Voltage Drop - (K)

[Bigbri0104]

Hello,
I'm trying to understand 24-volt fire alarm single phase voltage drop calculations vs. single phase 120/240 voltage drop calculations.
I understand the purpose for calculating voltage drop, but I'm having a little difficulty understanding why the constant (K) is not included in voltage drop calculations for fire alarm circuits.

Fire alarm circuit ex. 2 x I x L (R-per ft/1,000) = VD

120/240-volt circuit ex. 2 x K x L x I/ CM = VD

I'm assuming the reason K is not in the fire alarm equation is because it's a DC circuit. Just want to be sure.

Thanks Brian

 

[don_resqcapt19]

K” = Direct Current Constant: This is a constant representing the direct current resistance for a 1000 circular mils conductor that's 1000 ft long, at an operating temperature of 75°C. The direct current constant value you should use for copper is 12.9 ohms and 21.2 ohms for aluminum conductors.

It really is the same formula, but just a different method of getting the resistance of the conductor.

 

[winnie]

Don has it. In one equation you are given R per 1000 feet of wire. In the other equation you calculate R using the resistivity constant and the cross section of the wire.

Jon

 

[Hv&Lv]

Fire alarm circuit ex. 2 x I x L (R-per ft/1,000) = VD

120/240-volt circuit ex. 2 x K x L x I/ CM = VD

In your equations above, K is the same as (R-per ft/1,000)

 

[Carultch]

K is the resistivity of the material, just expressed in units of Ohm-kcmil/kilofoot, instead of Ohm-meters.

 

[Bigbri0104]

Good morning

Thanks for breaking down the equation for me much appreciated.