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Old 06-06-2015, 05:55 PM   #1
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Default Wire size chart

We have all referred to the voltage drop charts when looking at wire sizes, but most that I have seen don't address things like, location heat, conduit or loom, etc so you know what the voltage will do, but still not sure if the amps are OK.

The Blue Sea "circuit wizard" takes a lot more of that into account, giving ampicity and voltage drop. Pretty surprising how much loom or conduit reduces the ampicity.

http://circuitwizard.bluesea.com/#

I would guess these would be very conservative ratings as they are from boat folks.
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Old 06-07-2015, 11:12 AM   #2
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That's a good link

It's kind of surprising how small of a gauge can carry the amps (open air) if you ignore the voltage drop (don't ignore the voltage drop).

Basically we need to size wire to take into account both the load carrying ability and acceptable voltage drop.

What voltage drop is acceptable? I see 3% for 12VDC noted on a few sites. Blue Sea notes 3% but also mentions 5% and even 10% for non critical & non feeder loads.

Do you add the positive and the ground wire to come up with the Length of Conductor? I do.
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Old 06-07-2015, 12:24 PM   #3
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The open air numbers are really high, in loom way low from what I expected. It does appear that there are probably systems out there that run hotter than they should. I did run the 80 amps on 4ga that Roadtrek uses and it was good on all criteria.

3,4,5% is what I have been running across with some solar wanting 1 or 2%. I think maybe it is best to just see what the voltage turns out to be, and see it it will work for what you are doing. Lighting is way different than charging for instance.

It is my understanding you need to use both + and - wire lengths.
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Old 06-07-2015, 01:32 PM   #4
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Yes, you do need to include both + and - lengths. This is one reason why using a proper chassis return is good practice. The metallic structure of a vehicle provides the equivalent of a HUGE wire; enough so that it is usually negligible. Has to be done properly, though. Best to stick to factory-engineered ground points.
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Old 06-07-2015, 05:02 PM   #5
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I have been playing with the calculator to see what influences the ampicity, and how much. Since ampicity is heat controoled, loom is a biggy, but the time is an interesting thing.

The ampicity effect by time seems to go from on to off at 10 minutes.

I did our setup of 120 amps (on each #4), in conduit, and it said I needed a 1ga at 11 minutes run time. At 10 minutes it said 6ga would do.

Looks like a safe compromise, probably super conservative, temp rise specification, with one size fits all.

Using that information, if we ever see amps that high, I can just run 10 minutes and shut off a while at the separator, or just idle where we only get about 110-120 amps. The alternator will also lose a about 30 amps or more capacity when it gets hot, which happens really fast at high outputs, so we should be plenty safe.
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Old 06-09-2015, 11:51 AM   #6
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The chassis return is an excellent point.

For anyone whose rig doesn't display amps in or amp out a Clamp-On Ammeter like this: http://www.classbforum.com/forums/f6/clamp-on-ammeter-999.html is a handy tool.

I'm hoping / wishing Gerry has one ...........
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Old 06-09-2015, 02:07 PM   #7
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On thing to always remember with chassis grounds is to check them with you ohmmeter when you do them. You should see nothing (unless you have a super sensitive meter). The chassis itself is indeed a very large wire, but it is also made of relatively low conductivity material in steel (10-20% of copper). For that reason, the connections to the chassis need to be of higher surface area than they would be on the copper or brass connections we are used to, for the same amps. Big bolts and washers with conductivity paste are a good ideal on high amp connections, especially if you can get to both sides of the chassis and use a big bolt, washers, and nut.
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Old 06-09-2015, 02:41 PM   #8
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Yes. Which is why it is best to use one of the engineered ground points provided by the chassis manufacturer. They can be found throughout the vehicle and address these concerns, while also avoiding drilling new rust-prone holes into your vehicle.
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Old 06-09-2015, 03:01 PM   #9
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Quote:
Originally Posted by avanti View Post
Yes. Which is why it is best to use one of the engineered ground points provided by the chassis manufacturer. They can be found throughout the vehicle and address these concerns, while also avoiding drilling new rust-prone holes into your vehicle.
I agree, the big problem comes when you are trying to do something like hook up the negative battery cable from the coach, or some other really high amp circuit. The factories don't use anything in the rear that comes close to being able to carry 200+ amps. For those I like to use a big bolt through a flange in a main frame member, which is thicker so it has more capacity at the connection. In our current setup, I did use a removed seatbelt mount, which is a large welded in nut. That connection checks very well.

On the new setup, I am going to try to keep the really high amp negatives directly to the shunt connection on the negative battery cable, rather than to the chassis and then back to the shunt. The fewer connections, the better, and those cables will not need to be very long if I get the components located right. At that point, the chassis ground connection to the shunt should only have the engine alternator as a high current source.

Good reminder-I need to order some more conductivity paste, I am out of it.
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Old 06-09-2015, 03:32 PM   #10
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Speaking of shunts, some might be interested in a trick I used to mount a shunt directly on the battery, avoiding they typical short cable run:



Details can be found here:

http://sprinter-source.com/forum/sho...3&postcount=80
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