VD calculation in question

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brian ploskunak

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Need some help verifying my calculations. I have a 120/240 1 phase service that is 236' from transformer. The calculated load is 382 amps. Using 2KID/VD, I am coming up with only using 350 MCM? (copper wire) What is wrong with my calculations?

2 x 12 x 382 x 236 / 7.2 = 300,506.
 
Thanks for the link, Trevor! Having trouble with the VD wire requirement being less than the ampacity requirement which in this case would be 500 MCM.
 
Thanks for the link, Trevor! Having trouble with the VD wire requirement being less than the ampacity requirement which in this case would be 500 MCM.
Click to expand...

It does seem rather odd. Here is another calculator that produced the same results. Note this quote from the calculator:

This Calculator will determine the minimum size conductor needed based on a 3% Voltage Drop. It does not take into consideration other factors such as minimum ampacity based on the National Electric Code Table 310-16. ( Allowable ampacity of conductors ). Consult The Electrical Code for further details.
Click to expand...

http://www.elec-toolbox.com/calculators/voltdrop.htm
 
Brian,
Having trouble with the VD wire requirement being less than the ampacity requirement which in this case would be 500 MCM.
Click to expand...
Actually with a calculated load of 382 amps, the 500 kcmil is too small. You need to redo the calc and drop 2 amps or use 600 kcmil.
Don
 
Voltage Drop and current carrying capability are related, but two different animals when it comes to sizing conductors. You have to look at both, especially long runs. Typically, anything under 200 ft usually isn't an issue, but as they approach this distance, and certainly longer distances a quick check is prudent.

The long and short of it, the amount of copper required to carry the current is based on surface area of the copper (kcmil). As the current flows, heat is generated due to the resistance in the copper. The maximum continuous currrent the cable can take without exceeding the temperature limits is what is supplied in Table 310.16, and others. The heat is also why conductor amapcity ratings must be decreased when multiple conductors are in the same conduit. (heat from one set is superimposed on the other)

The voltage drop is based on the resistance of the copper and the maximum amount of current that will be flowing through it. I x R_cable = VD of cable.
This voltage drop is subtracted from the starting voltage to get final (load) voltage.

So, although the cable may meet the voltage drop requirements, it may not have enough surface area to adequatley carry the current.

Rule of thumb: Size conductors based on current, check voltage drop, and UPSIZE if required. NEVER downsize.
 
I was not looking to downsize the wire to the voltage calculations. I was concerned that the VD calculation comes out to much less than the ampacity requirement. Typically, VD calculations require you to increase the wire size on long runs but from what you are telling me, that is not always the case.
 
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