How to Calculate Voltage Drop: The 2KID/CM Formula Explained

Why Voltage Drop Shows Up on Every Electrical Exam

Voltage drop is one of those calculations that comes up again and again, both on the job and on the exam. If a conductor run is long enough, the voltage at the load ends up noticeably lower than the voltage at the source. Motors run hot, lights dim and equipment stops working properly. Exam writers love testing it because it checks whether you can handle a formula and look up a table in the codebook.

The Electrical Code Coach covers the topic in Lesson 5.4 of their video series. They use one simple, easy-to-remember formula, and they walk through it step by step on a real exam-style question.

The Formula: 2KID Over CM

Search online for a voltage drop formula and you’ll probably find this one:

VD = 2KDI ÷ CM

That’s a perfectly good formula. The Code Coach just shuffles the letters so it’s easier to remember:

“I’ve rearranged it so it’s easier to remember. I do it like this: 2KID over CM. Easy to remember and easy to plug in.”

It’s all implied multiplication: 2 × K × I × D ÷ CM. The letters stand for:

• 2: the constant multiplier for single-phase circuits. For three-phase, use 1.732 instead
• K: the K factor, which you can think of as a multiplier for your wire material
  • Copper = 12.9
  • Aluminium = 21.2
• I: the current in amperes
• D: the distance in feet
• CM: the area of the conductor in circular mils

You Only Need the One-Way Distance

This formula takes the distance once. Some other formulas make you double the length to account for the out-and-back path, but here the 2 multiplier already does that for single-phase circuits. Whatever distance the question gives you, plug it in as it is.

“You don’t have to put it both ways. It’s whatever distance the question says. You can plug it in one time. That’s why I love this formula.”

That’s one less place to make a mistake when you’re under time pressure.

Memorising the K Factors

You won’t find the K factors in the NEC, so you need a plan for having them on exam day. The Code Coach suggests three options, depending on what your state allows:

1. Bring Ugly’s Electrical References into the exam, if it’s permitted. The K factors are listed in it
2. Write them in your codebook, if your state allows notes in the book
3. Write them on your scratch paper as soon as the exam starts. This is what the Code Coach did

The third option works anywhere. Before you read a single question, write “Cu = 12.9, Al = 21.2” and “1φ = 2, 3φ = 1.732” at the top of your scratch paper.

Finding Circular Mils in Chapter 9, Table 8

The circular mils value comes straight from the codebook. Go to NEC Chapter 9, Table 8 (Conductor Properties), which sits towards the back of the book with the other Chapter 9 tables.

There’s a lot going on in Table 8, but as with any NEC table, you only need the part your question is about:

• Left-hand column: conductor size. You’ll see the usual sizes (14, 12, 10 AWG), then the larger AWG sizes up to 4/0, then the kcmil sizes (250, 300, 350 and up)
• Area: Circular Mils column: this is the value you need. Make sure you’re reading from the Area section

Don’t Worry About Solid vs Stranded

For the smaller sizes, such as 14, 12 and 10 AWG, Table 8 lists two rows: one for solid and one for stranded. Look across and you’ll see that the circular mils value is the same for both. The stranding doesn’t matter for this calculation. Just read the circular mils.

The kcmil Shortcut

When you get past 4/0, the circular mils column has no entries. That’s because for 250 kcmil and larger, the size already is the circular mils value, given in thousands:

• 250 kcmil = 250,000 circular mils
• 300 kcmil = 300,000 circular mils
• 350 kcmil = 350,000 circular mils

Add three zeros and you’re done.

Worked Example: 200 A Circuit on 4/0 Copper

Here’s the exam-style question from the lesson:

What is the voltage drop of a 200 A circuit using 4/0 copper conductors that is 225 ft long on a 120/240 V system?

Step 1: Write the Formula Down

The Code Coach recommends physically writing the formula on your scratch paper. Then draw a long division line so you have space for the inputs on top and the circular mils underneath. Working one piece at a time keeps the numbers from getting mixed up.

Step 2: Fill In the Top Line

Write an ”×” between each value as you go:

1. Single-phase or three-phase? A 120/240 V system is single-phase, so the multiplier is 2
2. K factor: the conductors are copper, so K = 12.9
3. Current (I): the circuit is 200 A
4. Distance (D): the run is 225 ft

So the top line reads: 2 × 12.9 × 200 × 225

Step 3: Look Up the Circular Mils

Go to Chapter 9, Table 8, run down the left-hand column to 4/0, then across to the circular mils column. The value is 211,600.

“Any time it’s a long number like that, I don’t try to figure out my thousands or tens. I just say 2-1-1-6-0-0 and I write that down.”

That’s a useful habit. Read long numbers digit by digit so you don’t drop or add a zero.

Step 4: Run the Numbers

On your calculator:

• 2 × 12.9 = 25.8
• 25.8 × 200 = 5,160
• 5,160 × 225 = 1,161,000
• 1,161,000 ÷ 211,600 = 5.48

Round it to 5.5 volts of voltage drop and pick the matching answer.

Putting the Result in Context

The question only asked for the voltage drop in volts. In the field, though, you’ll usually want it as a percentage too. On a 240 V circuit, 5.5 V works out to roughly 2.3%.

The NEC’s informational notes in Article 210 (branch circuits) and Article 215 (feeders) recommend keeping voltage drop to 3% for a branch circuit or feeder, and 5% overall for feeder and branch circuit combined. These are recommendations, not enforceable requirements. Still, exam questions sometimes ask whether a circuit is within the recommended limits, so it’s worth knowing where those notes are.

Common Mistakes to Avoid

• Using 2 on a three-phase question. Read the system type carefully. If it’s three-phase, use 1.732
• Mixing up copper and aluminium K factors. Check the conductor material in the question before you write down K
• Doubling the distance. With this formula, plug in the one-way length only
• Reading the wrong column in Table 8. Make sure you’re in the Area: Circular Mils column, not the diameter or resistance columns
• Forgetting the three zeros on kcmil conductors

The Bottom Line

Voltage drop calculations look intimidating, but they follow the same pattern every time:

1. Write down 2KID ÷ CM (or 1.732KID ÷ CM for three-phase)
2. Fill in the K factor, current and one-way distance
3. Look up the circular mils in Chapter 9, Table 8
4. Multiply across the top, divide by the circular mils, and round

Practise this enough and it becomes one of the easiest points you’ll pick up on the exam.

How NEC Mastery Fits Into This

The Code Coach’s method works because it’s repeatable: same formula, same table, same steps. The way to make it automatic is to work through lots of problems until you no longer have to think about it. NEC Mastery is built for that kind of practice.

• 8,000+ exam-style questions, including voltage drop and other calculation problems, so you can practise single-phase and three-phase, copper and aluminium, and AWG and kcmil sizes until the formula is second nature
• Detailed explanations referencing specific NEC articles and tables, so every time you review an answer you’re reinforcing where Chapter 9, Table 8 lives and how to read it
• Timed mock exams weighted to your exam type, so you can see how fast you are at calculations and build the confidence to work through them under real exam pressure
• A companion to your codebook: NEC Mastery pairs with your NEC so you learn to look things up quickly, not just memorise answers