Quick Hits

🏘️ Quick Hit: Two-Unit Buildings Get a Sneaky Comparison

NEC Article 120 — “Load Calcs Won't Kill You” 🔌⚡

Episode 34 — 120.85 Two Dwelling Units

🏠 WHAT HAPPENS WITH A DUPLEX?

You've already seen the optional calculation method for 3 or more dwelling units in 120.84.

But what about a building with exactly two dwelling units sharing a feeder or service?

That's where 120.85 comes in.

And the rule is surprisingly simple:

🧮 COMPARE TWO CALCULATIONS

For the two dwelling units, you can compare:

① STANDARD METHOD

The calculated load determined using the Part III standard calculation method.

VS.

② THREE-UNIT EQUIVALENT

The calculated load determined by treating the installation as though it were three identical dwelling units using the applicable 120.84 method.

📉 USE THE LOWER NUMBER

Here's the important part:

If the standard Part III calculation is higher than the 120.84 calculation based on three identical dwelling units, you're permitted to use the lower calculated load.

Think of it as a comparison:

Part III standard calculation

⬇️

COMPARE

⬆️

120.84 three-unit calculation

Lowest permitted value wins.

🧠 WHY DOES THIS MATTER?

A duplex can sometimes produce a higher calculated load using the standard method than you'd expect when compared with the optional multifamily calculation.

Section 120.85 provides a way to make that comparison rather than automatically accepting the higher number.

It's essentially a built-in two-unit comparison method.

🏘️ SIMPLE EXAMPLE

Imagine a two-unit building sharing one feeder.

STANDARD PART III CALCULATION

100 kVA

120.84 THREE-UNIT-EQUIVALENT CALCULATION

85 kVA

Because the 120.84-based calculation is lower:

85 kVA

is the permitted calculated load under the 120.85 comparison.

🎯 THE EXAM TRAP

Don't confuse this with:

❌ Always using Part III

❌ Always using the 120.84 calculation

❌ Averaging the two calculations

The key is:

COMPARE THEM.

And when the 120.85 condition applies, the lesser calculated load is permitted.

⚡ QUICK MEMORY TRICK

2 UNITS?

Think:

“Compare it to 3.”

Two-unit building → compare the standard calculation against the applicable three-unit 120.84 calculation.

Lower number?

That's the one you're looking for.

🎯 REVIEW QUESTION

Per 120.85, if the Part III standard calculation for a duplex is HIGHER than the applicable 120.84 three-unit-equivalent calculation, which number is permitted to be used?

A) Always the Part III number

B) The lesser of the two

C) Always the 120.84 number

D) The average of both

👇 Comment your answer!

✅ ANSWER

B) The lesser of the two

The whole point of the 120.85 comparison is to permit the lower calculated load when the applicable conditions are met.

🔗 KEEP LEARNING

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🧭 ARTICLE 120 NAVIGATION

← Episode 33 — The Multifamily Demand Factor Table

→ Episode 35 — Next Article 120 Quick Hit

← Back to Article 120 Quick Hits

📚 ARTICLE 120 — PART IV

Episode 25: Welcome to Part IV — Optional Calculations

Episode 26: Who Even Gets to Use This Shortcut?

Episode 27: Bucket #1 — General Loads

Episode 28: Bucket #2 — Heating & AC (Part 1)

Episode 29: Bucket #2 — Heating & AC (Part 2)

Episode 30: Bucket #3 — EVSE + Putting It All Together

Episode 31: Adding Load to an Old House

Episode 32: Multifamily Buildings

Episode 33: Multifamily Demand Factor Table

Episode 34: Two-Unit Buildings ← YOU ARE HERE

⚠️ EDUCATIONAL NOTICE

This Quick Hit is an educational explanation based on the author's understanding of the NEC. It is not a substitute for the official NEC, applicable local amendments, manufacturer instructions, or the determination of the Authority Having Jurisdiction (AHJ).

Always verify the requirements applicable to your specific installation.

The National Electrical Code® and NEC® are registered trademarks of NFPA. JoshTheSparky.com is not affiliated with, sponsored by, or endorsed by NFPA.