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Old 08-27-2014, 08:22 AM   #1
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Wiring for Dummies?

Electricity has always been a bit closer to magic than science for me so hopefully this will be an easy question to answer (and hopefully for me to understand!).

I just made a home for my new Puma 3cfm compressor in the back of the van in the little cubbyhole under the rear closet (50 layout). I have it rigged up with alligator clips so I can pull it out and run it off of the battery or another vehicle, but it's heavy and I'm thinking of hard wiring it in.

This is how I'm thinking it should go, please correct me if I'm wrong:
Positive off of the compressor to suitable fuse in 12v fuse box.
Ok so far? If so, I'm assuming I can ground the negative wire to a bolt right next to the compressor that ties the closet/bed unit to the frame? Or does he negative have to rout back to the fuse box as well?

While looking at the fuse box I realized I may have been living with a false understanding of how 12v wiring works... Looking at the battery and seeing the positive and negative cables coming off, I've always thought of the electrical system in a car as a closed loop, like the circulatory system. Electricity flows out of the battery, throughout the vehicles starter, radio, etc and back to the battery.
That's how it looked to the eight year old me and I've been interpreting all of my automotive electrical knowledge off of that assumption ever since...

I've replaced plenty of batteries over the years, but never followed that negative wire to see where it goes. Don't tell me it grounds straight to the frame 18 inches away?

Thanks for any illumination you can offer. Very happy with the self sufficiency we're going to have being able to air up on the spot...

-Mark...
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Old 08-27-2014, 09:00 AM   #2
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Re: Wiring for Dummies?

Short answer to your question is yes the negative cable will attach to the nearest large metal piece; frame, inner fender structure etc etc. Most every other circuit in the automotive realm have their grounds also attached to a ground source, in theory the entire body and complete drive train are electrically connected to each other. (In semi-rare instances this isn't true but for this discussion it works.)

Keep this simple electrical tid bit in mind: "shorter is better..........".

Since you don't state (or know?) the amp draw of your compressor its tough to advise which gauge wire to run to your eventual mounting and wiring location. Most likely your finished compressor circuit will begin attached directly to the positive battery terminal, through a suitable fuse then back to the mounting location. A switch capable of handling the initial and sustained current flow will be necessary unless the compressor has a holding tank and pressure switch.

I'm thinking the fused power or battery wire would be no less than 8 gauge which tends to require soldering or crimping tools most of us don't own. (I have them but that's part of being a partial wiring geek AND collector of tools. )

Hope this helps you get started.
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Old 08-27-2014, 09:12 AM   #3
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Re: Wiring for Dummies?

You are correct on all of your points.

The current flow is closed loop back to the battery; the chassis/frame is just part of the return path.

Think of it this way: Everything could certainly have a return (negative black) conductor back to the battery, but things are simpler (and cheaper) if each electrical component is grounded to the chassis/frame and the frame is connected to the battery negative. This is conceptually a "ground buss". The frame is just one giant negative conductor.

Here is some electrical trivia.......way back in the pioneering days of electricity the direction of current flow needed to be defined, and although it was not fully understood, there were two possibilities; positive to negative or negative to positive......and the pioneers guessed incorrectly. Electrical current actually flows from negative to positive.

http://www.allaboutcircuits.com/vol_1/chpt_1/7.html
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Old 08-27-2014, 12:19 PM   #4
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Re: Wiring for Dummies?

Everybody is on track here. Things like starters are often just grounded to the frame. Sometimes more sensitive equipment is wired to tap off a ground block that has a negative wire that runs to the battery but is also grounded to the frame as a secondary backup ground. The frame is able to handle a heavy amp load where wire is rated for its size. But frame grounds can have issues with connectivity and a direct connection to the battery may be a better choice in some cases.

I'm sure the instructions for the compressor designate the correct wire size to be used. As mentioned just hooking to a fuse block can be an issue if for example the compressor requires 2 str copper but the fuse block is fed by a #4 str copper wire which is half the size (and lower ampacity) of the #2 that feeds the compressor.

In some cases the negative fuse block is also only rated for a specific amount of amperage, & you can also run into problems if you "over amp" it even if it's tied to the frame and/or ground. This applies to any connector, lug or connection point... positive or negative.
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Old 08-27-2014, 02:25 PM   #5
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Re: Wiring for Dummies?

Quote:
Originally Posted by daveb
But frame grounds can have issues with connectivity and a direct connection to the battery may be a better choice in some cases.

Issues like rust.., which is an insulator (poor conductor). That's why it doesn't hurt to clean up the ring lug to frame joints on battery cables once in a while.

Safety standards for many things (like medical devices) require several things for chassis grounds:

ring lugs (no spade lugs) so that even if the joint loosens, the ring lug will stay connected to ground, ie not fall off.

Internal or external star lock washers between the ring lug and the chassis-these "bite" into the base metal providing a low resistance path to ground over time when oxidation occurs.

There also cannot be a surface finish (paint, anodize, etc) around the grounding point since these inhibit conductivity.
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Old 08-27-2014, 03:08 PM   #6
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Re: Wiring for Dummies?

Quote:
Originally Posted by boywonder
Quote:
Originally Posted by daveb
But frame grounds can have issues with connectivity and a direct connection to the battery may be a better choice in some cases.

Issues like rust.., which is an insulator (poor conductor). That's why it doesn't hurt to clean up the ring lug to frame joints on battery cables once in a while.

Safety standards for many things (like medical devices) require several things for chassis grounds:

ring lugs (no spade lugs) so that even if the joint loosens, the ring lug will stay connected to ground, ie not fall off.

Internal or external star lock washers between the ring lug and the chassis-these "bite" into the base metal providing a low resistance path to ground over time when oxidation occurs.

There also cannot be a surface finish (paint, anodize, etc) around the grounding point since these inhibit conductivity.
And why many electronic sound equipment installations suggest having a complete separate ground that avoids contact with a frame ground altogether. Besides wire resistance and ground loop potentials, the frame itself can have its own resistance. I doubt a compressor would have an issue though. Many winches are grounded through the frame only but I've seen trouble from just what you describe.
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Old 08-27-2014, 08:04 PM   #7
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Re: Wiring for Dummies?

I found this on the Ihttp://www.ebay.com/itm/PUMA-12-VOLT...-/280790586347

Specifications:

-MODEL: PD1006
-DEMENSIONS: HEIGHT- 16.5" LENGTH- 17.5" WIDTH- 8"
-HORSE-POWER: 1 HP PEAK - 3/4 HP RUNNING
-150 PSI MAX / 135 PSI WORKING PRESSURE
-1.5 GALLON TANK
-12 VOLT DC POWER SUPPLY
-1 YEAR WARRANTY
-OIL-LESS
-AMP DRAW: 30 AMPS

I believe the standard Sportsmobile fuse block has a 40 Amp reset-able circuit breaker, putting a (30 amp circuit ) would pretty much use up all your available power. I am not sure what the Fuse Block is rated for, I expect more then 40 amps.

Then you get down to the good stuff.
The size of the wire is going to determined by
  • Voltage
    Length of wire (that includes both the supply and return path)
    Voltage loss
    Type of load (variable or fixed) In this case its variable because it is motor.
    Wire insulation Temperature rating


    The you have the derating items
    Duration
    Conduit or sheathing
    and others

The good news is there are web pages that do most of this for you unless you love math and want to crank on the formulas your self.

Bluesea had a good circuit and fuse selection wizard
http://circuitwizard.bluesea.com/
They also have apps for Android and IOS

You can change the parameters and see what effect it will have on the AWG rating.

The other issue is one that Dave mentioned previously, if I don't go directly to the battery then I have to make sure the circuit behind your new circuit is capable of the extra load. Say, if you wanted to tap a new separate fused circuit prior to the 40 amp resetable breaker, You need to make sure that wire can now accommodate your existing 40 Amp load (and possibly what ever else is on it)

Also if you have any kind of battery monitor with a shunt, you will want to make sure your ground connection is on the non battery side of the shunt. (if you go to the frame it will be that way by default)

-greg
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Old 08-28-2014, 05:38 AM   #8
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Re: Wiring for Dummies?

Okay now that we have some hard facts here's what *I'd* do in this case......

Either use a positive battery terminal aftermarket junction block to tap a single dedicated 4 gauge extra heavy duty jacketed cable fused through an ANL/AMI fuse holder like this: http://www.bluesea.com/products/5005...r_-_35_to_300A

From there back to the compressor permanent mounting location secured along the way to anything solid and not subject to movement. There are many, many places along the frame such a cable could be run and fully secured in place. In fact I would consider lengths of PVC plumbing as a chase in spots that might be the least bit vulnerable to road hazards.

Once the compressor was firmly mounted in place the ground connection would be something like a split body rivet nut. These give incredible hold to the body sheet metal & when installed correctly don't come loose under any conditions. Minimum 8 gauge ground wire with ring tongue terminal, internal tooth "star washer" and 1" or slightly longer bolt securing it all together. After tight apply some sort of battery terminal protectant, anything to discourage corrosion.

These steps don't address switching for the compressor which would be fairly simple all things considered.
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Old 09-01-2014, 03:22 AM   #9
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Re: Wiring for Dummies?

Thank you everyone for all of the insight! This little gem was particularly insightful:

Quote:
Originally Posted by boywonder
The current flow is closed loop back to the battery; the chassis/frame is just part of the return path.
The audible 'pop' you may have heard was the light-bulb going off over my head when I realized the frame everything is grounded to is just one big wire leading back to the battery...

Thank you JWA for the straight forward description of how you would go about wiring this up. Since O'Reilly only caries wire up to 10 gauge (took me a bit, but now I have it straight that smaller numbers mean thicker wire ) I'll be ordering up all of the fittings and wire straight off of the house battery instead of coming off of the fuse box.

I'll try to put something together when everything is complete to show how it all turned out. I've got a trip down to Baja tentatively planned for the end of October where this compressor will come in very handy. Between my Staun deflators and the compressor I'll be comfortably running the proper pressure between the black top, wash board and sand.

-Mark...
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Old 09-01-2014, 04:50 AM   #10
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Re: Wiring for Dummies?

Glad you can use anything I offered up Mark---hope your install goes easy!

We're here to help if we can.............don't be afraid to ask.
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