Mike Dolt Enterprises
Mike Dolt Enterprises
User: GFourteen, Location: Connecticut
Asked on: Monday, March 28, 2011 at 10:34 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:24 am EDT
Question: I have a #3 THW Cu, 3wire 120/240v single phase feeder 219' in length. If I wanted to find the Ampacity per phase I should use this formula right?I= VD*CM/D*KCM- cmil of 52,620D- 219'K- (approx) 25.8VD-(Voltage Drop) should I use 120v or 240v to get my 3% from 210.19(A)FPN:why use one over the other? If this is a stupid question I'm sorry, I just don't understand
Formula
Yes, you should use the formula
I= VDCM/DK to calculate the Ampacity per phase.
Voltage Drop (VD)
You should use
120V for the Voltage Drop (VD) to calculate the Ampacity per phase. This is because the National Electrical Code (NEC) specifies a maximum allowable voltage drop of 3% for branch circuits in Section 210.19(A)FPN.
Reasoning:
The Ampacity calculated using 120V represents the current that can flow through the feeder while maintaining a voltage drop of no more than 3% (120V * 0.03 = 3.6V). This ensures that the equipment connected to the feeder will receive a voltage within the specified operating range.If you were to use 240V for the Voltage Drop, the calculated Ampacity would be higher, potentially exceeding the actual Ampacity of the feeder. This could lead to overheating and potential safety hazards.By using 120V for the Voltage Drop, you are ensuring a conservative estimate of the feeder's Ampacity and maintaining compliance with NEC requirements. Flag for review