Wire size chart for DC

[got_nailed]

I’m looking for a chart on using copper wire with DC current. My plant use 12, 24, 48, and 96 DC. Some of it is up to 250 amps at 48 volts. We do have a wire size chat at work but it only covers 12 volts and up to 100 amps for DC.

I did run across some wiring that is IMO way under sized for amps it caring and the insulation are in bad shape. I’ve replace all the questionable wiring with larger wires but I still don’t know if it’s the right size for the loads (I’m planning on rerunning the wires once I get the needed wire sizes that should have been run).

I have done a few searches on goggle and on here but I can’t find what I need.

 

[barbeer]

I believe that 310.16(.15 or .17?) would be applicable.

 

[don_resqcapt19]

You need to look at the voltage drop as well as the ampacity of the conductors. A conductor sized for a 1% drop at 120 volts will have a 2.5% drop at 48 volts with the same current.
Don

 

[got_nailed]

Thank you for the NEC code violations numbers. I did site down and do some reading. I?ll have to talk to some of the electricians at work and get more info.

I?ll end up have to do a case by case to get some of this stuff replaced.

 

[dereckbc]

Got nailed,

I have been designing DC plants for a few decades now; and there is no easy answer to your question. NEC 310.16 is the bare-bones minimum you can get away with but for the most part entirely inadequate for most DC applications because of the low voltage nature of the beast.

Telephone companies have developed their own tables with minimum wire size vs breaker/fuse size. These charts are proprietary in nature, but far exceed any NEC requirement. For example a 20-amp breaker minimum cable size is 6 AWG. However the charts have distance limitation of around 50-feet

The only way to be precise is to use voltage drop calculation. To use the calculations you first have to have standards for maximum voltage drop allowed on the various nominal voltages of 12, 24, and 48 volt. To complicate it further is the distribution system has to broken down into components and voltages assigned to each segment like battery bables, battery buss, primary distribution, and secondary distribution.

You start by the industry standard of total allowable voltage drop from the battery terminals under full discharge current to the far end equipment. It looks like this:

12 = ? volt.
24 = 1 volt
48 = 2 volts
96 = 4 volts

As you may notice this is very tight tolerances. Now it gets worse because the part of the system you are working with is probable the secondary distribution, and you do not get the full voltage drop allotment, only a fraction of it because the upstream system get a fare share.

So if my assumption of secondary distribution is correct you max allowable voltage drop are:
12 = ? volt
24 = ? volt
48 = 1 volt
96 = 2 volts

So now all you need is a formual now that you know the max allowable voltage drop and the maximum discharge load current which is:

CM = (22.2 x I x D)/Vd

Where:
CM = Circular Mills of copper cable
I = Maximum load current at full discharge
D = One-way distance in feet
Vd = Maximum allowable voltage drop.

So lets say we have a 12-volt system on a 50-amp breaker that will draw 35-amps at full discharge voltage and the one-way distance is 60 feet. Plug in the values into the formual:

I = 35
D = 60
Vd = ?

And you get CM = 186,480 circular Mills. Go to NEC Chapter 9, Table 8 and look to see what size cable you need without being smaller comes out to 4/0. One final check go to NEC 310.16 and make sure the cable is equal to or larger than that required for the OCPD of 50-amps on the cable type of RHW which is # 8 AWG. 4/0 is larger than required so you are good to go.

Easy huh?

 

[hardworkingstiff]

Easy huh?

Yea, when you do it. LOL.

 

[got_nailed]

Last night at work I did change out a bunch of cabling. I used the chart on http://www.houwire.com/products/technical/article310_17.html and did the temp corrections; then I went up one size (all of what I did was less than 6 foot runs). There is still a lot to do but I have t wait till stuff is down so I can get in them to do this type of work.

For all of the long runs (stuff that?s not mounted to the machine) I?m trying to get them to use relays and put in a transformer at the other end for there loads. We have some pump controllers that are 400 feet form controller to pump. I did also right it up to use the old controller panels to send signals to a new controller panel and use AC pumps. I do think after looking in to the cost of updating the DC wiring and what it would cost to do it with AC pumps and the second control board they will go with the AC.

With a few more slow nights at work I can have it all on paper with cost and how to control the down time on the switch over. I do know that with using AC pumps and not having the old DC system we will have less down time with the problems we keep getting. Even if the old box calls for the pump to kick on and it don?t you could use the new pump boxes to pump out the chemicals as needed. We have had some lines down for 36 + hours because of not getting the pumps to work and get the chemicals out of cold storage.

I?ll have that on my desk tonight dereckbc.

Thank you for your help.