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Old 08-19-2015, 09:49 PM   #1
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Wire size short/long run

As an example a 12v 200 amp load requires 1/0 ga for a 20ft run from the battery bank. Since the distance between the posts on the battery bank are inches, is there any reason the large cable is needed for inter-bank connections? My current setup has large cables for the run to the starter and the same size cable between the posts in the battery bank.
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Old 08-19-2015, 10:15 PM   #2
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I would keep them the same size.

In theory you could downsize considerably but I do not do it in practice.
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Old 08-20-2015, 12:59 AM   #3
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Use the Blue Sea wire app, it will work out how large your conductors should be and you can also figure out breaker size requirements. It's science, not voodoo.
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Old 08-20-2015, 01:16 AM   #4
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Two different issues to be considered. Ampacity is the maximum current capability of the cable and is basically limited by the melting temperature of the insulation. If the current exceeds this limitation the heat generated by the current flow may melt or ignite the insulation, either condition being an extreme safety hazard.
The second issues is voltage drop. The longer the cable run the greater the voltage loss -- you might put 12v in but at the end of a 20 ft run carrying "x" amps you only get 10.5v. Not necessarily a fire hazard but your refeigerator might not like it much.
Both need to be evaluated when sizing cable.
Your inter-battery cables see pretty high loads when starting, charging, and operating an inverter.
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Old 08-20-2015, 03:11 AM   #5
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Quote:
Originally Posted by ksanders View Post
I would keep them the same size.

In theory you could downsize considerably but I do not do it in practice.
this can be handy for cables and drops


Marine Wire and Cable: Voltage Drop Calculator
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Old 08-20-2015, 10:17 AM   #6
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I just upgraded my house bank to a pair of 6V golf cart batteries. These batteries power the house loads including a 2,000 watt inverter. If the inverter were putting out maximum power (which it never is, maybe 1,000 watts for the coffee maker) then it would pull about 220 amps DC.


I had a #1 gauge jumper in my electrical parts spares so I used that for the jumper. #1 is rated for 245 amps continuous outside of the engine room which was the case in my installation. The voltage drop in that 12" jumper is trivial even at a couple of hundred amps.


So it works for me.


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Old 08-20-2015, 10:53 AM   #7
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Best practice is to use the same gauge throughout; look at it as one continuous cable, which in many ways, that's what it is. Would you put a smaller gauge jumper in the middle of your main run from the batteries? Of course not. I look at the manual for a Magnum inverter and it says the interbattery jumpers "must" be the same gauge size as the main leads. Look at the users' manuals for Trojan and Rolls, they only spec one gauge size for a certain system maximum ampacity and no mention of sizing down the interbattery connection. Why mess around with something like this?
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Old 08-20-2015, 11:17 AM   #8
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From some of the posts in this thread there seems to be some misunderstanding of electricity and its properties.

All wire has a certain resistance per foot. Pushing a current through that wire causes a voltage drop. The resistance and the current cause heat to be generated as well.

Wire size tables are based on providing for a certain maximum voltage drop to the load.

But thats not what the OP asked. The OP asked if he could use a smaller jumper (I'm guessing a foot or less here) between cells on his battery bank.

Yes yes he could do that, of course he could. So long as that jumper did not create a unacceptable voltage drop, and so long as the jumper did not generate heat in excess of the insulation thermal ratings.

We do not recognize it, but this technique is used all the time by manufacturers...

Thake a windlass for example. The battery cable you install might be for example 1/0. Ever look at the wire leads on a windlass??? Well, I bet they are not as large as the cable you pulled from your battery bank.

Thats because a short run of a smaller size cable is perfectly acceptable because it does not create a significant voltage drop.
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Old 08-20-2015, 11:26 AM   #9
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Quote:
Originally Posted by ksanders View Post
From some of the posts in this thread there seems to be some misunderstanding of electricity and its properties.

All wire has a certain resistance per foot. Pushing a current through that wire causes a voltage drop. The resistance and the current cause heat to be generated as well.

Wire size tables are based on providing for a certain maximum voltage drop to the load.

But thats not what the OP asked. The OP asked if he could use a smaller jumper (I'm guessing a foot or less here) between cells on his battery bank.

Yes yes he could do that, of course he could. So long as that jumper did not create a unacceptable voltage drop, and so long as the jumper did not generate heat in excess of the insulation thermal ratings.

We do not recognize it, but this technique is used all the time by manufacturers...

Thake a windlass for example. The battery cable you install might be for example 1/0. Ever look at the wire leads on a windlass??? Well, I bet they are not as large as the cable you pulled from your battery bank.

Thats because a short run of a smaller size cable is perfectly acceptable because it does not create a significant voltage drop.
It is rampant behind a lot of dash panels and electrical too...do just have to do the calculations if unsure.

Some multiple battery connections use copper strips instead of cable also...less prevalent in boat installs...probably due to the thought of vibration or shifting.
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Old 08-20-2015, 01:18 PM   #10
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OK, suit yourselves. At least make sure you downgrade the circuit protection to protect the smaller cable. Why someone would want to limit capacity of an important system like this to save a couple of bucks is beyond me, but hey, that's just me I guess.
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Old 08-20-2015, 04:28 PM   #11
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Just my understanding of how it works...whether it is shortchanging or not is not my understanding...just what is required o do the job...not just the "bigger is better" ploy.


Like plumbing...wiring is not just one size beginning to end...it is about "runs" and what is required for what.
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Old 08-20-2015, 08:29 PM   #12
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But you guys are confusing a power source (the battery bank formed by the interconnects), and the entire circuit for delivering that power from beginning to end, with power consumers, such as a windlass motor (which i invite you to open up and see where the power goes).

There's a reason why you will be very hard pressed to find an inverter manufacturer, or for that matter a battery company, that if not outright warning against doing so, makes any mention of it being kosher. I idly went through a whole bunch of them today and couldn't find a one. And yes, all that aside, when it comes to high power, and potentially dangerous electrical issue like this bigger is better.

But like I said, go ahead, it's not my boat, and you're feelin' lucky, aren't ya?
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Old 08-20-2015, 09:17 PM   #13
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I am not missing anything that I know of...have seen it done in quite a few applications through the years.


I too looked at a bunch of sites and couldn't find any mention of sizing jumpers...you have any links that I could read referencing sizing jumpers?
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Old 08-21-2015, 12:26 AM   #14
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I'm not sure if this is any indication of anything at all, but in the past, batteries had exposed inter-cell connecting straps. Take a look at this battery and note the size of those straps. You can see that the sectional area was substantial, but no larger than absolutely necessary in a mass-produced battery. It seems logical that this would also apply to inter-battery connections.



While no longer exposed, they are still substantial in more modern designs.

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Old 08-21-2015, 01:52 AM   #15
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I appreciate everyone's comments but I'm not asking for opinions. I was hoping some EE would have the answer.
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Old 08-21-2015, 02:23 AM   #16
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I appreciate everyone's comments but I'm not asking for opinions. I was hoping some EE would have the answer.
All you are ever going to get is an opinion.

My first response to you was correct and based on ohms law, something I have spent a lifetime thinking in terms of.

I would tend not to do it, but there is no reason why you couldn't.
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Old 08-21-2015, 03:00 AM   #17
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Well, that was a long pause in the conversation.
Terry,

This is straight out of the Trojan Battery User's Guide. The cable sizes for series and parallel connecting cables under 6' are in the table below. I can't imagine a more accurate or reputable source for the recommended size of these jumpers than the battery manufacturer. Plenty of EE's on staff I'm sure, not to mention lawyers. I'm sure you could call their tech support department for more info as well.
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Old 08-21-2015, 09:46 AM   #18
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Quote:
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As an example a 12v 200 amp load requires 1/0 ga for a 20ft run from the battery bank. Since the distance between the posts on the battery bank are inches, is there any reason the large cable is needed for inter-bank connections? My current setup has large cables for the run to the starter and the same size cable between the posts in the battery bank.
Getting back to the OP, let's say he is sure the load will never ever never exceed 200 amps, in other words, he's feeling lucky . Assuming he is using 105C rated boat cable, and the batteries are in the engine room, he thinks he could step down to 1 gauge (208 amps) from 1/0 (242 amps.) . The difference in price at GenuineDealz.com is $1.20 a foot. Take the extreme, a guy feels he can get away with stepping down from 4/0 (378amps) to 2/0 (280 amps); the price difference is a roaring $3.39 a foot. I mean really, is this even worth the argument, let alone de-rating the whole system and increasing risk? When you see this kind of stuff on a boat, you start wondering what other corners are getting cut for the sake of a ten or twenty dollar bill...

As for links, go pull up the manuals for the larger Magnum or Xantrex inverters, which explicitly say to use the same gauge, for starters. Or look at the Trojan and Rolls user guides where the prescribe a certain size and no mention of a different one for interconnects right there on the page where they discuss various battery bank configurations. They're all available online. But what do those guys know?

Larry M: great point, but apparently those battery builders don't know squat either, they could reduce costs and no one would be the wiser, according to the wisdom here.
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Old 08-21-2015, 10:05 AM   #19
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The answer I believe to the OP is no...they don't have to be the same size as the runs.

I have perused all the sites you suggested and more...but no specific links as requested, and find no mention of battery bank jumper sizing.

I looked at Rolls, Xantrex, Exide, Trojan and a huge number of golf cart sites. All seem to leave it up to " installer discretion" . I could have missed the exact location so a specific point of info would be appreciated. If I had one, I would be eager to share with others that may have the same issue.

The dollar agreement is a good one...but not necessarily the "best"answer to the question.

Again...I am all ears for a tech electrical answer...... or a link to one so I can pass along that info in the future.

But for now, I am with Kevin Sanders with the...no they don'thave to be....but better biggrin if you want to or already have the wire and fitting on hand or on order.
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Old 08-21-2015, 10:17 AM   #20
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The answer I believe to the OP is no...they don't have to be the same size as the runs.

I have perused all the sites you suggested and more...but no specific links as requested, and find no mention of battery bank jumper sizing.

I looked at Rolls, Xantrex, Exide, Trojan and a huge number of golf cart sites. All seem to leave it up to " installer discretion" . I could have missed the exact location so a specific point of info would be appreciated. If I had one, I would be eager to share with others that may have the same issue.

The dollar agreement is a good one...but not necessarily the "best"answer to the question.

Again...I am all ears for a tech electrical answer...... or a link to one so I can pass along that info in the future.

But for now, I am with Kevin Sanders with the...no they don'thave to be....but better biggrin if you want to or already have the wire and fitting on hand or on order.
Somehow I got this posted on the wrong thread last night.



This is straight out of the Trojan Battery User's Guide. The cable sizes for series and parallel connecting cables under 6' are in the table below. I can't imagine a more accurate or reputable source for the recommended size of these jumpers than the battery manufacturer. Plenty of EE's on staff I'm sure, not to mention lawyers. I'm sure you could call their tech support department for more info as well.
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