Apprentice path · Stage 3: The theory you cannot fake later · Lesson 13

Voltage drop: why the run length matters

Wire is not a free connection -- it has resistance, so some of your voltage is used up just getting to the load.

Why this matters on the job

A long run that tested fine at the panel can leave a motor or a heater running on less voltage than it needs, and the symptom looks like a broken load rather than a wiring problem.

How to think about it

  1. Remember the conductor itself has resistance, and more length means more of it.
  2. Multiply the current by the resistance of the loop to get the voltage dropped in the wire.
  3. For a two-wire DC circuit, count both conductors -- the one going out and the one coming back.
  4. Subtract that drop from the source voltage to get what the load actually receives.

Worked through

A 12 volt DC source, a 5 amp load, and a total wire-loop resistance of 0.20 ohms. The drop in the wire is current times loop resistance: 5 x 0.20 = 1 volt. So the load is not getting 12 volts. It is getting 12 - 1 = 11 volts. One volt has been spent on the wire.

Check yourself: Same circuit, but the run is made longer so the loop resistance becomes 0.40 ohms. What reaches the load now?

Answer: The drop is 5 x 0.40 = 2 volts, so the load receives 12 - 2 = 10 volts.

Voltage drop is current times the resistance of the loop. Double the loop resistance and, with the same current, you double the drop -- so twice the wire costs you twice the voltage.

Watch out for: People count the wire out to the load and forget the wire coming back. The current goes all the way round, so both conductors are in the loop. Count one and your answer is half of the truth.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.