Basic Rewire Advice

[Doryboy]

  Well I'm finally about to dive into a very basic rewire and needed some guidance on wire gauge and fusing for the batteries. Also if you see something on the diagram that needs changed, I would appreciate any advice there too. I'm just trying to put together a base electrical system that is safe and can be added onto as I go.

  Right now everything is wired off one starting battery, which was a complete mess of wires and terminals stacked on each other that I have been cleaning up and/or terminating.

  It's pretty bare bones, no added electronic equipment besides lights, bilge, horn, GPS and a 12V socket. I'm gonna run 6 AWG wire to the helm to allow for additional items in the future (VHF, washdown, USB ports, wiper etc...). The existing wire to the helm is only 12 & 14 gauge to 2 fuse blocks.

  I don't plan on installing a pot puller or downriggers so really no need for a +power hub in the stern which is why I chose a PowerPost for the grounds instead of a dual bus bar or safety hub.

I'm adding the following Blue Seas items:
Dual Circuit Plus Switch 6011
285 Series 50 Amp Breaker
PowerPost Plus
6 AWG to helm fuse block 15' run
12 Circuit ST Blade Fuse Block
Hopefully an ACR later on

Yes I do realize I won't be charging the house battery with this switch but I chose it because of adding the ACR in the future. My plan is to just to top off the house battery with a charger after each trip.

Both batteries will be in the starboard stern next to each other.
What gauge should I run from the battery's positive terminals to the switch and the negative terminals to the PowerPost? 4AWG?
Do I need to add the MRBF terminal fuses to both batteries or just the house? Fuse size recommendations?
If it helps the motor is a 2001 Johnson 70HP 2 Stroke. The cable from it says 4/2 on it, that is just duplex 4 AWG, correct?

[DevMah]

The positive wires from the batteries to the switch must be at a minimum the same size as your motor cranking wires. So, if you have selected 4 AWG to your motor then all of the wires to the switch must be the same or larger. The negative jumper from the house battery to starting battery must be at a minimum the same size as your motor cranking wires.
 
The Negative from starting battery to power post the  AWG 4 should be good as this only feeds the accessories. You could just use AWG 6 also.

I always recommend MRBF for safety reasons, however if your connections are short and you can also just sleeve the wires. If you do go the MRBF way, then use the dual MRBF holder as you will need this if you are planning to add an ACR later.


Dev

[Barnaclebob]

6awg to the helm is probably overkill, 8 will be cheaper and much easier to work with.  According to the Blue sea systems circuit wizard, 8awg with a 20ft total conductor length and 10% acceptable voltage drop can handle 80 amps.  It would take a LOT of helm accessories on all at once to draw that much power.  Many people think you take the total amperage of the fuses on your panel to figure out the amperage of the feeder wire but thats not necessary.  As long as the breaker protects the wire and doesn't trip from the actual current draw then you are good.  There's probably rule about actual continuous current draw not exceeding 80% of the breaker but you are still far away from that with 8 awg and a 50A breaker.

I agree with Dev, 4AWG for all of the battery to switch cables.  Gregs marine wire is a great place to buy them.  Super fast custom battery cables at prices cheaper than you can DIY.

I don't prefer the use of a MRBF fuse on any starting wires.  Having that connection be fused is fixing a problem that doesn't really exist in practice and has a different set of potential issues IMO.

http://circuitwizard.bluesea.com/

https://gregsmarinewiresupply.com/

When you do add an ACR, im a big fan of the Blue Sea Systems batterylink charger.  That way you can take care of your ACR and charging needs all in one device.

[disposable]

I'm trying to remember what I did... and the real professionals will surely correct me if I'm mistaken, but...

I made a "worst-case busy night with flooded bilge theoretical total draw" (highly improbable but what happens if I was running my bait tank and both bilge pumps and my bait tank+squid+spreader+anchor lights, with the fish finder and radio and radar on...) and based my wires/MRBF off that. I don't remember the sizes, but my helm run was pretty overbuilt... and of course, I've NEVER come even remotely close to that maximum draw... but it's worth considering that worst case scenario. also consider your potential expansion (especially if you plan to feed an anchor winch/drum from that helm panel). finally, don't forget that the "total run" of your wire is from the source, to the helm panel, AND BACK to the battery bus/"ground" (that changed my wire size by one gauge, I think).

[DevMah]

6awg to the helm is probably overkill, 8 will be cheaper and much easier to work with.  According to the Blue sea systems circuit wizard, 8awg with a 20ft total conductor length and 10% acceptable voltage drop can handle 80 amps.  It would take a LOT of helm accessories on all at once to draw that much power.  Many people think you take the total amperage of the fuses on your panel to figure out the amperage of the feeder wire but thats not necessary.  As long as the breaker protects the wire and doesn't trip from the actual current draw then you are good.  There's probably rule about actual continuous current draw not exceeding 80% of the breaker but you are still far away from that with 8 awg and a 50A breaker.

Humm...

8/2 duplex tinned marine per foot is $2.01 per foot x 20'= $40.20 at Greggs
6/2 duplex tinned marine per foot is $2.84 per foot x 20'= $56.80 at Greggs


https://gregsmarinewiresupply.com/product/6-2-awg-duplex-tinned-marine-wire/
https://gregsmarinewiresupply.com/product/8-2-duplex-tinned-marine-wire/


You save a whooping $16.60.
IMO
$16.00 is a McDonald's lunch nowadays...
its always better to over design than under especially if the cost is minimal...

Please note that a you need to add both negative and positive cable lengths (complete run) in your calculation as disposable is has mentioned.
In my general experience you are looking at approx. 30' of total run for a 17'  boat by the time you snake the wire up and around to the helm.



FYI
I like to use the duplex as it has the added outer jacket that adds one more line of protection from abrading and chaffing.


Dev

 

[Doryboy]

Thanks for all the input.

Yes, it's probably oversized with the current equipment. I put in 20 amps on a 30' roundtrip into the blue seas calculator and it recommended AWG 6. This was with a 3% drop in the calculation though. I really don't know how to calculate it when some loads would be critical and other are not. It would be nice working with AWG 8 vs AWG 6 however.

The price difference for custom 15' cables through Greg's was (AWG 8) $16.91 vs (AWG 6) $23.33, so the extra $13 for the pair wasn't a big concern.

I did consider the duplex for the added protection it provides but I would like the terminal ends on the bigger gauge to be done by professionals and not have to buy the tool for crimping that large of cable. I will add some sort of protection around them up to the helm.

Ok, so AWG 4 for all battery connections will be fine then, that's kinda what I thought. I'm assuming AWG 2 would work also but not necessary . I know a repower will eventually happen down the road so if AWG 2 helps prepare for that transition. I would be open to just doing that and being done with it.

[disposable]

Well, if a rewire is in your future... that kinda changes things a bit. In my humble opinion, I'd rather spend twice as much if I didn't have to do that job twice... idk about you bot for me there were some tight squeezings and endless zip-ties and a couple busted knuckles... the dollar difference is is real but ultimately it's all just boat bucks... but if you can do something now to save an uncomfortable afternoon next year, DO THAT!

[davidsea]

    When in doubt, Dev knows....

  Looking at your diagram, either the 6011 or 5511e would be great - The off/on/combine  will let you shut everything down, separate, or combine your circuits to charge both batteries when running.    It will also make adding an ACR later very easy.  Plan ahead when installing the switch to allow mounting the ACR directly below, and it will take only 2 short battery cable jumpers (and a small ground wire) to do it.    The 50a 7183 breaker is good as well, mounted as close as possible to the house side of the switch. (Mine is mounted just above the switch and the ACR just below) ....image follows....
  When wiring, remember that both battery-to-switch cables need to be the same gauge, big enough to handle the start current from either battery in the 'combine' position. 
    My personal preference is single-point ground wiring at the stern, where ALL grounds - from motor, both batteries, and the (-) feed to the helm - attach to a single post or bus near the batteries - like a PowerPost+.   Mine is visible where all the black cables converge just below the bottom of the bait well.
    The only change you'd need to make would be running separate (-) cables for each battery, rather than daisy-chaining them, and moving the motor (-) cable to the post. 
  The other preference, which has gone back and forth here on the site, is the need for a MBRF-type device on the start battery.  I don't use one, since I feel the extra point of possible connection failure in the start/run circuit isn't worth it - if the wiring is done properly.  If your boat makes sparks and blows fuses regularly, then  you'd best have a fuse there , too....   

   

[Doryboy]

Thanks for image on how you positioned everything and tips for where the ACR can go later on. It looks like your ACR is connected to the inputs on the switch instead of the batteries? It's hard to tell, did you fuse those connections?

Ok, easy change for the - cables. I was only daisy chaining because I had seen where people recommended to put the outboard's ground straight to the starting battery and I was unsure if I
could stack all 4 terminals onto the PowerPost.

I think your photo is actually where I got most of my layout from during all my forum searching. One other question now that I think of it, is how long/type of screws did you use for affixing the breaker, switch, Powerpost+, and ACR?

[davidsea]

  Yes, the ACR cables jump directly to the battery switch.   Doing this eliminates extra wires and connections, and the batteries are essentially connected direct, not fused.   (This is also a good place to connect the output wires from a dual-bank onboard charger.   If you do this, make sure the charger leads have fuses close to the switch terminals, since the small-gauge charger leads are then essentially connected direct to the battery.)
  ALL of the battery leads, both pos and neg, and also the ACR jumpers at the switch, should be of the same gauge.   The 'combine' feature on the switch means that if the start battery is low or dead, the full starting current to the motor will come from the house battery and cables, and the full charging output will go through the ACM.
  Some have used a starboard sheet to mount the electricals, but since the sheet still has to be screwed to the liner anyway, I've just mounted the 3 parts (switch, ACR, 50A breaker) direct.   As long as you drill small pilot holes first to avoid splintering the liner, #8 or #10 coarse-thread SS sheet-metal screws seem to work just fine.   The foam behind the liner is fairly thick, but offers no real screw-holding ability, so I've used screw lengths that penetrate no more than an inch behind the liner.  If you're nervous about depth, run a piece of wire through the pilot holes to check before running the screws in.  My powerPost is mounted to the transom behind and just above the batteries - no worries about depth there.

[headduck]

Fyi... This is Dev's job... and hes nice enough to do it here for free. Thanks again sir. 

[amazing grace]

Great thread with really helpful conversation. Good luck with the rewire doryboy.

[markymark]

Instead of using starboard for the component mounting plate I took AJs advice and bought a used cutting board at goodwill for a buck.