Voltage Drop Calcs

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Re: Voltage Drop Calcs

The formula for caculating DC resistance is:

R = (K/A) x 1000 ohms per 1000 ft.
R ohms per 1000 ft
K volume resistivity in ohms-cmil/ft
A cross setional area in cmil

Using the given resistance in table 8 you can determine the value of K. The value for #12 was 12.6 and for 4/0 it was 12.6. The value of K is at 75C. K = 10.57 at 25C. The value of K should remain fairly constant at any temperature.

I don't think the formula can be used with table 310.16. Voltage drop at any amperage varies with the distance. If the distance is 10 ft you may get an answer of #12 cu instead of #6.

[ April 16, 2005, 02:48 PM: Message edited by: bob ]
 
Re: Voltage Drop Calcs

I've always assumed "coated" to mean "insulated".

Looking at it, I can't actually be sure of that.

But I would presume that any kind of metallic coating would change the conductors resistance differently for every different type of metallic coating.

The same would probably be true for different insulation types but I would think to a much lower degree.

And also, I think it would be termed a "clad" conductor.

Edit: I'm pretty sure copper means 100% copper.

[ April 16, 2005, 02:54 PM: Message edited by: physis ]
 
Re: Voltage Drop Calcs

The term "coated" in NEC table 8 is bound to mean "tinned".

I do not know if they literally used tin or nickel or both, but back in old days, where the insulation was rubber-based, they had to coat the conductor strands to keep the rubber from attacking the copper. The DC resistance of this coating was higher than copper.

Thus, I believe it is correct to use "uncoated" for most modern "plastic"-insulated wire.

[ April 16, 2005, 09:13 PM: Message edited by: kiloamp7 ]
 
Re: Voltage Drop Calcs

Hello Izak: Your question is not an uncommon one. The best approach to solving your problem, is to perform a voltage drop calculation solving for the minimum size conductor that will provide the maximum voltage drop allowed. Once the minimum size conductor is arrived at based on allowable voltage drop, then check Table 310-16 for the minimum conductor size based on the rating of the terminals to which the conductor will be connected to. Remember current-carrying capacity of a conductor is based on insulation temp. rating, ambient temp, and number of conductors in a raceway or cable. All of the above factors must be considered.
 
Re: Voltage Drop Calcs

Well.

Now I simply must know what coated means. No two ways about it.

I'm gonna say that there ain't no way insulation ain't a coating.

I can also say that when I dealt with much smaller stuff, tinned conductors were not at all uncommon, in fact the opposite was true.

And I think the term tinned comes from the tin content in solder. The ratio, such as 60/40 refers to tin and lead. That, and people called everything tin in the '50's. So called "tin cans" are not made out of tin. Tin's a wierd metal, very soft, you don't see it by itself much.
 
Re: Voltage Drop Calcs

Hi Folks, Since I am writing an article for the NEC Digest, I happen to have my Old Code books handy. your question about the term "coated" that appears in Table 8 Conductor Properties is a term that took the place of "Tin'd" which first appeared in the 1947 NEC and last appeared in the 1981 NEC. "Coated" means "Tin'd".
 
Re: Voltage Drop Calcs

Alright, well, I guess I can let you have this one kiloamp7. But I'm not happy about it. :D
 
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