How to Size Electrical Cables

[CapetaniosG]

To properly size electrical cables, there is a formula we can use. Here it is.

Acmil= (KxIxL)/V Where:

Acmil is the cross-sectional area of the wire in circular mils

K=10.75 representing the resistance of copper in mil-foot

I= is the maximum Amps in the circuit we're designing

L= the length of the conductor (wire) in the circuit in feet.

V= is the maximum allowable voltage drop at max. load

Here is an example: We want to size the electrical wire for our new windlass drawing 100 amps and we want to limit the voltage drop from the battery
(24 Volts) to the windlass to 5% or 1.2 volts (0.05x24=1.2volts). We measured the routing of the wiring from the battery to the windlass to be 20 feet or 40 feet of cable run. The required cross-sectional area of the wire is:

Acmil= (10.75x100x40)/ 1.2= 35,883.333 circular mils. We will choose AWG #4
wire which has Acmil= 37,360.

You can apply the formula to any circuit as long as you know the max. amps, cable run and the max voltage drop you want to have in the circuit. The rest of it is simple Arithmetic.

For reference:

AWG14 = 3,702 Circular mils
AWG12 = 5,833 Circular mils
AWG10 = 9,943 Circular mils
AWG 8 = 14,810 Circular mils
AWG 6 = 25,910 Circular mils
AWG 4 = 37,360 Circular mils
AWG 2 = 62,450 Circular mils

Have I confused some of you with the new math? If I did, my sincere apologies. I didn't mean to. Pull out your Algebra book and refresh and stop spending so much time boating. Guys I just consumed a dozen clams and half a dozen beers. The clams were local, the beer from Holland.

CapetaniosG
Hatteras 53MY

 

[spartonboat1]

So after all that consumption, were you really wired? Sorry couldn't help it

 

[CapetaniosG]

Spart do you have any clams in lake Michigan? or just just Carp.

CapetaniosG

 

[dwaynec]

Do you think this should go into the technical FAQ? Seems like a lot of good info. It sure has been useful to me. Thanks.

 

[Nick]

Good addition to the FAQ.... assuming the beers and or the clams have not affected your math calcs!

So the beer from Holland we can guess. Did the clams come from Alice's or Southold or...??

Capt you must really miss Rhodes and your mistress by now!! (Lady Ntina of course)

Nick

 

[CapetaniosG]

Nick- I do miss Rhodes and Lady Ntina. I'll be there in a fiew days and all will be well. The clams, I racked myself from Goose Neck, Southold. Boy they are they good with a cold beer. If I can log on from my lap top in Rhodes I'll keep in touch with you all. Most of you will be hauling out while I'm crousing the Greek Islands.They are a true paradise. Eat your heart out.

Nick with that name you must have some Greek blood in you. Is that so?

CapetaniosG

 

[Nick]

If you are still in town this weekend, feel free to stop by Brewers Stirling Harbor E dock before you leave, "Ocean K'Motion" or email me your tel.

Yes my family came from the Islands of Spetses and Kefalionia. Merchant marine heritage!

American born!

Nick Andrianas
andrian1@optonline.net

 

[CapetaniosG]

Spart_ If you ever pass by Benton Harbor Michigan with your boat (it is on the Eastern shore of the lake) and see two dome like structures next to one another, that's the D. C. Cook Nuclear Plant that I helped built back the seventies. I was the Lead Structural Engineer for it. The electricity from that plant (2200 MW) may be feeding your boat. So pay your electric bill because I want my retirement check to keep coming.

CapetaniosG

 

[Nonchalant1]

If the distance from the batt to the windlass is 20 ft, you used 40 ft cable run. Is there a reason it is doubled? Maybe because with all the turns and corners it actually takes 40 ft of cable?

Doug

 

[jim rosenthal]

No it is because the actual current path is forty feet long. Twenty feet from the battery to the load, and then twenty feet back. All of those feet contribute resistance to the circuit, and therefore they all get counted.

 

[CapetaniosG]

Dr. Jim is correct, as always.

CapetaniosG