Impedance Values Tables

Look up the NEC, National Electrical Code, code book(s) they have all of the needed formulas there. I don't have one handy, but I know that they have these formulas in the back of the book. Don't know if they have anything online though. Goodluck. I looked for some info in my old school books and came up empty, sorry.

OHMS= K X L divided by cmA ;where K is a constant,10.4 for copper and for aluminum is 17.0;L is the length in feet;and cmA is circular mil area. 500mcm=500000cmA

Thanks very much!

Take heed!

Numbers offered in the above posting, are the Specific resistances of Cu and Al at 28⁰C and are NOT the impedance!.

Your cable impedance will be Z=R+jX, where Z, is the complex resistance (impedance) in Ohms, R is the specific Resistance at a given Temperature, X is the reactance whether capacitive or inductive, and is greatly customer dependant. Looking at the formula,( j)you can easily deduct that this is not a simple arithmetic sum, but rather a vectorial one.

What are you trying to calculate? if just the voltage drop,and assuming that your customer is pure resistive (never) than the above formula is ok. But if cosine φ is what you are worried about than you need to know other factors as well, and they are not always under your control.

Hope it helps a bit.

Wangito.

Thanks for the heads-up! I thought the values might be a little off when calculating voltage drop.

I'm trying to figure voltage drop for a 5000 sq ft house, with 2 AC 5 ton units! Plus the other service off the same XFRMR is a 4000 sq ft house with 1 AC 5 ton unit! I got a 25 kVA transformer feeding both houses. The 5000 sq ft has about 220ft of 350mcm Cu, and the other service is about 200ft of 500mcm Cu.

This one has been bothering me for a couple of days cause I'm 100% sure there is a table that lists the resistance values for the conductors - any diameter.

I may actually have to go to the book store and see if the American Electrician's Handbook has it (which I think it does) or find some contractor that might have it.

Thanks for your help,

Here's what I suggest:

1. Don't worry about voltage drop. If you have a notable drop over a several hundred feet of wire, boy you are in trouble.

Go straight to the AMPCITY (Amperage capacity) tables found in Any electricians handbook and install the correct cable type and size, and just forget about impedance. Once you have the correct cable with the correct current safety margins, all your other problems will automatically be solved.

Te NEC book will provide you with all the necessary info.

Wangito.

Yeah, for sure the NEC will have all of the answers for. There will be grounding and ampacity tables there for you to look at and get answers from.

Thanks a lot guys. I finally found it at the back of the NEC 2005 ed. It's pretty comprehensive.

a 25 kva transformer sounds real small for your 5000 and 4000 square foot house? sounds to me you would need at least a 45 kva transformer.

wondering;