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Author Topic: Conductor Size Question  (Read 2104 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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« 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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« 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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« Reply #15 on: May 07, 2012, 03:20:25 AM »

The electrical engineers I used to work with agreed with the swaged connector, but just remember that every electronic device ever made uses copious quantities of solder connections with copper as one side at least...they don't corrode to notice.  Resin core solder contains little to no lead now anyway.

You can always double up cable connections, each sized for half the load.  As noted, voltage level is what inverters love, so minimizing voltage drop is important.

Brian
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« Reply #16 on: May 07, 2012, 05:40:11 AM »

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.

The OP has some 2/0 cable which should be good to 300 amps for his distance and his inverter will consume 208 amps at the full 2500 watts.  Yes, the inverter can provide more than 2500 watts in a surge condition, but the inverter will probably shutdown if the surge lasts too long.  4/0 cable would be a good thing if the OP had to buy cable.

The reason I recommended having the ends done is because most folks don't have a crimper for large cable.  I'm lucky that I have a friend who has contacts in the electrical trade and he got me a hydraulic crimper to crimp my 4/0 cable.  I happen to own a crimper that does 8 AWG up to 1/0, but I got it cheap on Ebay.
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« Reply #17 on: May 07, 2012, 07:59:39 AM »

FUSE? Properly sized wires with a properly sized fuse normally do not get hot enough to smell!

 Improperly sized wires with no or wrong size fuse cause fires.  Insurance company looks sees no fuse, and with a big smile says very sorry but claim denied!
                                                                                                JIm
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« Reply #18 on: May 07, 2012, 08:17:21 AM »

FUSE? Properly sized wires with a properly sized fuse normally do not get hot enough to smell!

 Improperly sized wires with no or wrong size fuse cause fires.  Insurance company looks sees no fuse, and with a big smile says very sorry but claim denied!

The OP may have had the right size fuse, but the wrong sized wire.  The fuse and wire size obviously were not matched up.

Can you cite any case were the lack of a fuse caused an insurer to deny a claim?  There are plenty of old houses were the wiring met electric codes of the day, but would never pass current electrical codes.  Lots of houses have old fuse panels where the occupant has either put in an oversized fuse or replaced the fuse with a coin or other metal object.  Is every fire claim on these old houses going to be denied because of fuse panel issues even if it wasn't an electrical fire?

(And yes, I have the appropriate sized breaker on my inverter cable although I should probably add a fuse too.)
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« Reply #19 on: May 07, 2012, 09:34:16 AM »

Good question.  I did a quick Google search and I have to sort through 50,000 ads by attorneys that earn their living by fighting claims denied by insurance company's.  So maybe I am just perpetuating an urban myth?
                                                              JIm
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« Reply #20 on: May 07, 2012, 09:49:24 AM »

My understanding is what happens with insurance is they'll pay your claim even if had no fuse or whatever, but then they either drop your coverage, or they drop coverage for electrical fires or whatever issue you had that caused the fire.  You can switch insurers, but there is a master database of insurance claims out there that most every insurer uses.
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« Reply #21 on: May 07, 2012, 10:55:03 AM »

Thank You Again, All,

I've decided that for safety's sake I will go ahead and invest in the 4/0 cable. Thanks to all and thank you belfert for the reference to genuinedealz. They will make up my cables including from alternator to battery-bank (thank you rv_safetyman) and battery-bank-to-inverter, plus provide fuse blocks sufficiently sized for each, etc.   

BTW, I did not have any type of in-line fuse from alternator-to-battery-bank or from battery-bank-to-inverter. I very appreciate your collective concern and advice on this important electrical matter.

Kevin
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« Reply #22 on: May 07, 2012, 12:13:36 PM »

The fuses should be placed as close to the battery bank as practical.
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« Reply #23 on: May 07, 2012, 07:21:54 PM »

Thank You Again, All,



BTW, I did not have any type of in-line fuse from alternator-to-battery-bank or from battery-bank-to-inverter. I very appreciate your collective concern and advice on this important electrical matter.

Kevin

All of the Trace inverters I have seen call for a type T fuse  $$$  (Disclaimer) I have not seen all Trace inverters!  Most I know use ANL type fuse's instead, Less   $

 You might want to do a web search and see if a manual is available for your inverter.  JIm
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« Reply #24 on: May 07, 2012, 07:32:19 PM »

Trace used a T-Tron jjn-300 if you don't find one I have a new one I was going to post in the spare tire section here is a guy who is the best on inverters and supplies www.onestopcustomshop.com located in  CO.you can wade through a lot of bs on the boards by speaking with him don't let the name fool you

good luck
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« Reply #25 on: May 08, 2012, 06:30:58 AM »

I spent some time researching this and decided on ANL. Our resident expert agrees so I have quoted him.  HTH  JIm
 



Sean:
One thing you might consider is changing from Class-T style fuses to ANL style, which are readily available in 300a ratings.  ANL fuses are less than $15 in quantity one, and I expect you could get a package of three or four for $10 apiece.  Here again the lug posts will be smaller than the Class-T posts, but at least you can get the larger lugs onto the posts.

-Sean
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« Reply #26 on: May 08, 2012, 06:38:55 AM »

How often does the fuse blow in 25 years I never had one blow
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« Reply #27 on: May 08, 2012, 07:26:33 AM »

  How often does the fuse blow in 25 years I never had one blow 

     I certainly hope at least as often as you need it!
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« Reply #28 on: May 08, 2012, 08:00:54 AM »

Class T fuses are really not that expensive.  Yes, they are more than ANL, but not that bad.  Bestconverter.com sells a 200 amp class T fuse with holder for $43 and replacement fuses for $17 each.

ANL does have the advantage of being available at some Best Buy stores and at most car audio places.
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« Reply #29 on: May 08, 2012, 01:50:52 PM »

The ones we had access to were about the size of the "CREDIT CARD" (72" , 75# very heavy duty compound bolt cutters) we carried on the fire department ladder trucks.  The swagger made short work out of low friction loss welder cable crimps up to and including 000000 size.  Wow.  Was also fun using them to crush peanuts and walnuts and stuff. (big toes?)  HB of CJ (old coot) Smiley
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« Reply #30 on: May 08, 2012, 06:23:47 PM »

Hi luvrbus,

I have never had a circuit breaker blow on my AC system in 23 years of bus ownership but I sure would wire a bus without them either.

Good luck, Sam
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« Reply #31 on: May 08, 2012, 06:29:57 PM »

I wouldn't either Sam but just the way the post was going about buying and the cost of the fuse I thought I missed out on something lol 
When the electrical stuff is to much for me here on the board I just go to the shop pull out some old BCM magazines from the 90's put my feet on the desk open a Miller's lite and read George Myers articles he had a way to even make dummies like me understand nothing fancy just facts 

good luck
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