voltage drop

[jumpinjohnny]

In the journeyman/contractor test prep K in the vd equation is 12.9 for copper. Can anyone tell me how this number was derived?

Also, we are allowed a 3% drop. We vary voltage at the source. If we have 120, with a 3% drop we could drop to 116.4. If we have 124v at source then we could drop to 120.28 v.

with this in mind, why couldn't a person start with 124v and drop to 116v and still be okay?

[ September 15, 2005, 07:29 PM: Message edited by: jumpinjohnny ]

 

[dillon3c]

Re: voltage drop

JJ,
go to Table 8,Chapter 9

Run your finger down to 1000kcmils.

from 1000kcmil run your finger horizontally to the right of page,you will see 0.0129=(12.9) under Copper Uncoated,(ohm kFT)=1000ft

*notice the (0.0212)= 21.2 for ALuminum also..

 

[infinity]

Re: voltage drop

Also, we are allowed a 3% drop.

Using the word 'allowed' makes it sounds as if the 3% voltage drop is a mandatory requirement. It isn't. It is part of FPN's for feeders and branch circuits which are not enforceable as part of the NEC. They merely appear for informational purposes, as explanatory materials, or as suggestions.

So in answering your question you could start with 124 volts and end up with 116 volts and be OK. Not the best design but not a code violation either.

 

[jumpinjohnny]

Re: voltage drop

maybe not the best design, but if I plug in all the numbers and come up with a number 6 to drop no more than 3.6v with 120v source then I reverse the calc. and plug in cm's for #8 and come up with a 4v drop.... Whats $ tenths of a volt

 

[oliver100]

Re: voltage drop

In NYC Con Edison regularly drops the voltage during the summer. So what voltage drop we are talking about - POCOS or ours? Moreover, what would be the base load for VD calculations. The size of the main breaker?

 

[bob]

Re: voltage drop

In the journeyman/contractor test prep K in the vd equation is 12.9 for copper. Can anyone tell me how this number was derived?

The value K is the volume resistivity of a copper wire weighing 1 gram and 1 meter long.
K is a multiplier to convert wire of different
cross sectional area in CM to resistance.
It is important that you know that K varies with temperature. K at 25C = 10.575 while K at 65C = 12.9. The temperature of the enviornment will dictate the voltage drop. The resistance of a conductor = 1.02(K x 1000)/A in cmil = ohms/1000ft. 1.02 is a factor for stranded wire
and is equal to 1 for solid conductors.

 

[sceepe]

Re: voltage drop

Also, we are allowed a 3% drop.

The allowed voltage drop is set by the energy code not the NEC. ASHRAE 90.1 requires no more than 3% for branch circuits and 2% for feeders!