9.5 Mobile Homes, Manufactured Homes & Mobile Home Parks

9.5 Mobile Homes, Manufactured Homes & Mobile Home Parks

Quick Reference: The ICC E3 outline puts Miscellaneous Other Special Occupancies in the Special Occupancies area and calls out mobile homes and RV parks by name. RV parks are covered in Section 9.4; this section carries NEC Article 550, which governs mobile homes, manufactured homes, and mobile home parks. Article 550 is a favorite exam source because it stacks a load calculation, a location rule, and a grounding rule that inverts what candidates learn everywhere else in the Code.

1. Scope and Terminology (NEC 550.2)

Article 550 does not apply to a mobile home installed on a permanent foundation and permanently connected as a conventional dwelling — 550.4(A) directs that case to the general Code rules once the running gear is removed and it is permanently attached.

2. Service Equipment: Location, Rating, and the One-Feeder Rule

NEC 550.32(A) — Location. Mobile home service equipment must be located adjacent to the mobile home, and not mounted in or on the home. It must be within sight from and not more than 30 ft (9.0 m) from the exterior wall of the mobile home it serves. A permitted alternative is a disconnecting means mounted in or on a properly grounded pedestal or meter pole within that same 30 ft.

NEC 550.32(B) — Not on the home. The service equipment shall not be installed in or on the mobile home. (The recognized case for a manufactured home allows the disconnecting means to be installed on the home where the requirements of 550.32(B) are met and the home is factory-equipped for it.)

NEC 550.32(C) — Rating. Mobile home service equipment must be rated not less than 100 amperes at 120/240 volts, and provisions must be made for connecting a mobile home feeder assembly by a permanent wiring method.

NEC 550.33 — Feeder. Each mobile home is permitted to be supplied by only one feeder assembly. That feeder must consist of four continuous, insulated, color-coded conductors: two ungrounded conductors, one grounded (neutral) conductor, and one equipment grounding conductor. There is no three-wire mobile home feeder in the current Code.

+-----------------------------------------------------------------------------+
|             MOBILE HOME SUPPLY ARRANGEMENT (NEC 550.32, 550.33)             |
+-----------------------------------------------------------------------------+
|   UTILITY  -->  MOBILE HOME SERVICE EQUIPMENT (pedestal or meter pole)      |
|                 * NOT in or on the home            (550.32(B))              |
|                 * Within sight, <= 30 ft from wall (550.32(A))              |
|                 * Rated >= 100 A at 120/240 V      (550.32(C))              |
|                 * Neutral-to-ground BOND made HERE (550.16)                 |
|                            |                                                |
|                            |  ONE feeder assembly only     (550.33(A))      |
|                            |  FOUR conductors: L1, L2, N, EGC (550.33(B))   |
|                            v                                                |
|                        MOBILE HOME DISTRIBUTION PANELBOARD                  |
|                 * Neutral bar ISOLATED from equipment ground (550.16)       |
|                 * NO neutral-to-ground bond inside the home                 |
+-----------------------------------------------------------------------------+

[!WARNING] The grounding inversion. In a conventional dwelling the main bonding jumper is inside the service panel that also distributes branch circuits. In a mobile home it is not. Under NEC 550.16 the grounded (neutral) conductor and the equipment grounding conductor must be kept separate inside the home: the neutral bar in the mobile home distribution panelboard is insulated from the enclosure, and the bond is made only at the mobile home service equipment ahead of the feeder. Finding a bonding screw or a neutral-to-ground strap in the mobile home panel is a rejectable defect — it puts normal neutral return current on the chassis and on every metal surface in the home.

3. Mobile Home Park Load Calculations (NEC 550.31)

Park distribution is the arithmetic half of Article 550, and the sequence is fixed.

Step 1 — Establish the per-lot load. Park electrical wiring systems are calculated at 120/240 volts on the larger of:

1. 16,000 volt-amperes for each mobile home lot, or
2. The load calculated under NEC 550.18 for the largest typical mobile home that each lot will accept.

The 16,000 VA figure is a floor, not a demand value. It is applied per lot regardless of how small the homes actually are.

Step 2 — Apply the Table 550.31 demand factor. Feeder and service conductors are permitted the following demand factors based on the number of mobile home lots supplied:

Step 3 — Add non-lot loads separately. Park amenity loads — laundry buildings, community rooms, offices, pool equipment, street and area lighting — are not part of the lot count and do not get the Table 550.31 demand factor. They are calculated under Article 220 Part III and added to the demanded lot load.

Worked example: 25-lot mobile home park

• Per-lot load: No home larger than 16,000 VA is contemplated, so the 16,000 VA floor governs.
• Gross lot load: $25 \text{ lots} \times 16,000\text{ VA} = 400,000\text{ VA}$
• Table 550.31 demand factor: 25 lots falls in the 22-to-40-lot tier at 24%.
• Demanded lot load: $400,000\text{ VA} \times 0.24 = 96,000\text{ VA}$
• Park service current (120/240 V single-phase): $96,000\text{ VA} \div 240\text{ V} = \mathbf{400\text{ amperes}}$
• Then add the separately calculated laundry building, office, and site lighting loads before selecting the service size.

[!NOTE] Do not borrow the RV park table. Article 551 recreational vehicle parks use a completely different basis: a per-site load that depends on the receptacle mix under 551.71, and demand factors from Table 551.73(A) that bottom out at 41% for 36 or more sites. Applying the RV table to a mobile home park — or the 550.31 table to an RV park — is a classic cross-article error. Section 9.4 covers the RV park side.

4. Inside the Home: Branch Circuits and Protection (NEC 550.13, 550.18, 550.25)

Although factory wiring is inspected under the HUD label rather than by the local AHJ, plans examiners still see Article 550 interior requirements on site-built additions, on park-model conversions, and on the calculations that justify the feeder size.

• NEC 550.18(B) sets the method for calculating the total load of the mobile home itself — the analogue of the Article 220 standard method, using 3 volt-amperes per square foot for general lighting and receptacles based on the outside dimensions exclusive of the tongue, 1,500 VA for each 20-ampere small-appliance and laundry circuit, and nameplate ratings for fastened appliances with a 75% demand factor where four or more such appliances are present.
• NEC 550.13 governs receptacle outlet placement inside the home, and 550.13(B) requires GFCI protection consistent with the dwelling-unit rules of 210.8(A).
• NEC 550.25 requires arc-fault circuit-interrupter protection for the branch circuits specified for dwelling units, applied to mobile and manufactured homes.
• NEC 550.15 limits interior wiring methods and requires bonding of metal parts, including the bonding of metal air ducts, the metal chassis, and metal piping systems back to the equipment grounding terminal.

5. Worked Plan Review: 40-Lot Park Expansion

Submittal: A developer proposes adding 40 mobile home lots served by a new 120/240 V single-phase distribution system, plus a 22,000 VA community laundry building and 9,000 VA of site lighting. Drawings show pedestals located 42 ft from the nearest home wall on several lots, 3-wire feeder assemblies from pedestal to home, and a note directing the electrician to "bond neutral to ground at the mobile home panel."

Findings:

1. 550.32(A) violation. Several pedestals exceed the 30 ft limit and are not within sight of the home they serve. Relocate the pedestals or reconfigure the lot layout.
2. 550.33(B) violation. Three-wire feeder assemblies are not permitted. Each home requires four continuous insulated color-coded conductors: two ungrounded, one grounded, one equipment grounding.
3. 550.16 violation. The bonding note is backwards. Delete it and add a note requiring the neutral bar in the mobile home panelboard to be isolated from the enclosure, with the system bond made only at the mobile home service equipment.
4. 550.31 calculation. $40 \times 16,000\text{ VA} = 640,000\text{ VA}$; 40 lots is in the 22-to-40 tier at 24%, giving $640,000 \times 0.24 = 153,600\text{ VA}$, or $153,600 \div 240 = 640\text{ A}$. The laundry building and site lighting are added outside the demand factor: $153,600 + 22,000 + (9,000 \times 1.25 \text{ continuous}) = 186,850\text{ VA}$, or about 779 A. Verify the proposed service against that figure, not against the demanded lot load alone.

6. Plans Examiner Verification Checklist: Article 550

• Scope: Confirm whether Article 550 or the general dwelling rules apply (550.4(A)).
• Service equipment location: Adjacent to the home, within sight, not more than 30 ft from the exterior wall, not in or on the home (550.32(A) and (B)).
• Service equipment rating: Not less than 100 A at 120/240 V (550.32(C)).
• One feeder: Only one feeder assembly per home (550.33(A)).
• Four conductors: Feeder assembly has two ungrounded, one grounded, and one equipment grounding conductor, all continuous, insulated, and color-coded (550.33(B)).
• Grounding separation: Neutral bar isolated inside the home; bond at the mobile home service equipment only (550.16).
• Park load basis: Larger of 16,000 VA per lot or the 550.18 load for the largest home the lot accepts (550.31).
• Demand factor: Correct Table 550.31 tier applied to the lot count — and NOT to amenity loads.
• Amenity loads: Laundry, office, pool, and site lighting calculated separately under Article 220 Part III and added after the demand factor.
• Article confusion: Confirm Table 550.31 was used for mobile home lots and Table 551.73(A) for RV sites.