Quick Hits

🏫 Quick Hit: Schools Get Their Own Sliding Scale

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

Episode 35 — 120.86 Optional Method for Schools

🏫 ANOTHER OPTIONAL CALCULATION METHOD

Article 120 doesn't stop with dwelling units.

Section 120.86 provides an optional feeder/service load calculation method specifically for certain school buildings with electric space heating, air conditioning, or both.

Instead of using the standard Part III calculation, an eligible school can use Table 120.86.

And unlike some of the earlier optional methods, this one is built around:

📉 LOAD PER SQUARE FOOT

The more connected load there is per square foot, the smaller the demand factor applied to the additional load.

⚡ WHAT LOADS GO INTO THE CALCULATION?

The connected load used with Table 120.86 includes the applicable loads serving the school, including:

💡 Interior and exterior lighting

🔌 Power loads

🔥 Water heating

🍳 Cooking equipment

⚙️ Other applicable loads

🌡️ The larger of the air-conditioning or space-heating load

The idea is to determine the building's connected load first, then apply the applicable calculation method from Table 120.86.

📊 THE SLIDING SCALE

Table 120.86 divides the connected load into ranges based on VA per square foot.

🟢 0–3 VA/ft²

100%

The first portion is counted at its full value.

🟡 OVER 3–20 VA/ft²

75%

The portion within this range receives the 75% factor, with the applicable adjustment included in the calculation.

🔴 OVER 20 VA/ft²

25%

The portion beyond 20 VA/ft² receives the much lower 25% factor, again with the applicable adjustment included.

🧮 WHY THREE TIERS?

Think of the calculation like a staircase:

FIRST 3 VA/FT²

100%

⬇️

NEXT RANGE — 3 TO 20 VA/FT²

75%

⬇️

ABOVE 20 VA/FT²

25%

The calculation recognizes that a very large connected load per square foot doesn't necessarily mean the entire connected load will be operating simultaneously.

🧠 THE IMPORTANT PART

Don't read the table as:

“The entire building gets one percentage.”

Instead, the calculation is tiered.

Different portions of the connected load fall into different ranges.

That's similar to the way you've already seen other NEC demand-factor calculations work throughout Article 120.

⚪ WHAT ABOUT THE NEUTRAL?

The optional school calculation doesn't eliminate the neutral calculation requirements.

The neutral load can still be determined using:

120.61 — Feeder/Service Neutral Load

So remember:

Optional building calculation ≠ optional neutral calculation.

🔌 WHAT ABOUT INDIVIDUAL FEEDERS?

There's another important distinction.

The optional method determines the applicable building feeder/service calculated load.

Individual feeders within the building still have their own ampacity requirements under the applicable NEC rules.

In other words:

BUILDING SERVICE

⬇️

120.86 optional calculation

INDIVIDUAL FEEDERS

⬇️

Still subject to the applicable standard requirements.

The individual feeder isn't automatically sized from the building's optional calculation.

🚫 THE PORTABLE CLASSROOM EXCEPTION

Here's the easy exam trap.

Portable classroom buildings?

NO.

Section 120.86's optional method does not apply to portable classroom buildings.

So don't see:

🏫 School

🌡️ Electric heating/AC

and automatically assume:

✅ 120.86

You still have to check whether the installation falls within the section's scope.

🎯 REVIEW QUESTION

Per 120.86, does this optional calculation method apply to portable classroom buildings?

A) Yes, always

B) No — portable classrooms are excluded

C) Only if they have AC

D) Only if connected load exceeds 20 VA/ft²

👇 Comment your answer!

✅ ANSWER

B) No — portable classrooms are excluded

That's one of the straightforward scope limitations in 120.86.

🧠 MEMORY TRICK:

“School ≠ automatically 120.86.”

Check the scope first.

⚡ QUICK RECAP

120.86

WHO?

🏫 Applicable school buildings with electric heating, AC, or both.

WHAT?

An optional feeder/service load calculation method.

HOW?

📊 Table 120.86 uses a tiered VA-per-square-foot calculation.

THE TREND?

More connected VA/ft² → lower percentage on the additional load.

NEUTRAL?

⚪ Still determined under the applicable requirements, including 120.61.

PORTABLE CLASSROOM?

🚫 Excluded.

📚 WHERE THIS FITS IN ARTICLE 120

We've now worked through several optional calculation methods:

🏠 120.82 — Single dwelling unit

🏚️ 120.83 — Existing dwelling unit

🏢 120.84 — Multifamily dwelling units

🏘️ 120.85 — Two dwelling units

🏫 120.86 — Schools

And Article 120 isn't finished yet.

🔗 KEEP EXPLORING

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🧭 ARTICLE 120 NAVIGATION

← Episode 34 — Two-Unit Buildings

→ Episode 36 — 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: Existing Dwelling Units

Episode 32: Multifamily Buildings

Episode 33: Multifamily Demand Factor Table

Episode 34: Two-Unit Buildings

Episode 35: Schools ← 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.