. They are much easier to work with than the Blue Sea switches. The BEP switches are surface mounted and can connect to fuse holders and busbars with a short connector bar.

BEP Marinco Pro Installer 770-EZ

SKU: BEP-770EZ Model 770-EZ

baymarinesupply.com

Home | BEP

www.bepmarine.com

 

[JBP-PA]

I wonder if your estimate of usage is perhaps a bit high. On my boat, with LED cabin lights, running modest instruments (tablet chartplotter, a couple MFDs, VHF, no radar or AIS), charging a couple devices, I only use 5, maybe 10 Ah per day. 120 watts of partly shaded solar was more than sufficient. When I added a portable fridge, I had to add an additional 140 watts, but that was more than enough.

 

[KCofKC]

A possible supplier for the required fuse holder might be Del City, they have lots of harness type of parts. I have used them for a lot of items.

 

[JohnBisMe]

In regard to various questions and comments, most of the wire I have purchased to put into the boat is 10 gauge tinned marine-grade wire... I had some 12 gauge, too, that was going to go to individual devices from the panel. I have some 8 gauge to go from the charger to the battery and from the battery to the panel. I figured it was thick enough? It's possible I looked up the amp capacity for the wrong type of wire, too. It's also possible that I overestimated my power usage. As I went through devices, I rounded things up by a half-amp or so to make sure I was covered, and if you round up enough devices, the error could become significant. Basically, I hope to run 1) A VHF radio 2) A Raymarine wind/depth/speed indicator and small display, a compass light, nav lights, steaming light, deck light and anchor light (all used as needed and all LED), LED cabin lights (with a couple of LED reading lights near the berths), a couple of 6" fans, a bilge pump (1100gph) and MAYBE if I'm feeling rich and sporty, a small 2-speaker radio/stereo. Also, I may have found a fuse and block I can use...my plan is similar or identical to something suggested above... to have a short cable from the battery to the fuse block, then to a battery switch I've already installed, then on to the main panel and bilge pump.i

 

[Davidasailor26]

most of the wire I have purchased to put into the boat is 10 gauge tinned marine-grade wire... I had some 12 gauge, too, that was going to go to individual devices from the panel. I have some 8 gauge to go from the charger to the battery and from the battery to the panel. I figured it was thick enough? It's possible I looked up the amp capacity for the wrong type of wire, too.

Those wire gauges all sound pretty reasonable given the types of loads you’re talking about. For the actual ampacity check the temperature rating of the wire’s insulation then use a table like the one at http://assets.bluesea.com/files/resources/reference/21731.pdf

 

[KCofKC]

Sounds to me a bit of overkill but that is never a bad thing. But do run it past the capacity chart that David shared and I think you will find with all of it your in pretty good shape!

 

[Captain Larry-DH]

Thank you both for your advice! Based on your recommendations, I have ordered a 17amp Victron waterproof bluetooth-capable charger (I know you said 20, but the 17 fits my needs and budget and is pretty close, and should charge 3x-plus faster than what I had previously purchased. I'll forego attaching the outboard alternator, even though I'd already bought the ports and connections. That's a hard decision to take since I got the engine specifically for the alternator (which made it more expensive), but since both of you recommend against it would be foolish to fly in the face of experienced advice.

I've looked at some solar panels, and have found a flexible 55w panel that could fit on my foredeck (so long as no one wants to sunbathe there or we're not taking in the headsails). I'll figure out a way to make it removable so we can get it out of the way except when we want to charge. Together with the little panels, that would give me 75w of charging power, which based on what you've told me would get me closer to 20-25ah per day, so I'd only be running a 10-20ah/day deficit and would be good (I assume) for a weekend trip without TOO much concern about conserving.

Thanks again very much for the help!

If that flexible panel is rated for the bend radius of your foredeck, you should find out of it’s also rated for walking on. I’ve had good luck with semi-rigid panels mounted to the deck with 3M VHB 2-sided tape at the perimeter. Leave gaps between the tape strips to facilitate water drainage and minimize trapped water.

 

[JohnBisMe]

One last question I had would be about protecting devices when the battery is being charged... both my charger and my solar controller seem to put out around 14 volts to charge my 12 volt battery. But while this charges the battery, does this hurt my 12v devices? Can I only charge the battery when my devices are disconnected, or does the extra volt or two in the circuit charging the battery really not hurt things on the other end? (Or do I need some sort of device between my loads and all my charging that makes sure it never goes above 12 volts?)

 

[Davidasailor26]

All of the “12 volt” loads on the boat are generally designed to handle anywhere from about 10 V to 15 V, at least. 12 volts is just the nominal voltage, but everyone designing those systems know the voltage will be less when the battery gets discharged and more when it’s charging, whether that charge be from solar, a shore power charger, or an alternator.