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Author Topic: Conductor Size Question  (Read 2111 times)
Kevin
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« on: May 06, 2012, 01:01:19 PM »

Hello All,

I currently do not have generator (installed) in my bus. I have a Trace 2500W inverter, six Trojan 6V deep cycle house batteries, and a switch on the dash to charge house batteries from 230 amp engine alternator while running down the road.

Several years ago I announced on this forum that I was going to attempt to run my Dometic Penguin 13.5 roof air from house batteries/inverter, w/batteries charging from the engine alternator.

I first experimented by starting up the bus and running at fast idle. With house batteries charging from engine alternator, I turned on the air and all seemed well - so went for a test drive of approximately 20 miles. The air was nice and cold and I was in process of patting myself on the back - while simultaneously cursing myself for not having thought of this brilliant idea years before - when I began to smell something burning. Oops.

I pulled over and checked the inverter by laying my hand on top. It was warm but not hot. I then touched the conductors (4 ga, like automobile battery cable) leading from my house battery bank to inverter and they were very hot! Disappointed, I returned home and noticed that someone (forget whom, sorry!) had left a comment warning me that I'd better have some pretty hefty cable from battery bank to inverter in order to do what I wanted to do.

Okay, so I'm just getting around to trying this again. Based on the equipment I have above, will someone please advise as to what size (minimum) conductor I should use from my battery bank to inverter in order to run my roof air while traveling down the road? Battery bank is approximately 4' away from where inverter is mounted, btw.

Many thanks,
Kevin

p.s. I did a search and see that others are doing just what I want to do, but I did not see anything re: conductor sizing.



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Van
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« Reply #1 on: May 06, 2012, 01:15:22 PM »

Keven, I used 0000 cable to my inverter from the batts. what guage wire are you useing to the AC unit?

     Van
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« Reply #2 on: May 06, 2012, 01:19:41 PM »

  Keven, I used 0000 cable to my inverter from the batts. what guage wire are you useing to the AC unit?          Van 

    Yeah, purely a guess but 0000 cable for the battery-to-inverter cable (you don't say explicityly but a 24-volt system will help a lot here).  Like Van says, are you using "Romex" for the 120V to the AC?  What gauge?  What kind of amps do the units you're using pull?
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« Reply #3 on: May 06, 2012, 01:41:26 PM »

I used 4/0 welding cable due to flexability and insulation, crimped and soldered terminals on it for the DC  and  for a/c wiring use  the 12 ga wire.
Dave
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« Reply #4 on: May 06, 2012, 02:37:50 PM »

There are many online voltage drop calculators that will help size wire based on voltage, amps, and distance.
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« Reply #5 on: May 06, 2012, 03:22:16 PM »

Voltage used is critical, step 1.0 in this conversation.  Your load uses watts but the wire cares about amps - so voltage is the key to deciding what amperage you need to design towards.  4 gauge and what you are doing is a recipe for a fire if you are 12 volt, so...  a little math.  These are round numbers, so some estimates and rules of thumb involved.  13.5KBTU AC, safe figure 15 amps at 120 VAC (it's a little less than that).  That is 1800 watts.  Figure in the inverter losses, up to 2000 watts.  At 12 volts (a little low on voltage if you are running off the alternator, but worst case if you are running off batteries ) that is 167 amps.  Minimum wire size for 160 amps is 00 gauge ( http://www.powerstream.com/Wire_Size.htm ).  If you run 24 volts you halve the current to 85 amps, so you can use 2 gauge.  Now - sizing for the minimum load is one way to go, but sizing for maximum load is a far better way to go, since wire costs very little in the greater scheme of things.  You have a 2500 watt inverter. That is 200 amps at 12 volts, but wait there's more!  Your inverter has a burst load ability probably double it's constant rating.  That is going to pull 400 amps, for maybe 5 seconds maximum.  You don't need to size your wire for that short load, but it would require 0000 gauge at least, so why not buy that?  Your wiring from the alternator to the batteries should be sized likewise, it will try to push 230 amps to your system if it gets asked, and will try to push 170 amps if the AC is on, so size it big if you can.

This is why 24 volts is good, you can cut down the wire size to 0 or 00 if you do.

Brian
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« Reply #6 on: May 06, 2012, 03:36:20 PM »

Hi Kevin,

It appears that you have a GM 4104 so your system is 12 VDC. If that is true, I like Brian's numbers. Also I am guessing that you didn't have a fuse in your original wiring between batteries and inverter or it would have blown. I highly recommend that you put a time delay fuse of appropriate size (250 amp I think would be right) in your new set-up.

Good luck, Sam
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« Reply #7 on: May 06, 2012, 03:40:09 PM »

Multi strand copper with the lug connections swagged, but not soddered.  Sodder will corrode over time.  Be sure to slip over the heat shrink covering first.  Considering how short the runs are and the small price difference between 00 and 0000 and adding whatever may be added down the road, perhaps just use 0000?  FWIW.  HB of CJ (old coot)
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« Reply #8 on: May 06, 2012, 04:27:36 PM »

Correct, electrical lugs WILL corrode if you use acid core solder, however with soldering wire you ONLY use rosin core solder, no corroding, ever.
Dave
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Kevin
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« Reply #9 on: May 06, 2012, 05:28:49 PM »

Thank You All!

It's a PD 4104, sure enough, and 12V. I am using 12-3 wiring for AC load to roof air. I have been given (as in FREE!!! Grin) a length of 2/0 AWG fine-strand copper wire that I was hoping would be big enough to use as conductors from battery bank to inverter - in order to run just the 13.5 air conditioner while rolling down the road (we're headed to the beach next month) but evidently that is not quite going to do. Cry

Since I know bugger-all about electrical (as my first post so obviously indicates) I sure am glad I've got you guys to save me from myself! I rather doubt that 8' (two lengths of 4') of 4/0 is going to break me financially, so that's what I'll be shopping for.

The Trace 2500 ran the air pretty fine for that first 20 miles or so prior to the 4 ga battery cable almost catchin' a'fire! Weee-Doggy! I guess I was that close to roastin' weenies road-side over the charred remains of my beloved '04. Seriously, this is no laughing matter, and I do appreciate all the knowledge and your willingness to share on this forum.   

This inverter is the old Trace 25 Year Anniversary model. It has served us well since purchasing used some 12 years ago. BTW, I see some widely differing viewpoints regarding MSW vs. PSW inverters, and their respective abilities to power a 13.5 roof AC. I suppose just because mine did for a 20 mile run doesn't necessarily mean that it will continue to do so, or should be used to do so? Any thoughts?

Thanks again,
Kevin
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« Reply #10 on: May 06, 2012, 05:39:28 PM »

I ran the numbers through some wire sizing calculators.  I figure 208 amps at max 2500 watts using 12 volts.  For 4 feet even at 300 amps under surge conditions 2/0 should be plenty.  4/0 would be overkill.  If you do go with 4/0 I would just have the cables made by someone like Genuinedealz.com who can put the ends on for you.  They charge $2 per cable to install the ends and they will even add heat shrink on request.
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« Reply #11 on: May 06, 2012, 06:11:37 PM »

Sizing wire close to it's rating always makes me feel uncomfortable because when the realities of the harsh life kick in, a little corrosion, some strands of conductor being shed when making up the ends, contact area in lug ends not 100%...

That heat shrink lets a careless assembler cover a ton of crap work... Nobody cares as much as you do to the results.

For the small amount of money extra, well worth up-sizing a bit.

happy coaching!
buswarrior

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« Reply #12 on: May 06, 2012, 07:04:01 PM »

Well here I go so fire away but I use the solder slugs on the cable ends no corrosion and the solder is only in that area of terminal end it will not wick like regular solder, heat shrink it good for ever not like a crimped end that over time will have corrosion
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« Reply #13 on: May 06, 2012, 07:48:56 PM »

The other issue you will need to address are the cables from the alternator to the house battery bank.  You will need to size that cable for at least 150 amps and probably better to size it for 200 amps.  It is best to run both ground and hot cables rather than rely on the bus ground system.

Be sure to have fuses on both the cable to the inverter and the cable to the alternator.

Jim
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« Reply #14 on: May 06, 2012, 08:14:00 PM »

Invertors seem to like invertor-to-battery cables with less than 1% loss.  Most of the charts I have seen indicate a 3% to 5% friction loss which is too much.  Sorry guys....ALL lead sodder corrodes copper wire...doesn't matter what type.  Properly swagged connections are the best. Now if you wanted to run silver wire/silver sodder  you would be OK.  $AUGHH$  There you have it.   HB of CJ (old coot) Smiley Smiley Smiley
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