NEC Section 90.9 - Units of Measurement: Real Talk

Welcome to the Wild World of Metric (and the Units We Actually Use)

Look, I know you opened the Code looking for real help, not a boring essay on millimeters versus inches. But stick with me for a second—this section matters more than you think, especially when you're trying to figure out if you can legally jam that conductor through a conduit.

Here's the deal: The NEC officially loves the metric system. It's all about those SI units (Système International—fancy French for "International System"). Think millimeters, meters, amperes, and all that jazz. The Code wants to be modern and international. Good for them.

But here's what actually happens on American jobsites: Nobody's walking around saying, "Hey, hand me that 12.7-millimeter drill bit!" Nope. You're asking for a half-inch bit because that's what's stamped on every tool in your truck. The NEC knows this, which is why they give you both measurements.

The Setup: Metric First, American Second (In Parentheses)

Everywhere in the Code, you'll see measurements written like this:

600 mm (24 in.)
1.8 m (6 ft)
13 mm (½ in.) trade size conduit

The metric number comes first (because the Code writers are trying to be proper), and then immediately after in parentheses, you get the good old inch-pound units we've been using since Benjamin Franklin was shocking himself with kites.

The key point? Article 90.9(D) says you can use EITHER measurement system and still be Code-compliant. If the inspector checks your work with a tape measure in inches, you're good. If some fancy engineer shows up with a meter stick, you're still good. They're both legal.

Hard Conversion vs. Soft Conversion (Yeah, It Matters)

This is where it gets slightly technical, but I'll keep it simple:

Hard Conversion: This is when they actually redesigned the product to metric sizes. For example, a 25mm raceway is NOT the same as a 1-inch raceway—25mm equals 0.984 inches. Close, but not identical. These don't perfectly match up.

Soft Conversion: This is just math—converting the same physical thing into different units. A 6-foot grounding rod is exactly 1.83 meters. Same rod, different measuring system.

Exception Time: When you're looking at tables in Annex C (that's the back of the book where they list conduit fill and wire sizes), they're allowed to put the trade sizes first instead of metric. Why? Because electricians think in "half-inch EMT" or "2-inch rigid," not "12 metric EMT." The Code writers had a rare moment of mercy here.

Real-World Jobsite Scenarios

Scenario 1: The Inspector With a Chip on His Shoulder

You're installing EMT above a suspended ceiling. The Code says it needs to be at least 2.1 m (7 ft) above the floor. You measure 6 feet, 11 inches because you're eyeballing it and figure "close enough."

Inspector shows up: "That's only 2.08 meters! Not compliant!"

You: "Buddy, 2.1 meters is 82.68 inches. I'm at 83 inches. We're good."

The inspector realizes he messed up the conversion and moves on.

Lesson: Always double-check conversions. Inspectors are human (barely, some days), and they make mistakes with metric just like everyone else. Keep a conversion app on your phone.

Scenario 2: The Engineer's Metric Madness

You get a set of electrical plans from an engineer who studied in Europe. Everything's in metric—cable tray heights, conduit sizes, box locations. Your crew is standing around confused because nobody knows what "32mm rigid" looks like in real life.

Solution: Article 90.9 says both systems are valid. Convert everything on the plans to inches BEFORE you start work. Write it on the plans in red marker if you have to. Yes, 32mm rigid is basically 1¼-inch (trade size). Close enough for installation purposes, and fully Code-compliant.

Lesson: Don't let an engineer's unit preference slow down your job. Convert, confirm, and install with confidence.

Scenario 3: Trade Sizes Aren't Real Sizes (And That's Fine)

Here's something that messes with apprentices all the time: "trade size" is a made-up designation. A ½-inch trade size EMT actually has an INSIDE diameter of about 0.622 inches. It's not a real half-inch.

The Code addresses this in 90.9(C)(1)—when the actual measured size doesn't match the name, we just use the trade size. Nobody's asking you to measure the inside of every stick of conduit. Just call it what it's called at the supply house and move on.

Lesson: Trade sizes are nominal (fake). Use them anyway. Everybody knows what you mean.

Scenario 4: Imported Equipment From Overseas

You're installing a German-made industrial machine. The nameplate lists everything in metric—voltage (230V), wire sizes (2.5 mm²), torque specs (10 Nm). Your apprentice is having a meltdown.

Reality check: European wire sizes (in mm²) aren't the same as AWG. A 2.5 mm² wire is roughly equivalent to 14 AWG, but not exactly. According to 90.9(C)(3), when industry practice uses one system, you're allowed to stick with it. If the equipment is listed and approved, install it per the manufacturer's instructions—even if those instructions are in metric.

Lesson: Don't try to force American standards onto foreign equipment. Work with what you've got, and make sure it's listed/approved for use here.

Key Takeaways (The Stuff You'll Actually Forget If I Don't Repeat It)

Metric is "official," but inch-pound is still fully legal. Use whichever system makes sense for your work. Both are Code-compliant (90.9(D)).
Conversions don't have to be exact. "Approximate conversion" is acceptable. If you're within a reasonable margin, you're fine. Don't stress about being perfect to three decimal places.
Trade sizes are fake. They're designations, not measurements. A ½-inch EMT isn't actually half an inch inside. Stop trying to make sense of it.
Tables in Annex C can list trade sizes first. This is the Code admitting that electricians think in fractions, not millimeters.
When the Code says "soft conversion is permitted," it means you don't have to redesign anything. It's just math. Relax.
Industry practice wins. If manufacturers, listing agencies, and standard practice all use one measurement system for specific equipment, the Code allows it. Don't fight it.
Both systems are on the test. Get comfortable with conversions. The test writers LOVE to throw metric questions at you just to see if you panic.

What to Study for the Exam

Alright, let's get serious for a second. If you're prepping for your journeyman or master's exam, here's what they're gonna hit you with:

1. Conversion Between Systems

You'll get questions like:

"A raceway must be installed at least 900 mm above the floor. What is this measurement in feet?"
Answer: 900 mm = 0.9 m ≈ 3 feet (close enough).

Study Tip: Memorize these quick conversions:

1 inch = 25.4 mm
1 foot = 304.8 mm (or roughly 305 mm, close enough)
1 meter = 39.37 inches (or roughly 3.28 feet)

They're NOT going to let you use a calculator with a conversion app. Know the basics cold.

2. Trade Size Questions

Expect something like:

"What is the metric designation for ¾-inch trade size EMT?"
Answer: 21 (metric designator), even though it's not actually 21mm inside. It's just the name.

Study Tip: Learn the metric designators for common trade sizes:

½ in. = 16
¾ in. = 21
1 in. = 27
1¼ in. = 35
1½ in. = 41
2 in. = 53

These show up all the time in conduit fill tables.

3. Code References to Units

They'll ask you to find a specific measurement in the Code and convert it.

Example:

"Per 334.15(B), NM cable must be protected by bushings when passing through metal framing. What's the minimum radius for bends?"
You look it up, find the measurement, and convert if needed.

Study Tip: Practice flipping to different Code sections quickly and identifying measurements. Speed matters on the exam.

4. Trick Questions About "Soft" vs. "Hard" Conversion

They love to ask:

"Are trade sizes an example of hard conversion or soft conversion?"
Answer: Neither—they're designated sizes (90.9(C)(1)).

Study Tip: Read 90.9(C) carefully. Understand when conversions DON'T apply (like with trade sizes, extracted material, and industry practice).

5. Compliance Questions

They'll hit you with:

"An installation is measured in millimeters. Is it Code-compliant?"
Answer: Yes (90.9(D)). Compliance with either system = Code compliance.

Study Tip: Don't overthink this one. Both systems are legal, period.

The Bottom Line

The NEC's love affair with the metric system doesn't mean you need to toss your tape measure and relearn everything in millimeters. It just means the Code is trying to play nice with the rest of the world.

For you? Keep using the units you know. Convert when you have to. And when an inspector or engineer tries to trip you up with metric measurements, calmly pull out your phone (or your Code book) and show them that both systems are equally valid.

Remember: You're not here to win a Nobel Prize in physics. You're here to wire buildings safely, pass inspections, and go home on time. The NEC's unit system is just another tool in your belt—use whichever end of the wrench fits best.

Now go forth and measure things. In whatever units make you happy.

Next time someone gives you grief about metric vs. inches, just smile and say: "Per 90.9(D), compliance with either measurement system constitutes Code compliance. Now let me get back to work."

Mic drop. 🎤