Wiring Gauge questions

[Derekxj]

Hey guys so a question for you experienced re-builders.

Im going to be using the boat for a little while, prior to dry docking it and gutting / rebuilding the whole boat.
Im currently working on sizing my main cables going to my new 12 circuit fuse block. there are no heavy duty electronics or gear.
Just your basic radios, lights, chart plotter, etc. I dont have anything serious. I currently have a battery switch and new dual battery setup in the back of the boat.
Im going to be now running a - and + up to my fuse block. most likely a legnth of around 15'. Should i be using 4 gauge? or can i get away with 6 for this?

A friend of mine who has alot of 12V electrical skill told me that i should me more than O.K. using 18G for everything on my boat. It would certainly make ordering easier also!!!

Does this sound about right? It seems a bit thin to me but again, no windless, no blowers.....the most draw i'll probably ever see is from the bilge which will ( hopefully!! ) rarely be on. The rest of my circuits will be a USB charge port. Cigarette lighter. bow / stern light. Some cabin lights.

 

[Silvertip]

Your friend is partially correct. Unlike a car, a boat takes considerable pounding so while in some applications 18 gauge is adequate for the current it must carry, but is not "durable" enough to handle the abuse. The feeder circuit from the battery/battery switch to the fuse panel should be #8 or 10 gauge because it will carry all of the current fed from the fuse block to the attached accessories/loads. 16 gauge could be used for most fuse block-to-accessories.

 

[UncleWillie]

10 Gauge is the common size used to the Dash Fuse block.
And should be fed through a 30 amp Fuse within 6 inches of the Battery.
Don't forget about the Ignition/Starter, Gages, Blower and Bilge Pump.
You may not need the 30 amp capacity, or the 12 circuits today, but you will not have to rewire in the future.

 

[canuckmark]

The length of the main and/or feeder wires is an important factor in determining gauge. Same as when you run an extension cord to, say, a table saw. If the cord was only 6 feet long, 12 or 14 gauge would be fine. But go 50 or 100 feet and the same gauge cord wouldn't be enough. It's all about loss and longer runs lose more.

I agree with the 10 gauge suggestion, that's what mine is. But the run is short too. At 15 feet you may want to bump up to 8 gauge just to be safe. There are online calculators and charts that may help if you search around a bit.

 

[Derekxj]

Thanks for all the input gents. Why is blue sea systems telling me to run nothing smaller than 4 gauge to my fuse panel from the battery??? I've already got a 120amp megafuse setup to go inline before my fuse block. I guess I'll run 16 gauge instead of 18. The price difference per spool is splitting hairs.

 

[Silvertip]

Stop! 16 gauge wire is fine for wiring the accessories to the fuse block. It is NOT ok to use 16 gauge between the battery and the fuse block. What on earth do you thing you need a 120 amp fuse in the feed? If you used 16 gauge wire that feed wire would melt before the 120 amp fuse ever blew. Just use #8 between the battery and the fuse block with a 30 amp fuse at the battery. Use 16 gauge for accessories.

 

[Derekxj]

yeah i would never in a million years use 16 gauge as a main branch cable.

the bluesea techs when i spoke to them thats what they recommended. could i get away with a 100amp? I want this system to be safe and reliable, i however already bought the AMG weather tight fuse holder and would like to use it being as im not making any connection right to the battery. my branch cable to the fuse block will be coming off of the battery switch common leg.

 

[Silvertip]

Gee Whiz: What accessories are you planning to install that requires #4 wire and 100 amp protection? I suppose the Blue Sea rep thought because you had a 12 fuse panel, with an average load of 10 amps each, that says 120 amp feed. Fact is, You will NEVER operate all 12 circuits at once, and the chance of all 12 having a 10 amp load is equally silly. So why not tell us what you have for accessories so we can actually help you. Even your house is not wired for 100% of Line Capacity.

 

[Derekxj]

Gee Whiz: What accessories are you planning to install that requires #4 wire and 100 amp protection? I suppose the Blue Sea rep thought because you had a 12 fuse panel, with an average load of 10 amps each, that says 120 amp feed. Fact is, You will NEVER operate all 12 circuits at once, and the chance of all 12 having a 10 amp load is equally silly. So why not tell us what you have for accessories so we can actually help you. Even your house is not wired for 100% of Line Capacity.

as for my loads as of now. 1. Gauge lights 2. Stern light. 3. Bow light. 4. USB charging socket. 5. Cigarette lighter socket 6. Cabin lighting ( only a few ) 7. A few courtesy lights on the deck. 8. Ship to shore VHS. 9. Bilge pump. Small 500 gph 10. Chart plotter 11. Fish finder 12. Stereo. I dont yet have a stereo or deck lighting, but will. Again I know most of these are low load but I want to plan ahead so if I tack on another small 6 circuit block perhaps I wont be undersized. I had told that rep what I would be running and he said if I went with 6 gauge I could probably get away with it but the wire may get a little warm at times.

 

[bruceb58]

You council easily run 10 gauge for the items you listed and not have an issue.

 

[canuckmark]

Just to expand on what silvertip was saying, the main fuseholder is meant to protect that main power line so it needs to be matched to the gauge of wire. In my old car audio days, in a typical amplifier run from the front battery to the trunk, an 8 gauge would need a 30 amp fuse protecting it. A 4 gauge would need a 60 amp fuse. You can go lower in fuse value, but it is unsafe to go higher.

So if you really want to use the 100/120 amp fuse you'd actually need to run a 2 or 0 gauge wire.

 

[UncleWillie]

1. Gauge lights <1A
2. Stern light. 2A
3. Bow light. 2A
4. USB charging socket. <1A
5. Cigarette lighter socket <10A
6. Cabin lighting ( only a few ) 2A
7. A few courtesy lights on the deck. 2A
8. Ship to shore VHS. 5A(TX) <1A(RX)
9. Bilge pump. Small 500 gph 5A
10. Chart plotter 2A
11. Fish finder 2A
12. Stereo. 5A

Total < 39A

You will never have all these circuits maxed out at the same time.
The Cigarette Lighter, VHF Radio(TX), and Bilge Pump are extremely intermittent use items.
Typical draw will be well under 15A. 10AWG and a 30 Amp feed will be fine.

Just checked the Main panel in my house.
The Master Beaker suppling the panel is 200 amps.
The 30 breakers in the Panel add up to 805 amps.
The Electric bill indicates a 20 amp average, with a 45 amp peak current per month.

 

[bruceb58]

And even if you did draw 30A, a 10 gauge wire will only have a 5-10% drop depending on distance for the rare instance that you had everything on.

 

[Derekxj]

1. Gauge lights <1A
2. Stern light. 2A
3. Bow light. 2A
4. USB charging socket. <1A
5. Cigarette lighter socket <10A
6. Cabin lighting ( only a few ) 2A
7. A few courtesy lights on the deck. 2A
8. Ship to shore VHS. 5A(TX) <1A(RX)
9. Bilge pump. Small 500 gph 5A
10. Chart plotter 2A
11. Fish finder 2A
12. Stereo. 5A

Total < 39A

You will never have all these circuits maxed out at the same time.
The Cigarette Lighter, VHF Radio(TX), and Bilge Pump are extremely intermittent use items.
Typical draw will be well under 15A. 10AWG and a 30 Amp feed will be fine.

Just checked the Main panel in my house.
The Master Beaker suppling the panel is 200 amps.
The 30 breakers in the Panel add up to 805 amps.
The Electric bill indicates a 20 amp average, with a 45 amp peak current per month.

Thanks for all the info!! Im the type of person that likes to run the VHF alot, mainly for weatherband and sometimes communication.
So are you saying what you listed are the sizes of fuses i should have in each fuse location for each circuit on my block? now........Where the heck could i make use of the really nice weathertight MEGA fuse holder that i bought?! lol.

 

[bruceb58]

No fuses will be higher. Also, some of those functions will be on the same fuse like the bow and stern lights. Chart plotter and fish finder would be on the same circuit, etc...

 

[Derekxj]

No fuses will be higher. Also, some of those functions will be on the same fuse like the bow and stern lights. Chart plotter and fish finder would be on the same circuit, etc...

Would it be a bad thing to keep those items seperated? Im a bit crazy and want EVERYTHING completely isolated to its own circuit.

 

[bruceb58]

Would it be a bad thing to keep those items seperated? Im a bit crazy and want EVERYTHING completely isolated to its own circuit.

Some things yes. Especially the bow and stern lights since they are on the same switch.