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.

I would keep them the same size.

In theory you could downsize considerably but I do not do it in practice.

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.

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.

ksanders said:

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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this can be handy for cables and drops


Marine Wire and Cable: Voltage Drop Calculator

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.


David

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?

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.

ksanders said:

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.

Click to expand...

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.

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.

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.

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?

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?

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.

I appreciate everyone's comments but I'm not asking for opinions. I was hoping some EE would have the answer.

meridian said:

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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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.

meridian said:

Well, that was a long pause in the conversation.

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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.

meridian said:

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.

Click to expand...

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.

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.

psneeld said:

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.

Click to expand...

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.
Attached Thumbnails

Which to me means that they could be different than "run" cables if the jumpers are less than 6 feet and the runs are longer...again...small patatoes but by suggesting that chart...then a series (jumper) cables of say 6 inches could be substantially smaller than ones at 6 feet.


That table really doesn't say much more than what I believe Kevin was saying and me that using loss and ampacity charts CAN be used to size jumpers...not arbitrarily using... "the same size as your "runs"" concept.....


But as has been said...do what you are gonna do....

psneeld said:

Which to me means that they could be different than "run" cables if the jumpers are less than 6 feet and the runs are longer...again...small patatoes but by suggesting that chart...then a series (jumper) cables of say 6 inches could be substantially smaller than ones at 6 feet.


That table really doesn't say much more than what I believe Kevin was saying and me that using loss and ampacity charts CAN be used to size jumpers...not arbitrarily using... "the same size as your "runs"" concept.....


But as has been said...do what you are gonna do....

Click to expand...

Exactly!

There is a huge difference between understanding the electrical and thermal properties of electrical conductors and quoting chapter and verse from a wire size table.

Again... I would probably not tend to undersize a battery jumper simply because I generally have the larger sizes and the crimpers on my boat all the time.

But It can be done and the loads would never know the difference.

Hmm! That Trojan table doesn't match other ampacity charts I have seen/used. Look at Ancor's table here: Conductor Sizes | Ancor Ancor makes marine, tinned wire and also should know a thing or two about ampacity. Their table is consistent with others I have seen so maybe Trojan's is based on different conditions/insulation.


But back to the conversation. Sure if I were going to order a new jumper for my recent dual 6V battery installation, I would choose 2/0. But because my house bank will never see more than 220 amps and 99.999% of the time less than 20 amps, I used a #1 jumper that I had on hand. It works, and it is safe.


David

ksanders said:

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

Click to expand...

Since wire size tables (as opposed to voltage drop calculators) typically do not reference the length of the wire run, that is not true.

Tables like the ones at Marine Wire and Cable: Voltage Drop Calculator are just based on the heat-dissipating capacity of the wire, which depends on its gauge, the temperature rating of the insulation, how many conductors are bundled together, and things like that -- all independent of length.

av8r and caltexflanc are correct about this.

QB said:

Since wire size tables (as opposed to voltage drop calculators) typically do not reference the length of the wire run, that is not true.

Tables like the ones at Marine Wire and Cable: Voltage Drop Calculator are just based on the heat-dissipating capacity of the wire, which depends on its gauge, the temperature rating of the insulation, how many conductors are bundled together, and things like that -- all independent of length.

av8r and caltexflanc are correct about this.

Click to expand...

OK....how do YOU base what wire size to use on a run?

I had his same discussion with my marine electrician. We will likely increase cable size in my house bank to 4/0 when we rewire the bank for a more optimal configuration this fall. The "optimal configuration" change is to ensure the batteries in the bank contribute equally to the loads on the system. The reason for 4/0 is because of the length of the cable run to the inverter charger and the associated voltage drop when under large loading. Since the current cable size is 2/0, I asked if the connectors between batteries could remain at 2/0 since the current between the individual batteries would be considerably less than that of the mail cables to the inverter charger. He indicated that while that could be done, he urged best practice would be to switch everything over to 4/0 all the way to the inverter charger.


Jim
Sent from my iPad using Trawler Forum

Interesting, this is one of the few things in boating where you don't have to guess, follow an old wive's tale, scatter chicken bones, listen to Bob who heard it from Sam who heard that somebody they heard of had a fire... More controversy than what quantity of bear spray you should pack in the bush. Cut and dried. Easy apps, good information in Calder etc.

Obviously, the only correct answer is to use only 4/0 cable, for everything. Everything, then there can be no problems.

Samr reasoning holds for a refit.

Buy a spool of #10 and most everything will be happy

Little downside in cost if you buy a big volume , same terminal ends for everything and with a set of stick on numbers from the Alarm folks , its easy to keep track, and create a schematic.

Only downside is #10 might weigh a bit more than #22 for a light.

What's the old saying? "The nice thing about banging your head against a wall is that it feels so good when you stop!"

I asked if the connectors between batteries could remain at 2/0 since the current between the individual batteries would be considerably less than that of the mail cables to the inverter charger.

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Not so, current, which is measured in amperes, will be the same all across all the battery-related cables.

But because my house bank will never see more than 220 amps and 99.999% of the time less than 20 amps, I used a #1 jumper that I had on hand. It works, and it is safe.

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It is "safe" only if the battery bank is fused correctly to protect the #1. No big sustained surge items in your system? No inverter?

Who here puts circuit protection on their jumpers between 6 V batteries?


yep...that wall is hard.....