Circuit, Wire, Fuse Size

[Stu Jackson]

Originally Posted by Ex-Calif

For the basics like how to crimp, voltage drop, long established theory books can be good and I have a lot of books. I even have all my old college texts - Physics, Math, Stats etc.

However the day it's printed the book is out of date.

Solar is still fairly new and evolving every day. MPPT controllers, modern boat 3-channel chargers, inverters, battery technology. Oh and a laptop stores great on board as opposed to the 4 bookshelves of books I have - LOL

Also this community is great for bouncing ideas off of - group decisions always make better decisions. Reading a book and not talking to anyone would still give lousy learning and lousy results IMHO.

Dan, of course, you're right about this. But, that wasn't your original question. That's all I meant. You didn't ask about solar, MPPT controllers, modern boat 3-channel chargers, inverters, battery technology... And the books on the subjects you originally asked about really haven't changed since they printed the books.

And I also note that you're right about the evolving tech on this stuff. And I know you wouldn't, but there are many who start out with "I don't know anything about....fill in the subject....but, I don't have time to read all the posts on this subject, so could you please all retype the same answers to the same question 'cuz I'm too lazy to do my own homework?"

That's where the internet can be infuriating for those of us who surely want to help.

Hope all goes well with your projects.

You're right, too, that 16 ga makes little sense.

[Ex-Calif]

Originally Posted by Stu Jackson

Dan, of course, you're right about this. But, that wasn't your original question. That's all I meant. You didn't ask about solar, MPPT controllers, modern boat 3-channel chargers, inverters, battery technology... And the books on the subjects you originally asked about really haven't changed since they printed the books.

I am with you here and believe me - I passed tests on this stuff for my license along with 2 years of classroom and practical.

However - I have read a lot too and I know the mantra - size the fuse for the wire. Now I admit this is the first time I have stripped an entire electrical system out - added plenty of circuits though.

So the mantra says - stick a 25amp fuse in.

But wait - Let's talk about the bilge pump. If it draws 3amp and I have a 25 amp fuse. What if it starts to bind - not catastrophic. It might be nice to have a fuse that blows before the motor catches fire...

Anyway - I had decided to go 14g/5a just about everywhere possible (consideringv-drop, ampacity and the unit load) but this community is wicked smart and using all these experts as a free sounding board? Priceless.

Also there are folks who haven't got 2 years of school and 30 years of technical under their belts (add up everyone posting? hundreds of years) - My whole 26 foot boat refit thread is partially about documenting here the progress - I know I could start a blog but the dynamic dialog is great for me.

When MaineSail & SailorChick (true experts) jump in and reveal I could accidentally order aluminum lugs from Ancor, well, that could save me 3-weeks of downtime if I screw up - Awesome!

Oh - I still don't think one of my root questions has been answered and I actually am still a bit confused.

Wires in a bundle will have higher resistance due to interference with each other. They will also get hotter. That's why you size up.

But if a bundled 14g wire has a recommended max load of 10amp vs. 25amp free is the max fuse still 25a (heat capacity of the wire) or is it 10amps now?

[Ex-Calif]

Originally Posted by Ex-Calif

<snip>

I have been on the hunt for a free schematic drawing software - CNET is coming up bupkiss and I may have to do it in powerppoint or some kludge.

Any ideas on that one? (software should be free or nearly so and simple)

Found the "schematic tool" thread - here - Schematic Software - Page 3 - Cruisers & Sailing Forums

I ended up going with Expresssch for now.

Originally Posted by JK n Smitty

I use Draft Sight.

Thanks for the tip - I steered away cuz it looks like full blown CAD program (probably more than I need) and I could not open any user guides, tips and tricks and so on without registering and I just get bugged by companies that harvest my information just so I can express and interest in their product...

[Ex-Calif]

Practical Electrical
Here is a story that might explain why newbies have such a problem learning electrical.

Photo 1 - If you recall my original panel it was typical of a 30 y/o boat. Lots of stuff has been added, one by one over the years...

Photo 2 - The rats nest at the aft port side. Josh and George both wanted to grab the dykes and cut this all out - but notice - there are depth sounder and speed log sensor cables in there - care must be taken. Also In my naivete I am thinking I might be able to salvage some good wire - at least for spares...

Photo 3 - The first wires I dig out are the big ones. My alternator schematic has 4 wires but my alternator always had double wires on each post and I didn't know why! Note the little red pigtail - I think that went to the battery post. Note the expert use of electrical tape (both white and blue) with tape splices and so on - This is the main DC supply.

Photo 4 - Trace these "big wires" aft - Hmm.

Photo 5 - There's a duct under that vinyl - That's good to know - I might use that in the rewiring job.

Photo 6 - The wires come out forward at the old electrical panel - but they are cut off and bare! - One of these is alternator + and one is alternator ground, and they've always been connected to the alternator! Had they ever met, that would have been fun! I just wonder what year or what happened to need to abandon this circuit.

[Ex-Calif]

The rest of the story...

Photo 1 - Here is the rats's nest - Every circuit was a different wire type, wire size, some solid core and even some 3 gang Romex (house wire!) - The only marine wire I found was the bilge pump (that I installed!)

Photo 2 - This is why you don't use house wire on boats - note the black corrosion where the ends were in a terminal block. This wire is obviously not tinned.

Photo 3 - This is why you don't use automotive or Walmart terminal blocks - Note the corrosion. The yellow wire is the Seatalk data sire. "Why doesn't my Speed log work? And yes, this is my bad - I had seen this when I replaced the depth sounder a couple years ago - I was lazy but this tiny improvement would have been like spitting in the ocean.

Photo 4 - Here is the replacement DC supply circuit - I don't know what the heck kind of wire it is. But hey they insulated with electrical tape! they also marked the wire with masking tape so you know what the green wire is for!. They also have quite an assortment of connectors! What an abortion...

Photo 5 - One lonely wire left and a bunch of zip ties...

Photo 6 - But even this one gives us a boat wiring lesson. How to make splices. Fold the two circuits in opposite directions. And make sure you zip tie the electrical tape so it doesn't come off! Oh - I tried to take one apart - they were soldered. The one I got apart was failed at the solder joint and I could not get a photo as it disintegrated in my hand. I suspect that (once upon a time) fancy nav box used this wire loom and the PO probably thought the system failed when it was likely his installer's fault.

Photo 7 - IHNFI - I have No Effin' Idea! I don't know where it went or what it did. All I know is that ain't boat wire!

Photo 8 - All I could salvage. Depth gauge cable, Speed log cable, Volvo engine harness and bilge pump circuit. OK don't zoom in on my barrel splices. I am not proud...

Photo 9 - This is actually a marine grade terminal strip! However, someone patiently turned it into a DC bus. The green wire at the bottom right was DC supply and normally if it failed or any jumper going up all circuits above would be dead - But they got smart! The yellow/green stripe is also DC+ - redundancy!

Photo 10 - Nuff said...

[St. Elsewhere]

Originally Posted by Ex-Calif

However - I have read a lot too and I know the mantra - size the fuse for the wire. Now I admit this is the first time I have stripped an entire electrical system out - added plenty of circuits though.

So the mantra says - stick a 25amp fuse in.

But wait - Let's talk about the bilge pump. If it draws 3amp and I have a 25 amp fuse. What if it starts to bind - not catastrophic. It might be nice to have a fuse that blows before the motor catches fire...

I would suggest the following for a bilge pump:

1. It should have a dedicated circuit
2. Size the breaker/fuse to protect the pump
3. Size the wire for desired voltage drop, and to not exceed ampacity for the wire.

[Maine Sail]

Originally Posted by St. Elsewhere

I would suggest the following for a bilge pump:

1. It should have a dedicated circuit
2. Size the breaker/fuse to protect the pump
3. Size the wire for desired voltage drop, and to not exceed ampacity for the wire.

#1 Yes

#2 Yes - The secondary fuse, at the bilge switch, should not exceed manufacturers recommended pump protection sizing.

#3 Yes - However there should be a fuse sized to protect the circuit wiring within 7" of the + battery terminal. This fuse can be sized to the ampacity of the wire as the secondary fuse, in the switch, is the one sized to protect the motor from over heating and catching fire in a stalled rotor event.

#4 Most bilge pump circuits will have two fuses, one at the battery (to protect the circuit wires) and one at the bilge switch (to protect the pump)....

[DeepFrz]

As Gord May would say,“If you don’t have time to do it right the first time, how will you ever have time to do it over.”

[FlyingCloud1937]

Originally Posted by Ex-Calif

Oh - I still don't think one of my root questions has been answered and I actually am still a bit confused.

Wires in a bundle will have higher resistance due to interference with each other. They will also get hotter. That's why you size up.

But if a bundled 14g wire has a recommended max load of 10amp vs. 25amp free is the max fuse still 25a (heat capacity of the wire) or is it 10amps now?

Size the fuse/CB to 125% of the load. Then size the wire according to the voltage drop.

Part of sizing the wire also includes de-rating, you have to de-rate the cable based on insulation temp rating, whether it goes through an engine room, how many current carrying conductors in the bundle.

So a 14 ga with an initial rating may be 15 amps, but after de-rating its only 10 amps, so you would size the fuse for 10 amps.

But if your load is 15 amps, it will be under sized. So then you would need to up-size the wire to 12 ga. and re-run your sizing to confirm.

Lloyd

[hellosailor]

ex-
While you can just "fuse for the wire" there are other considerations, as you suspect.

Every wire has one ampacity "in open air" and another much lower one in heated or enclosed spaces, such as behind a bulkhead, under an insulating liner, or in an engine bay where there's extra heat poring into it.

So number one, fusing "for the wire" also means taking into account where the wire will be going. And how much of a safety factor you want, which plays into number two:

The intended load.

If you have a dedicated circuit for the VHF (which I'd personally call a critical piece of equipment that should have a dedicated circuit) then sure, maybe the wiring can take a 20A fuse. But if the VHF itself only needs a 5A fuse...I ask myself why on earth would I want anything more than that? If there's ever more than a 5A load on that wiring, something is burning, probably IN the radio, and I want the power to be cut off ASAP. So, I'd go with the 5A fuse. Is it "overprotecting" the wire? Sure. So what? It is also properly protecting the circuit, which is the wire and the load combined.

While wiring codes and agencies may write uniform standards to simplify life, you always have the choice of aiming higher.

Then there's the wire gauge versus voltage drop. Another case of standards versus personal choices. That pesky VHF makes another good example. It is designed to work on 13.8 volts, nominal alternator power. A 3% voltage drop in the wiring now cuts it to 13.386 volts, and yes, the output power and clarity will be affected somewhat. Of course, if you are trying to use the VHF because the engine died and your batteries are down to 12.2 volts...oops, you've only got 11.83v to the VHF, and at that point it won't be working as well at all.

OTOH a 3% drop in a lighting circuit will just mean your cabin lights are a little dimmer. Or not, if they're electronic and not voltage dependant at all!

Again, it is to your discretion to put an importance on these things, to your preferences.

I've spent a lot of time on a boat with a "forward panel" at the mast. It made sense, one run to the mast, then all masthead lighting located on that panel. Saved a batch of wire, too. But it also made it harder to send someone below and say "turn on the nav lights" since that meant "Which panel?" and getting light forward to find them. Again, personal preference. I like all the switches on one panel, one location. But then...do you still run one heavy ground forward? Or multiple line pairs? <G>

You'll find that whatever wiring you buy, often a 100' spool will cost less than two 25' spools, so it pays to just buy the bigger spools, and the worst thing that will happen is that you'll have wire on hand to use for next year's projects.

While you're doing the wiring, consider putting in some spare runs for equipment that you're thinking about adding in the future, or at least, run messenger lines there-and-back so you can just pull new lines through at a future date, if you need to.

Design software? A legal pad & #2 pencil. No learning curve, easy to edit, and you can get fancy after it is all done, installed, working and confirmed.

[Ex-Calif]

Thanks for all the thoughts and replies!

- Voltage drop and ampacity - I have always primarily considered ampacity but on a boat (especially mine) efficiency is paramount. I will be running 14g just about everywhere and sizing for 3%. I've got this all straight in my head now. One thing a lot of folks don't consider is they start with a 5a fuse, add something in downstream and don't reconsider the load - sometimes (wire capacity permitting) the fuse should get upgraded to 10a or something.

- Circuit -dedication. I am adding a 3 switch panel because a couple things were ganged. I won't have room for more switches but I am also adding a 10 gang fused unswitched bus. In the future if something is added it won't get ganged.

- Equipment protection - some electronics will have circuit protection and then equipment protection at the equipment - like the stereo.

- Forward distribution panel - I don't intend to put a switch panel. This is more about future maintenance. Pulling wire is always easier through conduit and straight pulls. I plan to have one straight conduit aft to forward. Then where the wire might take a 90 degree turn put in a terminal block. Cabin lights come to mind. One supply forward and then "spider" out 5 pairs across the roof. Same idea for all the mast wire, forward cabin power and nav lights (on the pushpit). If I need to add a wire or replace it the conduit will help.

- Schematic - I have a pretty good equipment list, the energy budget is coming together and I settled on expressch.exe for the schematic software. main distribution schematic attached. It's coming along. This boat will have documentation when the next guy gets it - LOL...

[Stu Jackson]

Nice, thorough well drawn diagram. You should be proud of it.

1. Why are you running your AO (B+) to the 1-2-B switch when you have an ACR?

1a. There are only 3 posts on the back of a 1-2-B switch, you seem to show more, like 6 little circles. What's that?

2. How do you run all 14ga wire for 3% drop?

[Ex-Calif]

Originally Posted by Stu Jackson

Nice, thorough well drawn diagram. You should be proud of it.

1. Why are you running your AO (B+) to the 1-2-B switch when you have an ACR?

1a. There are only 3 posts on the back of a 1-2-B switch, you seem to show more, like 6 little circles. What's that?

2. How do you run all 14ga wire for 3% drop?

1. - Great catch. That's why I am posting the schematic periodically. If the alternator and solar are on the house side (batt 1) then there is a chance I could run the engine with Batt 2 selected and fry the alternator. When batts are in all or 1 - the "standard mode" batt 2 will be topped up by ACR. When in all or 2 the charging systems will supply both banks or 2 only. I suppose the solar could go upstream of the switch and only supply batt 2 (house) - This could be a good strategy, as on another thread they are talking about how the tach can be blanked if the batts are "full" - if the engine is running and the batts are full I could go to bank 2 and the tach won't see the solar.

1a - This is my attempt to show the internal circuitry of the 1-2-all switch. If you zoom in you should see that the output is actually the center "dot" 2 pairs of dots are connected to each other inside the switch - As the pointer rotates the contacts are made and broken appropriately (I think)

2. I haven't completed the circuit list but wire run length has been integrated into the energy budget spreadsheet so I can get peak, average load, hours per day (hook, marina & daysail), then wire length, gauge, recommended fuse size and predicted V-drop. Right now the max run I have is like 20 feet and that is for cabin lights (switching to LED). DC buss supply and ground busses will be bigger gauge.

[FlyingCloud1937]

One thing you need to remember.

All critical circuits need their own dedicated circuit each. So anyt load that is required to operate the vessel be on it's own dedicated circuit. Ie no ganging Nav lights with any other cicruit, VHF dedicated, radar, GPS, engine electrics.

I even go as far as separate circuit for anchor light, then I lead the the nav lights switch to a gang fuse, so all running lights are fused separate. That way if you have a fault in one of the running lights, all others still work. Now if you are going to add a separate Tri-light then that should also be on it's own circuit.

Lloyd

Originally Posted by Ex-Calif

1. - Great catch. That's why I am posting the schematic periodically. If the alternator and solar are on the house side (batt 1) then there is a chance I could run the engine with Batt 2 selected and fry the alternator. When batts are in all or 1 - the "standard mode" batt 2 will be topped up by ACR. When in all or 2 the charging systems will supply both banks or 2 only. I suppose the solar could go upstream of the switch and only supply batt 2 (house) - This could be a good strategy, as on another thread they are talking about how the tach can be blanked if the batts are "full" - if the engine is running and the batts are full I could go to bank 2 and the tach won't see the solar.

1a - This is my attempt to show the internal circuitry of the 1-2-all switch. If you zoom in you should see that the output is actually the center "dot" 2 pairs of dots are connected to each other inside the switch - As the pointer rotates the contacts are made and broken appropriately (I think)

2. I haven't completed the circuit list but wire run length has been integrated into the energy budget spreadsheet so I can get peak, average load, hours per day (hook, marina & daysail), then wire length, gauge, recommended fuse size and predicted V-drop. Right now the max run I have is like 20 feet and that is for cabin lights (switching to LED). DC buss supply and ground busses will be bigger gauge.

[Ex-Calif]

Originally Posted by FlyingCloud1937

One thing you need to remember.

All critical circuits need their own dedicated circuit each. So anyt load that is required to operate the vessel be on it's own dedicated circuit. Ie no ganging Nav lights with any other cicruit, VHF dedicated, radar, GPS, engine electrics.

I even go as far as separate circuit for anchor light, then I lead the the nav lights switch to a gang fuse, so all running lights are fused separate. That way if you have a fault in one of the running lights, all others still work. Now if you are going to add a separate Tri-light then that should also be on it's own circuit.

Lloyd

Yup - When I got the boat the steaming light was on the same switch as the nav lights? I guess the PO always motored at night. To free up a switch I ganged the instrument power with the instrument lights - not ideal.

I also added a 12V socket. I ganged it in with the cabin light master. I had a 6 switch panel and am adding 3. I just won't have the panel space to add on another 6 although I would like to. I don't need it now but might in the future...

Final schematic isn't done and if I have to have 12 switches I will have to find the space even if it means rearranging the cabin build. Actually with the 3-panel I could have 15!

I am adding a stereo, foredeck light and want the cabin outlets switched so I still have some homework to do.

When is enough switches enough? Never - LOL...