Voltage Drop Down a Feeder

9 min · difficulty 5/10

A customer at the far end of a long feeder meters 112 V at the service on a nominal 120 V base. The substation is putting out full voltage and every meter agrees. Is that reading legal, and where did the missing volts go?

It is not legal, and the feeder itself spent them. Lesson 2 sized the drop across a single loaded conductor with Ohm’s law. A feeder is that same physics repeated the whole way out. Every segment carries the current bound for everyone downstream of it, and every segment adds a little impedance, so the drop accumulates with distance. Voltage leaves the substation high and falls the entire length of the feeder, worst at the last customer. That is why the far end reads low while the source reads full.

Two different numbers govern how low it may go, and merging them is a common mistake. The legal one is ANSI C84.1 Range A, which sets the service-voltage window at 114 V to 126 V on a 120 V base. Separately, most utilities hold their feeder design to roughly 5% total drop end-to-end as an internal planning margin. Range A is the regulatory limit the utility must meet at the meter. The 5% figure is a design target, not itself the legal line. A feeder can sit inside a loose 5% rule of thumb and still put a customer below Range A.

Voltage drop down a feeder, below the Range A floor A voltage-versus-distance curve starting at 126 volts at the substation and sloping smoothly down to 112 volts at the last customer. A shaded green band marks the ANSI C84.1 Range A window from 114 to 126 volts. A dashed red line at 114 volts is the Range A floor. The curve ends with a red point at 112 volts, below that floor, while the substation point at 126 volts sits at the top of the band. A gold bracket on the right marks the 14 volt total drop. Voltage falls the entire length of a loaded feeder 126 V ceiling 114 V Range A floor 126 V Substation 112 V Last customer 128 106 Voltage (V) Distance along the feeder 14 V dropped, substation to last customer 112 V < 114 V Range A floor
Voltage falls the entire length of a loaded feeder, and the last customer's 112 V lands below the legal 114 V Range A floor.

Now the hook resolves. The 112 V reading is below Range A’s 114 V floor, so it is out of spec even though it might satisfy a looser percentage guess. For a buyer, an operator, or a planning engineer, the lesson is the same. Range A is the legal window you are accountable for, not the 5% design target, and the last customer on the feeder is where you find out whether you made it. The next lesson covers the tools that pull that far end back up: regulators, capacitor banks, and conservation voltage reduction.

SAIDI and SAIFI targets drive capital planning, and capital planning drives what gets ordered and when. The Feeder tracks the reliability investment and regulatory signals behind those orders, free, every week.

Question 1 of 3

A feeder leaves the substation at 126 V (120 V base) and the last customer reads 112 V. How many volts were dropped across the feeder from the substation to that meter?

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.