HELP!! Wire size recommendations for 4120 cab power circuit

[jenkinsph]

I have read a few times about your having trouble finding the connectors for the 8 ga wire. Yandina (who makes battery combiners and trolling motor chargers) has some relays with 1/4" stud terminals on sale right now for $9.95 might be worth a look. Also BestBoatWire has been absorbed or bought out by GenuineDealz.com, when I was buying the wire and terminals for my boat rewire last spring this was a great source. I think I liked the way it was before the buy out though.


Catalog Frame Click on parts button to take you to the relay and 70 amp circuit breakers.

 

[eepete]

I agree on the dual #10s are less desirable than a single #8. Good deal on the alternator! I understand using the two relays, and since you have full control of dividing up the load you can make that work. Sounds like you are also well aware of the need for reasonably convenient accessibility to the fuses. I'd also take the overhead box, but I'm a "function wins out over form" kinda geek.

If you can get a 40 amp relay in the same form factor as a 30 amp, I'd do it. It will give you more margin for error and longer relay life.

I also think you are wise to just use the AC wiring current "specs" for wire size selection. Painless is supplying the entire system, so they can make that call. You are mixing and matching so play it safe.

I'd checkout Steve (jeninsph) link. IMHO, the single relay - single feed is desirable. Only one circuit to cut off, closer to "how things are done". But it's clear you understand the "building blocks" and can make it all work. Good luck with it, keep us posted, and pictures might help the next person to try this.

Pete

 

[kennyd]

Why do you need Butt connectors? I hate those things! Sure place for failure...

 

[arrabil]

You guys are applying AC in-home wiring standards to DC automotive problems. Take a look at your pickup truck, there isn't a single wire bigger than 12 gauge running to anything other than the alternator and starter.

You should use 12 gauge wire for a 40 amp relay. In fact that would be a standard 40A automotive circuit.

EDIT: I didn't read this quite right.... a 40 amp relay and 12 gauge wire go hand in hand... but you want all of this:
a set of 55W front lights, pair of 55w rear lights, strobe beacon, dome light, defroster fan, front wiper and rear wiper
on one 40A relay? Somehow I doubt that adds up. The defroster is likely near 40A all on its own. Each 55w light is 5A. Each wiper motor is probably 5A. The strobe and dome light are probably around 2A.

Normally you run each switched circuit to one relay. Why even worry about ignition on? Run them from the battery through a relay to a switch you put on the dash and then you can turn on the dome light or beacon or rear lights without the ignition. You'd be surprised how useful that can be (the tractor lights are wired like that already on my 4200). And use the inline maxi fuse holders with weatherproof hoods ($2.50 at TrueValue) and save yourself a pile of trouble.

Only the defroster really needs a relay. Most switches are rated at 12V/10A so you don't even need a relay for the rest if you don't want. I wired two extra 55w lights directly to the headlights circuit in the ROPS. On my 4200 the wire was already designed to handle the extra load.

 

[arrabil]

Here is the wire guide I've been using for ages....

 

[OtterBrookStables]

Why do you need Butt connectors? I hate those things! Sure place for failure...

To connect a weatherproof 40A fuse holder ( w/ 10 Gauge leads) to the primary power wire.

 

[OtterBrookStables]

You guys are applying AC in-home wiring standards to DC automotive problems. Take a look at your pickup truck, there isn't a single wire bigger than 12 gauge running to anything other than the alternator and starter.

You should use 12 gauge wire for a 40 amp relay. In fact that would be a standard 40A automotive circuit.

EDIT: I didn't read this quite right.... a 40 amp relay and 12 gauge wire go hand in hand... but you want all of this:
a set of 55W front lights, pair of 55w rear lights, strobe beacon, dome light, defroster fan, front wiper and rear wiper
on one 40A relay? Somehow I doubt that adds up. The defroster is likely near 40A all on its own. Each 55w light is 5A. Each wiper motor is probably 5A. The strobe and dome light are probably around 2A.

Normally you run each switched circuit to one relay. Why even worry about ignition on? Run them from the battery through a relay to a switch you put on the dash and then you can turn on the dome light or beacon or rear lights without the ignition. You'd be surprised how useful that can be (the tractor lights are wired like that already on my 4200). And use the inline maxi fuse holders with weatherproof hoods ($2.50 at TrueValue) and save yourself a pile of trouble.

Only the defroster really needs a relay. Most switches are rated at 12V/10A so you don't even need a relay for the rest if you don't want. I wired two extra 55w lights directly to the headlights circuit in the ROPS. On my 4200 the wire was already designed to handle the extra load.

I knew there was some different set of "rules" for 12V wiring. You are correct - there is nothing in a car larger than 12 or 14 AWG. I spoke with a guy at NAPA today and learned a lot. First - you're right - no relay is required unless I want the switches armed by ignition on. I like the idea of having power available anytime - running or not. Unless you use a dedicated relay for each circuit, there is no isolation offered by a single relay activated by ignition on. That type of relay circuit does not protect a thing. An isolation relay is where the ignition-hot (or any other control voltage) is run through your switch which in turn activates the relay to switch the power on to the load. I am goung to run a single 12 AWG or maybe 10AWG wire fused at 40A at the battery and cascade it directly to my switches. Each switched circuit will be individually fused right after the switch. By the way here are the loads: Wipers are fused at 7.5A each - but that is to cover inrush current at start-up. The wipers will draw about a third of that when running - call it 3A each. The 220W of front and rear lights will draw 18A, the dome light - 1A, the strobe draws 2A, and the defroster fan is just a little 4-blad, 6" fan on a swivel that draws about 3A. Total if everything is on at once: 30A. None of these loads require an isolation relay.

Good feedback - thanks! Hope this helps out others.

 

[jenkinsph]

My first response on pg 1 was about 30 amps as I recall. And the OP wanted to install a relay that switched the power off/on with the ignition switch.

I'm sticking with the suggestions of the electrical engineer but everyone can do as they please! Enjoy your new cab.

 

[OtterBrookStables]

My first response on pg 1 was about 30 amps as I recall. And the OP wanted to install a relay that switched the power off/on with the ignition switch.

I'm sticking with the suggestions of the electrical engineer but everyone can do as they please! Enjoy your new cab.

Yep - you hit the total amperage right on. Actually, I am an electrical engineer - I just have zero experience with auto wiring materials, components or methods. I am in the semiconductor business and have done a fair amout of residential wiring over the years, but no auto/marine work or design. But, I am learning. The response I gave above a few minutes ago is my best interpretation of the situation, and I firmly believe it is technically sound. Since I have decided to have power on constantly rather than with the ignition, there is no longer any need for the relay. Makes life a heck of a lot easier! Makes no sense to overcomlicate or over-engineer a system is there is no added real value - perhaps percieved value, but in the end, the simple circuit I have decided on will function the same as the more elaborate circuit without cutting any corners in terms of safety or longevity.

 

[arrabil]

The isolation part of a relay is more practical than plain isolation. You don't want 40A of current flowing through the same switch where your finger is. So you use a relay to direct the current to the load while maintaining in-cab safety. My guess is this came about from early car fires more than any other reason.

EDIT: With that in mind, I would not run 18A of lights through one switch without a relay. You'll notice that the best switch you can buy at NAPA will be 12V/15A so... [and most likely you'll only find 12V/10A]