Quick Hits

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

🏠 Episode 47 — Mobile Homes: Everything Else Gets Added On

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

🎯 What You'll Learn

The lighting, appliance, and laundry calculation from 120.140(A) is only the beginning.

Section 120.140(B) builds the total calculated load for sizing the mobile home's power supply by stacking several additional loads on top of the result from subsection (A).

📖 The Quick Hit

🏠 120.140(B) — Building the Full Load

The total load used for sizing the power supply includes all of the following.

1️⃣ Lighting, Small-Appliance & Laundry Load

Start with the calculated load from 120.140(A)(5).

This is the number we calculated in Episode 46 using:

💡 Lighting

🔌 Small-appliance circuits

🧺 Laundry circuit

with the applicable demand factors.

That becomes the first piece of the overall mobile-home load.

⚙️ 2️⃣ Motors, Heaters & HVAC Loads

Next, add the nameplate ampere ratings for applicable motors and heating/cooling equipment.

This can include equipment such as:

🌀 Exhaust fans

❄️ Air-conditioning equipment

🔥 Electric heating

🔥 Gas heating

🔥 Oil heating

⚠️ Heating vs. Cooling

When both heating and cooling are present, omit whichever is smaller.

In other words, you don't simply add the full heating load AND the full cooling load together.

There is an important exception:

If the blower motor serves as the AC evaporator motor, it remains included even when the heating load is the larger of the two.

⚡ 3️⃣ Largest Motor — 25% Add-On

After identifying the applicable motors from item 2, take:

➡️ 25% of the current of the largest motor

and add that amount to the calculation.

This is the familiar largest-motor add-on concept:

Largest motor current × 25%

🚿 4️⃣ Household Appliances

Next, include the nameplate ampere ratings for:

🚮 Waste disposer

🍽️ Dishwasher

🔥 Water heater

🧺 Clothes dryer

🔥 Wall-mounted oven

🍳 Cooking units

📉 More Than 3 Appliances?

If there are more than three of these qualifying appliances:

➡️ Apply 75% to their combined nameplate current.

So five qualifying appliances don't automatically get counted at 100%.

You first total their nameplate currents, then apply the 75% factor.

🔥 5️⃣ Freestanding Range

A freestanding range is handled separately.

Its load is calculated using:

➡️ Table 120.140(B)

This is a dedicated table for the range portion of the mobile-home calculation.

We'll break that table down in the next episode.

🔌 6️⃣ Other Factory-Uninstalled Outlets & Circuits

Finally, include the anticipated load of any other outlets or circuits that are provided but haven't had their equipment factory-installed.

The idea is simple:

If the mobile home is supplied with a circuit or outlet intended for future equipment, the expected load still needs to be accounted for.

🎁 Future Air-Conditioning Provision

There's also an interesting future-load provision.

If the mobile home does not have air conditioning installed, but a 40-ampere feeder assembly is provided:

➡️ Allow 15 amperes per phase for future air-conditioning load.

So even though the AC equipment isn't currently installed, the calculation accounts for the capacity that has been provided for it.

🧠 Exam Takeaway

When you see 120.140(B), think:

➕ 120.140(A)(5) calculated load

➕ Applicable motor/heating/cooling nameplate currents

➕ 25% of largest motor

➕ Qualifying appliance loads

➕ Freestanding range via Table 120.140(B)

➕ Other anticipated loads

And remember the two big traps:

⚠️ Heating vs. cooling: omit the smaller load, subject to the blower-motor exception.

⚠️ More than 3 qualifying appliances: apply 75% to their combined nameplate current.

🎯 Test Yourself

Per 120.140(B)(4), a mobile home has 5 qualifying appliances consisting of a waste disposer, dishwasher, water heater, dryer, and wall oven.

What percentage applies to their combined nameplate total?

A) 100%
B) 75%
C) 50%
D) 35%

✅ Answer

B) 75%

The rule applies when there are more than three of the qualifying appliances.

With five appliances, combine their nameplate currents and apply:

75%

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🔎 NEC References

Primary: Article 120, 120.140(B)

Related: 120.140(A)(5) — Lighting, Small-Appliance & Laundry Load

Related: Table 120.140(B) — Freestanding Range Load

⚡ Series Progress

Article 120 — Load Calcs Won't Kill You

🏠 Part IX — Mobile and Manufactured Homes

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

➡️ Next Up

Episode 48 — Mobile Homes: The Freestanding Range Table

Next we'll break down Table 120.140(B) and see how the freestanding range load gets calculated.