Conductor size calculation based on voltage drop for single phase

[Atif Mehmood]

Dear all,

How can i calculate a conductor size based on derated ampacity if i have distance from source ( 17M) , source voltage (110VAC), Current (34.7A), single phase. As per NEC Voltage drop should not exceed more than 3%.
Kindly give one example for reference.

 

[JDBrown]

Dear all,

How can i calculate a conductor size based on derated ampacity if i have distance from source ( 17M) , source voltage (110VAC), Current (34.7A), single phase. As per NEC Voltage drop should not exceed more than 3%.
Kindly give one example for reference.

Different people use different methods. I use the method I was told to use by the City of Los Angeles.

Start by going to NEC Chapter 9, Tables 8 and 9. For conductors #2 AWG or smaller, use the DC resistance from Table 8. For conductors larger than #2 AWG, use the Effective Z Table 9. I'm guessing you use metric distances, so you'll use Ohms/km. Choose the wire with the minimum ampacity for your application, and find the corresponding Ohms/km from the tables.

Now it's just a matter of applying Ohm's Law, V=IR. In this case, V is the voltage drop over the length of the conductor, I is the current flowing through the conductor, and R is the resistance of the conductor. You'll need to calculate R using the Ohms/km from the tables in Chapter 9, and the length of your conductors. Remember that for single phase circuits the length of the conductor is twice the distance from source to load.

Once you have calculated the voltage drop across your conductor, you can calculate the percent voltage drop by dividing by your nominal voltage and then multiplying by 100%.

So as not to just do this for you, I'll provide an example using different numbers. Suppose the distance from source to load is 12 meters, the load is 56.1 amps connected to a 60 amp circuit breaker, and the voltage is 230 volts, single phase. Also assume we are using equipment with 75 degree terminations.

Using the 75 degree column in NEC Table 310.16, we can see that the smallest wire that can be protected by our 60 amp breaker is #6 copper. Going to NEC Chapter 9 Table 8, we see that uncoated #6 copper has a resistivity of 1.608 Ohms/km. We now determine the resistance of our wire by multiplying the resistivity by the length of the wire.

R = (1.608 Ohms/km) x (.012 km) x (2) = 0.038592 Ohms

Using this, we can calculate the voltage drop using V=IR.

I x R = (56.1 amps) x (0.038592 Ohms) = 2.165 volts. This is the amount of voltage dropped over the length of the wire. Now we convert it to a percentage of the nominal voltage:

( 2.165V / 230V ) x 100% = 0.94%

In this case, our voltage drop is an acceptably small percentage, so we can choose to use #6 AWG copper and we're done. If we had come up with a percent voltage drop that was unacceptably large, we would choose a larger wire size and start over.

Hopefully this has been helpful.

 

[kingpb]

Holy crap that is a lot of work, I just use the VD calc App from Southwire on my Iphone. LMAO :ashamed1::lol:

 

[JDBrown]

Holy crap that is a lot of work, I just use the VD calc App from Southwire on my Iphone. LMAO :ashamed1::lol:

Well, I don't actually do the calculation by hand. I wrote a simple little script to do it for me. I'd probably use something like Southwire's app, except for the fact that the Building Dept. told me they want the calculations done this certain way. I only outlined the whole process the way I did in hopes that it would help the OP understand the process. But maybe I went a bit overboard. :ashamed:

 

[darekelec]

That rule is a FPN thus not enforcable.Voltage Drop - A Closer Look!
John M. Birkby ~ July, 1999

National Electric Code Articles 210-19(a) FPN No. 4 and 215-2(d) FPN No. 2 state in part, "...and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, will provide reasonable efficiency of operation." Section 90-5, Mandatory Rules, Permissive Rules, and Explanatory Material is defined as follows:

(a) Mandatory Rules. Mandatory rules of this Code are those that identify actions that are specifically required or prohibited and are characterized by the use of the terms shall or shall not.
(b) Permissive Rules. Permissive rules of this Code are those that identify actions that are allowed but not required, are normally used to describe options or alternative methods, and are characterized by the use of the terms shall be permitted or shall not be required.
(c) Explanatory Material. Explanatory material, such as references to other standards, references to related sections of this Code, or information related to aCode rule, is included in this Code in the form of fine print notes (FPN). Fine print notes are informational only and are not enforceable as requirements of this Code.

 

[kwired]

Dear all,

How can i calculate a conductor size based on derated ampacity if i have distance from source ( 17M) , source voltage (110VAC), Current (34.7A), single phase. As per NEC Voltage drop should not exceed more than 3%.
Kindly give one example for reference.

When you say you have derated ampacity, I think of running larger conductors than usual because of ambient temperature, or number of conductors in raceway.

Voltage drop is something else, but is improved when you do have to derate your conductors.

Figure VD just like you would without a derated conductor, same load but different conductor size and it will give you a different VD if load or conductor size is changed.

And also NEC does not specify 3% or any specific amount of permissible voltage drop. The 3 and 5% figures mentioned are in unenforceable fine print notes or in 2011 they are called informational notes.