Determining Wire Gauge

[justinlw]

With 24VDC, and roughly 30 amps over a distance of ~270inches(round trip), what kind of guage will i need?

I'd prefer to not just be given the answer, but maybe one of you guys know a great site explaing how to figure this out. math websites, or charts acceptable.

i appriciate the help.

 

[Optikon]

With 24VDC, and roughly 30 amps over a distance of ~270inches(round trip), what kind of guage will i need?

I'd prefer to not just be given the answer, but maybe one of you guys know a great site explaing how to figure this out. math websites, or charts acceptable.

i appriciate the help.

Well, here is a table.

http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/wirega.html

It depends on what kind of voltage drop you can live with. Lets say, you can only tolerate 2V drop at 30A. That means the total wire resistance has to be less than 0.067 Ohms. The table shows you gauge sizes along with resistance per 1000 ft. So figure out what gauge can meet your voltage drop at 270 feet of that wire size. Really just ohms law & algebra.

The temperature affects wire resistance but unless you are operating in extreme environments, that wont matter much. Just make sure you have some margin built in. Oh, and these are DC calculations (assumed)

 

[stevez]

The voltage drop test is a good check but the amount of heat generated is also of concern. The type of insulation and the environment play into the ampacity of wires. A wire could pass the voltage drop test - yet generate so much heat that the insulation would melt or burn. If this were copper foil on a board it might overheat and delaminate.

From a table in a manual I found that 14 ga wire could have an ampacity of 45 amps for a single conductor, in free air, running at 200 deg C in a 30 deg C environment. That same wire, with not more than 3 conductors in a raceway or conduit would be rated for 30 amps at 200 deg C. If the allowable temp of the wire were limited to 60 deg C then the ampacities would be 20 amps in free air or 15 amps with no more than 3 conductors in a conduit.

I'd start with 10 ga wire then test the voltage drop. A lighter gage might work.

 

[justinlw]

thanks guys.

temp is diffently going to be a factor. its going to be in a metal conduit, under a tile floor.

from the looks of it, i'm going to start with 10ga, and keep 8 or 12 as a backup depending on stuff.

 

[Papabravo]

In winding coils and transformers a common figure is 700 CM/Ampere. A CM or circular mil is a unit of area equal to a one mil diameter circle, or pi/4 square mils. The circular mi area of a wire is just the square of the diameter.