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Reader Picks a Fine Point and Makes Suggestions Re: Voltage Drop

Sep 1, 1998 12:00 PM, By EC&M Staff

I have three requests for discussion of various interpretations of the NEC and methods of calculating voltage drops. (See March issue Back To Basics, “Calculating Voltage Drop.”)

First, the article states the NEC, Sec.210-19(a) (FPN 4) and Sec. 215-2(b) (FPN 3) recommends 5% voltage drop for feeder circuits and 3% for branch circuits. This seems to imply a recommendation of 8% for the total length of feeder and branch together.

However, the NEC doesn’t recommend those percentages. It recommends sizing the circuitry “to prevent a voltage drop exceeding” such an amount. The recommendation is for 5% and 3% voltage drops where applicable. What interpretations do your readers have of these percentages?

Second, “feeder” is defined as conductors from service to final branch circuit overcurrent protective device (OCPD); “branch circuit” is defined as circuit conductors from final OCPD to the outlet. Sec. 210-19(a) (FPN4) and Sec. 215-2(b) (FPN 2) recommend “maximum total voltage drop on both feeders and branch circuits to the farthest outlet not exceed 5%.”

FPN’s further recommendation is each component (branch circuit in Art. 210 and feeder in Art. 215) be “sized to prevent a voltage drop of 3%” on the respective component. I try to stay around 2.5% max for each component. How do your readers interpret Sec. 210-19 and Sec. 215-2?

Furthermore, the author writes of using the “R” (resistance) values of NEC Table 9 to determine voltage drop, but the examples all use wiring in the range of No. 10 and No. 12 AWG sizes—a conservative approach up to the range of size No. 2 to No. 1/0, where the resistance value equals the 85% PF impedance values shown in Table 9. Moreover, the impedance values are higher than the resistance values, and therefore more conservative.

I use resistance values to calculate voltage drop percentages when wire sizes are No. 1 and smaller, and 85% impedance values when wire sizes are No. 1/0 and larger. What methods do your readers use for voltage drop calculations?

Ned Thun
Durham, NCH

What's the Liability to Customer Who Shorts Out Utility Feeder?

I am hoping a reader has information on the following.

A customer of an electric power company caused a short circuit in a 12.4kV feeder line approximately 1000 ft from a substation while a swing loader shorted the lines. According to the power company, the circuit breaker and associated protective devices activated per system design to limit the current to allowable design limits. However, the 20MVA 110/12.4kV transformer failed and required complete overhaul including rewinding.

Is the customer liable for repair of the transformer or just the cost of the repair of damage to the feeder and lost revenue? The 35-yr-old transformer was moved several times.

Jack E. Parker, P.E.
Slidell, LA

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