A 240-volt single-phase source supplies a load that draws 30 amperes. The total resistance of the complete conductor path, out and back, measures 0.25 ohm. What voltage appears at the load terminals?
- 232.5 volts
- B240 volts
The source voltage appears at the load only when no current flows. Under load the conductors always take their share, which is why an unloaded measurement can look fine while the equipment still misbehaves.
- C247.5 volts
Adding the drop instead of subtracting it puts more voltage at the load than the source produces. Conductor resistance can only consume voltage.
- D225 volts
This doubles the 0.25 ohm as though it were a one-way figure. The problem already gives the total for both conductors, so applying the factor of two again overstates the drop.
Why A is correct
Thirty amperes through a quarter ohm of conductor resistance drops 7.5 volts, and that drop is subtracted from the 240-volt source to give 232.5 volts at the load. Because the problem states the resistance for the complete out-and-back path, the drop is calculated directly without doubling anything.
What this question is testing
Tests whether you can apply Ohm's law to conductor resistance as a series element, whether you subtract the drop from the source rather than adding it, and whether you read carefully enough to notice that the given resistance already covers both conductors.
On the job
This is what a voltmeter is really telling you when you meter at the panel and again at the equipment while the load is running. The difference is the conductor drop, and it appears only under load, which is why measuring a dead circuit proves nothing about voltage drop. The same arithmetic in reverse, measuring the drop and dividing by the current, gives you the resistance of a run and can locate a loose connection or a damaged conductor.
Memory technique
The wire takes its cut first, and the load gets what is left.
Exam tip
Read whether the resistance given is one way or for both conductors. Doubling something that is already doubled is the usual trap.
Where to look it up
Chapter 9, Table 8 gives conductor resistance per thousand feet, which is where a real total path resistance comes from. Voltage drop targets themselves are informational notes.