Washington voltage drop practice questions

What voltage drop is asking you

A voltage drop question gives you a circuit, its voltage, the load in amps, the run length, and the conductor, and asks how many volts you lose over the run, or whether that loss is within the percentage the NEC recommends. The governing reference is the informational note to NEC 210.19, which recommends a branch circuit drop no greater than 3 percent, and no more than 5 percent total across the feeder and the branch circuit together.

• Single-phase: voltage drop equals 2 times K times I times L, divided by the conductor circular mils. K is about 12.9 for copper, I is the current in amps, and L is the one-way length in feet.
• Three-phase: same setup, but swap the 2 for 1.732. Everything else stays put.
• The recommended figure: 3 percent on the branch circuit, 5 percent total on feeder plus branch, from the informational note to NEC 210.19. It is a recommendation, not a hard requirement, and the exam tests whether you know the figure.

That 3 percent is the number most exam questions hinge on, because the trap is computing the volts correctly and then forgetting to check the percentage against what the NEC recommends. Commit the formula and the 3 percent figure to memory so the only thing you reach into the book for is the circular mils of the conductor.

Worked example

Take a 120 volt single-phase circuit carrying 16 amps over 100 feet of run on 12 AWG copper, which is 6,530 circular mils. Walk it the way you would on the exam, one step at a time.

1. Write the formula. Single-phase voltage drop equals 2 times K times I times L, divided by circular mils, with K about 12.9 for copper.
2. Plug in the numbers. That is 2 times 12.9 times 16 amps times 100 feet, divided by 6,530 circular mils.
3. Run the math. The top comes to 41,280, and dividing by 6,530 gives 6.3 volts of drop over the run.
4. Check the percentage. 6.3 volts out of 120 is 5.3 percent, which is past the 3 percent the NEC recommends in the informational note to 210.19.
5. Make the call. The drop is too high, so you upsize the wire. A larger conductor has more circular mils, which drops the result back under 3 percent.

The number you have to defend is the 6.3 volts, the 5.3 percent, and the article it comes from. Once the formula is automatic, the only value you reach into the book for is the circular mils of the conductor, and the rest is arithmetic you can run on the calculator you brought.

Why this topic costs points

Voltage drop looks open book, because the article and the conductor tables are sitting in front of you. That is exactly why candidates lose points on it. The formula is not in the book in a form you can just copy, so guys who never drilled the setup stall on it under the clock. Three things go wrong.

• The formula is not memorized. Voltage drop is not a lookup, it is a setup. If you cannot write 2 times K times I times L over circular mils from memory, you waste minutes rebuilding it, and the NEC and theory section is 60 questions in 3 hours.
• The single-phase and three-phase setups get crossed. Candidates use the 2 on a three-phase circuit, or the 1.732 on a single-phase one. Read whether the circuit is single-phase or three-phase before you pick the multiplier.
• They compute the volts and stop. The question wants to know whether the drop is within the recommended 3 percent, not just how many volts it is. Forgetting to divide by the source voltage and check the percentage is the difference between a right answer and a wrong one.

That is why drilling voltage drop is not about memorizing one answer. It is about making the setup, write the formula, pick the right multiplier, plug in, then check the percentage, automatic, so you spend almost no time in the book at all.

Practice it inside the Washington bank

Reading the rule is not the same as drilling it under time. The free 15-minute Washington diagnostic scores you against the NEC and theory section and the Washington Laws and Rules section separately, so you see whether calculation topics like voltage drop are actually what is costing you points, or whether your weak spot is somewhere else. From there you drill the section that is dragging your score instead of grinding everything evenly. We will not quote you a voltage drop question count we have not verified. We route you to the diagnostic and the practice platform so you can rehearse the formula and the percentage check the way the exam asks for them.