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Old 01-21-2011, 11:34 AM   #1
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working with high curent DC


I am looking at using some deep cycle batteries as replacement for my UPS batteries and I suspect I need some rather thick cables if I want to go like 10+ feet away. Can I use 8 or 10 awg romex for this? It will either be 12V or 24V, not quite sure yet. UPS is 1000VA but for future proof sake, lets say 3000VA at near full load. I just want to make sure that when it's running on battery, the cables can handle this load for extended time. Also, how do I typically connect cables to batteries? would I need to buy a crimper and crimp some lugs?

I would probably run this in a conduit on the ground, just so it's neater. Also, any safety tips for using marine batteries indoors? I do know they produce a bit of hydrogen, should I vent this out or is it a very minimal amount, and is it only when they're charging?

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Old 01-21-2011, 01:32 PM   #2
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The Romex cables will take the same 20 amps for 12 gauge, 30 amps for 10 gauge, etc.

Ten feet should not be a problem but for much longer lengths the percentage voltage drop becomes significant.

THe number of volts dropped is the same for a given current flow (amperes) and a given piece of wire regardless of the supply voltage. (Volts equals amperes times resistance)

Use the lugs or clamps recommended for the battery. For automotive batteries, an auto supply store should have the clamps you need. Typically you screw the wire onto these.

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Last edited by AllanJ; 01-22-2011 at 09:52 AM.
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Old 01-21-2011, 05:20 PM   #3
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Oh yeah, that makes sense, can just use the same rules as with home electrical. Actually anyone know of a good chart for that? I know the guages required for 15 20 30 and 40 amps but anything beyond I'm not too sure. I'm guessing this will use 24 amps at most for a 1000va unit (600w), so I'll probably be fine with 10awg if the UPS needs 24v.
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Old 01-22-2011, 02:59 AM   #4
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The lower voltage will have far much more senstive to the voltage drop and with 600 watts @ 12 volts = 50 amps BUT how far the UPS to battery that will make the diffrence there so if you are only less than 5 feet you may squeak by with #6 AWG cable and for your distance { 10 feet } you may need to use #4 AWG cable to keep the voltage drop to minuim.


Most on line voltage drop chart most are sized for either 120 or 240 volts load but few may have listing for other voltage so you may have to goggle a little.

24 volts is better due you can get away with little less voltage drop issue.
The other thing is make sure you mark the junction box that you have low voltage cable there so in case someone else have a funny idea why.

And it may be a wise idea to add a fuse along the way.

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Old 01-22-2011, 09:50 AM   #5
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A 3 percent loss (voltage drop) out of a 24 volt supply is 0.7 volts. For 24 amps the resistance of the wire path can be at most 0.3 ohm.

Using 10 gauge copper wire (minimum to avoid overheating the wire) you are limited to 15 feet of cable (30' round trip) for 24 amps with 3% drop.
Using 8 gauge copper you are limited to 23'
Using 6 gauge copper you are limited to 36'

Double the distance, double the voltage drop, probably too much.
A tad more distance, perhaps end up with 4% drop, you may get away with that.

If you have to upsize for distance and voltage drop purposes, then you pretend that a continuous load is intermittent and not upsize any more for that.
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Old 01-23-2011, 05:43 AM   #6
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upsize your wire. AND VENT THOSE BATTERIES, unless you want to have a lot of fun I have by mistake had a battery explode in a similar happenstance, with enough force to break the battery case. Standing right by it when it happened, couldn't think for awhile
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Old 01-23-2011, 03:08 PM   #7
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For venting, could I get away with hooking up a 1 inch pvc pipe to the top of the enclosure, making it go up, then through the wall? Or would I need two (one intake, one exhaust). And is an inch thick enough? Since hydrogen is light I'm guessing it will be able to go upwards on it's own without any help. On the outside of the house I'd make the pipe do a 90 downwards and add a small screen to avoid any bugs from entering.

From what I understand I also need to add some distilled water to those every now and then, how do I know when I need to do that?
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Old 01-23-2011, 03:32 PM   #8
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Im sorry, how does 1000VA on a 24volt system come out to 30 amps......You size wire off of CURRENT (amps) by Apparent Power (VA) not Real Power (watts).......... VA and Watts ARE NOT the same thing nor interchangeable. Look into Power Factor, Reactive Loads and Resistive Loads/Inductive Loads.

Youre going to need wire capable of running about 50 amps or so. (1000VA) unit


The VA rating is the 120v output from the INVERTER side of the batteries. This means that the draw on the batteries will be greater than that due to inefficiencies.

Most of those units have about 1' pigtails of #4 Automotive copper.......I suggest doing some research into electrical theory and engineering before trying something that is potentially dangerous.
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Old 01-23-2011, 04:46 PM   #9
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Actually I always wondered, why is it that UPSes and transformers are rated in VA, but the watts are different? From my understanding watts = volts x amps, so what exactly is VA? When I want to know how much I can put on the UPS, I look at watts, because equipment is rated in watts, not VA. So If a server takes 200w then I know I'm using 200 out of the available 600.

I was calculating based on watts which came up to 25 so with inefficiencies I figured I could expect the max to be 30, at full load.

Also say I go with something much bigger, like #4 or maybe even #2, since I do plan to put in a 3000VA unit at one point, what is the proper way to connect wires this size, to the smaller wires in the UPS, as well as to the batteries?

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Old 01-23-2011, 05:10 PM   #10
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VA is a term used where the load is NOT connectted directily to the source, but is instead via a transformer.
The difference is things like inductive reactance , phase angles ,
Power factor and capacitve reactance.
All these things have a bearing on the final output power available.

The term Watts applies only when the load is connectted directily to its source, and NOT via any transformers.

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Originally Posted by Red Squirrel View Post
Actually I always wondered, why is it that UPSes and transformers are rated in VA, but the watts are different? From my understanding watts = volts x amps, so what exactly is VA? When I want to know how much I can put on the UPS, I look at watts, because equipment is rated in watts, not VA. So If a server takes 200w then I know I'm using 200 out of the available 600.

I was calculating based on watts which came up to 25 so with inefficiencies I figured I could expect the max to be 30, at full load.

Also say I go with something much bigger, like #4 or maybe even #2, since I do plan to put in a 3000VA unit at one point, what is the proper way to connect wires this size, to the smaller wires in the UPS, as well as to the batteries?
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Old 01-23-2011, 05:42 PM   #11
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I think I sorta get it, but could you point me to a resource to read further on this? I've never fully understood the coloration and it's probably something good to understand if I'm going to play with this stuff more.
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Old 01-23-2011, 08:55 PM   #12
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Red, I'm just an electrical hobbyist interested in unrelated systems, but I saw two things in your OP that caught my eye. 'High voltage DC', and 'romex'. I'm not going to give you a bunch of formulas and ratings. I'll just say that many times in my research I have seen it cautioned never to use a solid wire like romex for HVDC. It overheats. It's my understanding that's why stranded wire was invented in the first place. The original DC systems by Edison were horrible fire hazards.



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Old 01-23-2011, 11:00 PM   #13
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Its not even high voltage DC....He should be good to about 50 VDC or so.

High voltage DC to me is hybrids with their 600v motors......


But again, OP, I am asking how do you figure that 24VDC and 1000VA is about 30 amps?????????

DO NOT figure your formula from WATTS......Calculate from the largest of the two, or better yet just from VA........as it will usually be the largest.

VA is not NECESSARILY ONLY for transformer loads, but whats called reactive loads.

VA is greater when your Power Factor drops below 1.

You are calculating the 120v MAXIMUM WATTAGE of 600w. NOT what the batteries (24VDC system) will be supplying.

I really hope you stop and read this post before pursuing this topic further.....You have potential to cause a dangerous situation.

You are dealing with two distinctly different systems in one unit, you need to have the ability to distinguish the two and how their systems function.
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Old 01-23-2011, 11:05 PM   #14
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That is good to know, I figured that romex would be better then stranded. Guess if I use automotive cables I'll be better off then. I think I can buy them precrimped and still long enough. Guess I can always bolt two together and wrap with tape if I need longer right? I'd also add some of that anti corrosion stuff.

Is there any chance of a battery actually catching on fire or exploding to the point where it can ignite other sources such as joists above, if things go wrong? Or do they more or less just burst and start leaking acid? I'm hoping if anything goes wrong it can be contained within the battery box. I've been reading up and lot of people have done this, so I'm probably worried for nothing. I will do lot of testing and monitoring before I put this in production.

As for my venting, does my idea I posted sound ok, with using the 1 inch PVC pipe going outside?
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Old 01-23-2011, 11:20 PM   #15
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don't hack the cables like that. You can get any length of cable made or you can DIY if you buy a crimper.

a battery can explode with a lot of force. I know of one that blew the top of the battery off and it hit the ceiling in the vehicle 6 feet up. They tend to splatter acid all over as well. They need to be in an enclosure that would control all that mess.

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