Quick Hits

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

🏠 Episode 46 — Mobile Homes: A Whole Different Calculation System

Article: 120 — Load Calculations
Part: Part IX — Mobile and Manufactured Homes
Section: 120.140
Format: Quick Hit

🎯 What You'll Learn

We're at the final part of Article 120!

Mobile and manufactured homes have their own specific load-calculation method. Instead of using the methods from Parts I–IV, 120.140 provides a separate calculation system for mobile home feeder assemblies.

📖 The Quick Hit

🏠 120.140 — The Mobile Home Method

Section 120.140 provides a separate calculation method for mobile home feeder assemblies.

This method replaces the calculation methods from Parts I–IV for the situations covered by this section.

The method is based on a:

⚡ 3-wire, 120/240-volt AC-only supply

with the 120-volt loads balanced across the two ungrounded conductors.

So unlike the standard dwelling calculations we've been working through, mobile homes get their own dedicated calculation path.

🔢 (A) Building the "Net VA" Number

The first step is building the total lighting, appliance, and laundry VA.

1️⃣ General Lighting

Calculate the floor area using the outside dimensions:

Length × Width × 3 VA/sq ft

That gives you the lighting VA.

2️⃣ Small-Appliance Circuits

For each 20-ampere appliance receptacle circuit:

➡️ 1,500 VA

Multiply the number of qualifying circuits by 1,500 VA.

3️⃣ Laundry Circuit

The laundry circuit gets:

➡️ 1,500 VA

This is a flat value for the calculation.

4️⃣ Add Them Together

Combine:

Lighting VA

➕

Small-appliance circuit VA

➕

Laundry circuit VA

=

Total VA

That's the starting number before the demand factor is applied.

📉 Applying the Demand Factor

Once the total VA has been calculated, apply the following percentages:

First 3,000 VA

➡️ 100%

Everything over 3,000 VA

➡️ 35%

So the calculation follows this structure:

First 3,000 VA × 100%

➕

Remaining VA × 35%

=

Calculated VA

⚡ Convert VA to Amperes

After applying the demand factors, divide the resulting VA by:

240 volts

This gives the calculated amperes per leg.

🧮 Example

Suppose the lighting, appliance, and laundry total is:

4,000 VA

First 3,000 VA:

3,000 × 100% = 3,000 VA

Remaining 1,000 VA:

1,000 × 35% = 350 VA

Total calculated VA:

3,000 + 350 = 3,350 VA

Then:

3,350 VA ÷ 240V = 13.96 A

So the calculated load for this portion is approximately:

➡️ 14 amperes per leg

🧠 Exam Takeaway

When you see 120.140(A), think:

🏠 Mobile/manufactured home

⚡ Separate calculation method

🔌 3-wire, 120/240V AC-only supply

💡 3 VA/sq ft lighting

🔌 1,500 VA per 20A appliance receptacle circuit

🧺 1,500 VA laundry circuit

📉 First 3,000 VA → 100%

📉 Over 3,000 VA → 35%

➗ Divide the resulting VA by 240V

🎯 Test Yourself

Per 120.140(A), a mobile home's lighting + appliance + laundry total comes to 4,000 VA.

What percentage applies to the portion from 3,001–4,000 VA?

A) 100%
B) 65%
C) 40%
D) 35%

✅ Answer

D) 35%

The first 3,000 VA is calculated at 100%.

The remaining 1,000 VA is calculated at:

35%

So:

1,000 VA × 35% = 350 VA

📚 Keep Exploring

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View Sources & References:
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🔎 NEC References

Primary: Article 120, 120.140(A)

Related: Part IX — Mobile and Manufactured Homes

⚡ Series Progress

Article 120 — Load Calcs Won't Kill You

🏠 Part IX — Mobile and Manufactured Homes

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

🎉 Article 120 Finale

We've reached the final part of the Article 120 series.

From dwelling-unit calculations and demand factors to multifamily buildings, farms, health care facilities, marinas, RV parks, and now mobile homes — Article 120 is a lot more than just adding up the loads.

And that's exactly why:

⚡ Load Calcs Won't Kill You.