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Author Topic: Adventures in wiring  (Read 3410 times)
K7RBW
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Posts: 398




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« on: September 05, 2011, 08:36:47 AM »

... or "in for a dime, in for a dollar."

I just finished running some accessory wiring in my 2010 RAV 4 and thought I'd share.

Good news: there was a rubber pass-thru on the passenger side of the firewall behind the glove box that is used for wiring and the washer fluid tube, which had an unused nipple. (Bad news: battery is on the driver's side). So I clipped the nipple and had my entrance into the car.

I ran some 10-gauge "zip-cord" through it and connected it to the battery by way of 2 40-amp fuse holders and a power-pole disconnect. (I'm a little concerned that the power-pole is in the engine compartment and not waterproof, but it's easy to keep an eye on).

That all worked fine. Eventually, I'll install a radio, but for now I can just plug one in when I need it and take it out when I don't.

The 10-ga zip cord was difficult to pull through the rubber, so when I added a 14-ga line for the trailer wiring, I bought some liquid wax (cable lube). That made it MUCH easier to pull the wire and would have been SUPER helpful the first time (at least now I have a quart which should last for the next 10 cars Smiley ). But, I thought, if I'm going to go to all the trouble to install one wire for the trailer hitch, I might as well install an accessory power cable to the back while I'm pulling cable. There's already a lighter plug outlet back there that says it's rated for 10 amps, but it has, what looks like 18ga wire going to it so 10 amps seems a bit optimistic.

I got some 10-gauge primary wire and ran that to a power-pole accessory connector in the back and a 14-ga wire to the trailer connector adapter. These are all fused at the battery and also connected power poles with the first "zip-cord" cable making it possible to disconnect the whole harness at the battery after the fuses. Each wire coming off the battery has its own fuse.

The zip cord is terminated with a power-pole in the glove box with about 6 feet of wire spooled until I can figure out where I want to install the radio. Both pos and neg are connected directly to the battery through 40-amp fuses.

For the back two circuits, however, I just ran the positive wires and tapped the ground from a ground point on the body near the connector.

When I tested the two circuits under load (about 10amp, more or less) I noticed that the glove-box circuit had about a 400 mv drop under load, while the rear accessory circuit (about 10 feet longer) had a 300mv drop under load. (Test: motor running, battery charged, voltage measured at connector without load, and then with load. Battery voltage presumed constant by alternator, but I'll measure that again today.)

So it seemed odd, that the longer run would have less of a drop than the shorter one, but the shorter run goes through two fuses, rather than one, which could account for some drop. It's also possible that some of the drop can be attributed to the alternator loading down.  Practically, that's good enough for what it's for, but I'm curious about what's going on.

The other good news is I can now take all the trim panels out of the back half of the car, down to the sheet metal, in about 10 minutes. Ham radio is all about learning new skills Smiley
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K7RBW
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« Reply #1 on: September 05, 2011, 10:52:43 AM »

So I tried a little more scientific testing by measuring at the battery and at the connector to get a more accurate measurement of the voltage drop. I also used two different loads (I had two 100W lights plugged into a 300W DC/AC inverter)

For the accessory plug (35'run to the rear of the car)
With 1 light (about 10A DC) there was a .51v drop from the battery to the connector
With 2 lights  (about 20A DC) there was a 1.06 drop from the battery to the connector
The single 30A fuse had a .045v drop across it with two lights on.

For the front power cable (8' run from the battery to the glove box and then 7' of cable coiled up)
With 1 light (about 10A DC) there was a .53v drop from the battery to the connector
With 2 lights  (about 20A DC) there was a 1.37 drop from the battery to the connector
Each 40A fuse had a .035v drop across it with two lights on.

I'm a little puzzled as to why the shorter run has a 30% higher drop than the longer run. The longer run has 35' of 10 ga from the battery to the connector and 6' lead from the connector to the chassis ground point in the rear of the car. The shorter run has 2 15', 10-ga wires (zip cord) that run from the connector to the battery. I'm guessing that the chassis has a lower reistance than the wire.
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KB1UAS
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« Reply #2 on: September 05, 2011, 02:21:57 PM »

I think you answered the question. The shorter run has more resistance even though technically round trip it is less 10 gauge the low resistance of the frame makes for less drop.
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KI4SDY
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« Reply #3 on: September 06, 2011, 06:24:17 AM »

I would question the durability of zip cord as a long term installation in a constantly vibrating and heat generating environment like an automobile. In most cases, the insulation is too soft, melt temperature too low and the insulation dries out and cracks with age. China makes some really poor quality zip cord. Further, it is too easy to mix up polarity of the wires for new or older sight challenged hams, resulting in a dead radio. I would use something more rugged and color distinctive for polarity in a mobile. I am cheap, but not that cheap!  Wink
« Last Edit: September 06, 2011, 08:49:07 AM by KI4SDY » Logged
MAGNUM257
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Posts: 159




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« Reply #4 on: September 06, 2011, 12:23:30 PM »

I would question the durability of zip cord as a long term installation in a constantly vibrating and heat generating environment like an automobile. In most cases, the insulation is too soft, melt temperature too low and the insulation dries out and cracks with age. China makes some really poor quality zip cord. Further, it is too easy to mix up polarity of the wires for new or older sight challenged hams, resulting in a dead radio. I would use something more rugged and color distinctive for polarity in a mobile. I am cheap, but not that cheap!  Wink

Although the author did not specify black/red "zip-cord", I believe that is what he meant, at least that's what I have always known it to be. I have used zip-cord in varying guages and have never had a problem with the insulation breaking down. I agree that for a modern radio I would use something a little heavier. I use 8 guage zip.
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K1CJS
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« Reply #5 on: September 06, 2011, 01:17:58 PM »

Although the author did not specify black/red "zip-cord", I believe that is what he meant, at least that's what I have always known it to be. I have used zip-cord in varying guages and have never had a problem with the insulation breaking down. I agree that for a modern radio I would use something a little heavier. I use 8 guage zip.

There are many different sizes and various insulation types and colors of 'zip' cord.  The only reason it is called zip cord is that the two insulated wires that make it up have their insulation jackets fused together so that the wires stay together.  They're also made in such a way that a quick pull 'unzips' them.  Heck, I've even seen two gauge 12 foot jumper cables made out of a type of zip cord.

The idea that zip cord is a two conductor 18 gauge lamp cord is wrong.  That's what it started out as.  That isn't what it is now.
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K0BG
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« Reply #6 on: September 06, 2011, 02:43:29 PM »

I just can't get my head around the comments about the body and/or frame having less resistance than a properly sized power cable. I've measured the resistance in several vehicles I have owned, and the copper run was always less. I suspect folks argue about this, because vehicle manufactures use the body work as a return. That fact doesn't make it better, but it does make it cheaper.

The other issue, are the sensors for all of the electronics in modern vehicle (averaging about $8,000 per vehicle these days). The FREDs (Frustrating Ridiculous Electronic Device) are all but bullet proof, but the sensors are easily upset by ground loops. Wire yours as you will, but you'll never catch me using the body work!
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W5LZ
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« Reply #7 on: September 07, 2011, 06:15:15 AM »

I think part of that voltage drop 'problem' is the size of the conductors, state of the connections, and how the measuring is done.  While I really like 'power poles', they aren't 'perfect' every time (stuff happens).  All in all, there doesn't appear to be a 'large' problem with the installation.
I don't have a particular problem using a vehicle's metal body as part of wiring a power circuit.  Of course there can be problems with it, but those problems are usually fairly easy to 'work around'/correct.  I also usually use huge power lines to the battery.  It's usually a lot of 'over-kill', but who knows what else I may 'add' later, and another power line probably won't be needed.  Have I measured the voltage drop on those 'large' power lines?  Nope.  It hasn't presented a problem so far.  Most automotive power systems are above the required characteristic power requirements for equipment I own, so I don't worry about it.
 Paul
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K7RBW
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« Reply #8 on: September 07, 2011, 06:45:53 AM »

The zip-cord is the 10-ga stuff you get from PowerWerx. It's soft and flexible (making it a pain to pull through rubber grommets w/o lube Smiley ). All the wiring in the engine compartment is in a split loom and secured. Some is in a thicker spiral wrap because I can't see where it's run so I wanted extra abrasion protection.

Connector-wise, all are new connectors and crimped with the ratchet-crimping tool. I'm a little worried about the connectors under the hood because they aren't waterproof, so I'll just keep an eye on them.

But, I'm with K0BG, as to being puzzled why the circuit that uses the chassis ground would have less overall resistance. Once I figure out what radio I want to install and where, I'll cut the zip cord to length. And because the car is new (1-yr old) and all the electrical connections are squeaky clean, I would expect them to be pretty low resistance, but I also realize that I'm going through a lot of spot welds to get from the engine to the rear wheel well. I guess the good news is that my car has an efficient grounding system and good chassis connectivity. I hope that lasts as the car ages. (That should bode well for RF grounding as well, I would think)

Anyway, I'll keep an eye on things and at least I have taken some benchmark measurements to compare against in the future.
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N6AJR
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« Reply #9 on: September 07, 2011, 12:00:26 PM »

I don't know if it will help or not, but on mobile rigs I always ground the radio with a self tapping sheet metal screw and some baid at the radio, and the neg post of the battery to the fender/chassis, and the shield side of the coax at the antenna to a bolt on the chassis side of the car, as opposed as to the trunk lid, or door or what ever the mount may be attached.  if you have a through hole mount this one is probably not necessary.  this gives all the  ground sides in the vehicle at about the same potential.  works for me.
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KC2MMI
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« Reply #10 on: September 15, 2011, 09:30:56 PM »

In theory, wiring that goes through an engine compartment should be rated for that use. It will be oil resistant (because of petrochemical vapors) and heat resistant, since engines tend to run around 180F and engine bays will exceed that temperature if there's no cooling air, i.e. at shutdown.

In practice...you're fused, you'll probably get 10 years without any problems.

But as you noticed form the voltage drops, if there's any way to move up the wire size, you may want to do so. With the engine running presumably you have 14.4 volts from the alternator so losses are tolerable, but with the engine off you're working with 12.6 or 12.7 for a new fully charged battery, and 11.6 would be considered dead. SLI batteries (starting lights ignition) really do not like being deep cycled at all either.

On waterproofing and such? I've learned to use some type of goo on every contact. Usually "high temperature dielectric silicone grease" aka brake grease, ignition point grease, light bulb grease. It does not conduct, but your powerpole has metal-on-metal contact so that's no issue. You could use NoAlOx or a similar product, which has zinc powder in it to prevent galvanic issues--and also doesn't conduct despite the zinc.

Then finish off the connector by overwrapping with silicone tape or butyl tape, either one will make a good waterproof seal although it will mean you can't just pull the plugs apart, you'll have to cut or peel the tape.

But if you want to RELY on a connection, always use goo and sealant tape. In the engine bay of a car, neither is necessary for just a few years, but they're still good practice, good insurance.
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