Quick Hits

📊 Quick Hit: Using REAL Data Instead of Math

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

Episode 36 — 120.87 Calculations Based on Existing Loads

📈 WHAT IF YOU MEASURED THE BUILDING INSTEAD?

Most load calculations are based on calculated or theoretical loads.

But what if an existing building is already operating?

Instead of trying to predict the building's maximum demand from individual loads, 120.87 provides a method that can use actual recorded maximum-demand data for certain existing installations.

That's a pretty powerful concept:

REAL BUILDING DATA

instead of

THEORETICAL LOAD CALCULATION

But there are important conditions.

🏢 THIS IS FOR EXISTING INSTALLATIONS

Section 120.87 applies to existing installations where actual maximum demand can be established from recorded data.

This isn't a shortcut you automatically get to use on a brand-new installation.

The section establishes requirements that must ALL be satisfied before the recorded-demand method can be used.

📊 CONDITION 1 — HAVE REAL DEMAND DATA

Ideally, the installation has a history covering:

ONE FULL YEAR

of recorded maximum demand.

That gives the calculation a chance to capture seasonal variations.

Think:

☀️ Summer cooling

❄️ Winter heating

🏢 Normal occupancy

⚙️ Seasonal equipment

The goal is to capture the building's actual operating demand rather than looking at only one convenient period.

⏱️ THE 30-DAY EXCEPTION

What if you don't have a full year's worth of data?

The section provides an exception allowing a minimum 30-day recording period under the applicable conditions.

The recording uses the:

Highest 15-minute average demand

on the highest-loaded phase.

But the building has to be operating under conditions that properly represent its actual use.

The recording period must account for applicable heating/cooling and seasonal loads.

🚨 THE 30-DAY METHOD HAS A BIG LIMITATION

The 30-day exception isn't available in every situation.

If the installation has:

☀️ Solar generation

🌬️ Wind generation

or

📉 Peak load shaving

the 30-day exception is not permitted under the applicable exception.

That's an important distinction.

Don't see “30 days” and assume:

“I can just put a meter on it for a month.”

The conditions matter.

🧮 CONDITION 2 — THE 125% TEST

Once the applicable recorded maximum demand has been established, the calculation doesn't simply say:

“That's what the building used, so that's the new load.”

There's a safety margin.

The recorded maximum demand is multiplied by:

125%

Then you account for:

➕ New load being added

➖ Load being removed

The resulting value must remain within the applicable:

Feeder ampacity

or

Service rating

🔌 CONDITION 3 — PROPER PROTECTION

The installation also needs the appropriate overcurrent/overload protection.

FEEDER

Applicable overcurrent protection under 240.4

SERVICE

Applicable service overload protection under 230.90

The recorded-demand method isn't intended to bypass the normal protection requirements.

🧠 PUT THE THREE CONDITIONS TOGETHER

Think of 120.87 as a three-part gate:

① REAL DATA

You have qualifying recorded maximum demand.

⬇️

② 125% + CHANGES

The calculation remains within the applicable feeder/service rating after accounting for new and removed loads.

⬇️

③ PROTECTION

The feeder/service has the required protection.

ALL THREE HAVE TO WORK.

It's not a pick-and-choose method.

🏗️ WHY THIS MATTERS IN THE FIELD

This section can be especially interesting when dealing with an existing building and a proposed additional load.

Instead of looking only at what every individual piece of equipment could consume, the electrician may have actual operating data showing what the installation has historically demanded.

That can provide a much more realistic picture of the existing installation.

But the code's conditions still control.

📊 SIMPLE CONCEPTUAL EXAMPLE

Imagine an existing building has qualifying recorded maximum demand.

You determine the applicable recorded demand.

Then:

RECORDED DEMAND

⬇️

× 125%

⬇️

+ NEW LOAD

⬇️

− REMOVED LOAD

⬇️

COMPARE WITH FEEDER/SERVICE CAPACITY

If the resulting calculation exceeds the applicable rating, the recorded-demand method doesn't magically make the equipment capable of carrying the additional load.

⚠️ THE EXAM GOTCHA

The biggest mistake here is remembering only:

“120.87 lets you use actual demand.”

That's incomplete.

The better memory is:

REAL DATA + 125% + PROPER PROTECTION

And remember the special limitation on the 30-day exception.

🎯 REVIEW QUESTION

Per 120.87(1) Exception, the 30-day recording method is NOT permitted if the building has which of the following?

A) A newly installed water heater

B) A renewable energy system like solar or wind

C) More than 10 dwelling units

D) Central air conditioning

👇 Comment your answer!

✅ ANSWER

B) A renewable energy system like solar or wind

The 30-day recording exception has specific limitations, including installations with renewable energy generation and peak load-shaving systems.

🧠 QUICK MEMORY CARD

120.87

EXISTING INSTALLATION

📊 Use qualifying actual maximum-demand data.

PREFERRED DATA

📅 One year of recorded demand.

EXCEPTION

⏱️ Minimum 30-day recording under the applicable conditions.

SAFETY FACTOR

125%

THEN ACCOUNT FOR

➕ New loads

➖ Removed loads

ALSO CHECK

🔌 Feeder ampacity

⚡ Service rating

🛡️ Required protection

BIG 30-DAY GOTCHA

☀️ Renewable generation / 🌬️ wind / 📉 peak load shaving can eliminate the 30-day exception.

🏗️ WHERE 120.87 FITS

We've now moved from optional calculated methods into an optional method based on actual operating data.

120.82

🏠 Single dwelling unit

120.83

🏚️ Existing dwelling unit

120.84

🏢 Multifamily dwelling units

120.85

🏘️ Two dwelling units

120.86

🏫 Schools

120.87

📊 Existing installations using recorded demand

🔗 KEEP LEARNING

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🧭 ARTICLE 120 NAVIGATION

← Episode 35 — Schools Get Their Own Sliding Scale

→ Episode 37 — 120.88 New Restaurants

← 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

Episode 36: Existing Installations Using Recorded Demand ← YOU ARE HERE

Episode 37: New Restaurants

⚠️ 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.