Quick Hits

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

🏁 Episode 49 — 120.141 Allowable Demand Factors — SERIES FINALE

Article: 120 — Load Calculations
Part: Part IX — Mobile and Manufactured Homes
Section: 120.141
Format: Quick Hit

🎯 What You'll Learn

We've spent the last several episodes working through the load calculations for individual mobile and manufactured homes.

Now we take the final step:

What happens when multiple mobile homes are supplied by one park wiring system?

Section 120.141 provides the demand-factor method for calculating the load of a mobile home park.

And this is it — the TRUE SERIES FINALE of Article 120.

📖 The Quick Hit

🏘️ 120.141 — Allowable Demand Factors

When calculating the load for the park wiring system, the calculation is based on the larger of two values.

1️⃣ 16,000 VA Per Mobile Home Lot

The first option is:

➡️ 16,000 VA per mobile home lot

This establishes a baseline load for each mobile home lot.

2️⃣ Actual 120.140 Load

The second option is the actual load calculated under 120.140 for the largest typical mobile home that the lot will accept.

So the park calculation uses whichever is larger:

16,000 VA

OR

The actual 120.140 load for the largest typical mobile home

➡️ Always use the larger value.

This gives the park wiring system a calculation based on the homes the park is realistically designed to accommodate.

📊 Table 120.141 — The Demand-Factor Curve

Once the per-home load basis has been established, Table 120.141 provides demand factors based on the number of mobile homes.

The basic concept is diversity:

The more mobile homes connected to the park wiring system, the less likely it is that every home will simultaneously operate at its maximum calculated load.

🏠 1 Mobile Home

➡️ 100%

With only one home, there is no demand-factor reduction.

🏠🏠 2 Mobile Homes

➡️ 55%

The demand factor drops significantly once a second home is included.

🏘️ 5 Mobile Homes

➡️ 33%

At five homes, the applicable demand factor is 33%.

🏘️ 10–12 Mobile Homes

➡️ 27%

The demand factor continues to decrease as the number of homes increases.

🏘️ 22–40 Mobile Homes

➡️ 24%

The curve begins to flatten as the park gets larger.

🏘️ 41–60 Mobile Homes

➡️ 23%

The demand factor decreases slightly further.

🏘️ 61 and Over

➡️ Demand factor not available in the source material used for this Quick Hit

⚠️ The supplied source text does not include the percentage for the "61 and over" row.

Rather than filling in a missing NEC value from memory, the percentage is intentionally left unidentified until the complete table can be verified.

📉 Understanding the Curve

Table 120.141 follows the same basic diversity concept we've seen throughout Article 120.

A single mobile home requires the entire calculated load to be considered.

As more homes are added, the probability that every home will simultaneously operate at its maximum calculated demand decreases.

That's why the demand factor drops.

Think of the progression:

1 home → 100%

⬇️

2 homes → 55%

⬇️

5 homes → 33%

⬇️

10–12 homes → 27%

⬇️

22–40 homes → 24%

⬇️

41–60 homes → 23%

⬇️

61+ homes → [source value unavailable]

The interesting part is how quickly the percentage drops early on, then begins to flatten as the park gets larger.

⚠️ The Fine Print

There is an important limitation associated with this demand-factor method.

No demand factor is allowed for any other load in the park beyond what is specifically provided for in the Code.

In other words:

This table is not a general-purpose demand factor for every load at a mobile home park.

The demand factors in Table 120.141 apply to the mobile-home load covered by this section.

Other park loads must be calculated using their applicable NEC requirements.

🧠 Exam Takeaway

When you see 120.141, remember:

🏘️ Mobile home park

⚡ Park wiring system

📐 120/240V

➡️ Start with the larger of:

16,000 VA per mobile home lot

OR

The actual 120.140 load for the largest typical mobile home the lot will accept

Then:

📊 Apply the appropriate Table 120.141 demand factor based on the number of mobile homes.

And remember:

⚠️ Don't apply the mobile-home demand factor to other park loads unless the Code specifically permits it.

🎯 Test Yourself

Per Table 120.141, what demand factor applies to a mobile home park with exactly 3 mobile homes?

A) 55%
B) 44%
C) 39%
D) 33%

🔎 Think About It

Table 120.141 provides demand factors based on the number of mobile homes.

You need to identify the row that applies to 3 mobile homes, rather than simply choosing the nearest listed data point.

✅ Answer

The exact 3-home demand factor should be verified against the complete Table 120.141 source before publishing.

⚠️ The supplied table data in the source material provided for this series includes:

• 1 home → 100%

• 2 homes → 55%

• 5 homes → 33%

• 10–12 homes → 27%

• 22–40 homes → 24%

• 41–60 homes → 23%

• 61 and over → percentage unavailable

The intermediate 3-home value is not included in the supplied source data, so it should not be guessed or substituted.

📚 Keep Exploring

Continue the Article 120 Quick Hits:

🌐 www.JoshTheSparky.com/quick-hits

Test yourself with Brain Busters:

🧠 www.JoshTheSparky.com/brain-busters

Answer the Question of the Week:

❓ www.JoshTheSparky.com/question-of-the-week

Read Sparky's Journal:

📓 www.JoshTheSparky.com/sparkys-journal

Catch the latest Sparky Weekly:

📰 www.JoshTheSparky.com/sparky-weekly

View Sources & References:

📚 www.JoshTheSparky.com/Sources

🔎 NEC References

Primary: Article 120, 120.141

Related: 120.140 — Mobile and Manufactured Homes

Related: Table 120.141 — Allowable Demand Factors

⚠️ Source Verification Note

The supplied source material does not include the demand-factor percentage for the "61 and over" row of Table 120.141.

The supplied material also does not provide the intermediate value needed to verify the 3-home review question.

No percentage has been guessed or substituted.

Once the complete table is available, the missing values can be patched with the verified NEC information.

⚡ Series Progress

Article 120 — "Load Calcs Won't Kill You"

🏁 SERIES FINALE

Episode 43: Marinas: Shore Power Gets Its Own Discount Curve
Episode 44: RV Parks: Every Site Gets a Starting Number
Episode 45: RV Parks: The Bigger the Park, the Bigger the Discount
Episode 46: Mobile Homes: A Whole Different Calculation System
Episode 47: Mobile Homes: Everything Else Gets Added On
Episode 48: The Freestanding Range Table + One Last Shortcut
Episode 49: 120.141 Allowable Demand Factors — SERIES FINALE

🏆 49 Episodes. 9 Parts. One Article.

We've now reached the end of the Article 120 Quick Hit series.

Across 49 episodes, we've worked through:

✅ Part I — General Rules

Voltages, rounding, floor area, noncoincident loads, and power control systems.

✅ Part II — Branch-Circuit Loads

Dwelling and non-dwelling branch-circuit load calculations and outlet requirements.

✅ Part III — Feeder & Service Loads

Lighting loads, demand factors, dryers, ranges, EVSE, neutral calculations, and other feeder/service load calculations.

✅ Part IV — Optional Methods

Optional dwelling methods and specialized calculation approaches.

✅ Part V — Farms

Farm buildings and agricultural load calculations.

✅ Part VI — Health Care Facilities

Hospital receptacle loads and patient-care equipment circuits.

✅ Part VII — Marinas, Boatyards, Floating Buildings & Docking Facilities

Shore-power receptacles and their demand factors.

✅ Part VIII — RV Parks

RV site loads and park-wide demand factors.

✅ Part IX — Mobile & Manufactured Homes

Individual mobile-home calculations, freestanding ranges, and finally mobile-home park demand factors.

🎉 ARTICLE 120 COMPLETE

49 Episodes.

9 Parts.

One entire NEC article broken down into Quick Hits.

From the first calculation rules all the way through 120.141, we've followed Article 120 from beginning to end.

And now:

🏁 SERIES COMPLETE

NEC Article 120 — "Load Calcs Won't Kill You"

⚡ 49 Episodes.
📚 9 Parts.
🔌 1 Article.

🎬 BONUS 50TH POST — SERIES RECAP

A standalone recap graphic can close out the series:

"49 Episodes. 9 Parts. One Article."

Use it as a final trophy-case post and a pinned introduction for anyone discovering the series after the fact.

Article 120 — "Load Calcs Won't Kill You"

🏁 SERIES COMPLETE