Ecmweb 6664 Nec Prefab Homes Pr
Ecmweb 6664 Nec Prefab Homes Pr
Ecmweb 6664 Nec Prefab Homes Pr
Ecmweb 6664 Nec Prefab Homes Pr
Ecmweb 6664 Nec Prefab Homes Pr

NEC Electrical Requirements for Prefabricated Homes

Sept. 17, 2014
Article 550 provides the requirements for electrical work related to prefabbed housing units, commonly known as mobile or manufactured homes.

Article 550 generally includes manufactured homes in the term “mobile home” [550.2]. But in some cases, there’s a distinction — and it’s important. So how can you avoid confusion? If both terms appear in any subsection, the NEC is making a distinction; if only “mobile home” appears, then it’s not.

These homes come in many sizes, including high square footage configurations (Photo 1). But the traditional trailer-sized unit (Photo 2) is (for economic reasons) perhaps the most popular. Unfortunately, the compact size of the “trailer” means flammable possessions form a high fuel density (relative to other home types), and there’s less space around ignition sources — notably space heaters. Consequently, occupants of “trailer” homes are disproportionately represented in fire fatality statistics.

Photo 1. Units larger than the traditional “trailer” are often on their own lot, serving as a low-cost alternative to a stick-built home but with the same space and amenities (Photo courtesy of Brynn Belle Isle).

The NEC doesn’t directly tackle the size issue. But it does mitigate other risk factors, which exist regardless of unit size. After presenting some general requirements, Art. 550 begins risk mitigation by addressing the power supply.

Photo 2. A simple way to upgrade the functionality of a traditional “trailer” is to add a carport. This photo shows a more elaborate approach, whereby the builder wraps a shell around it to add covered parking and a storage room. This shell also has lights and receptacles (Photo courtesy of Brynn Belle Isle).

Structure power requirements

The power requirements could be confusing, because you find power requirements at the beginning of Part II in 550.10 and 550.11, but then Part III is titled “Services and Feeders.” The “secret” here is the requirements of Part II are for supplying power to the individual unit while the requirements in Part III are for distributing power in mobile home parks (Photo 3) and the services to individual units.

Photo 3. Mobile home parks are typically, but not always, dense collections of inexpensive “trailers" (Photo courtesy of Brynn Belle Isle).

The power supply to a mobile home must be a feeder assembly consisting of a listed 50A mobile home power supply cord or a permanently installed feeder 550.10(A). This requirement has two Exceptions. If the unit is:

1. Equipped with gas or oil-fired central heating equipment and cooking appliances. In that case, you can use a 40A supply cord.

2. Constructed per the seven conditions enumerated in 550.32(B). This is typically a larger “manufactured home” rather than a “trailer.” The service size is often comparable to that of a typical site-built stick home.

If you do run the listed cord, you must permanently attach it to the panelboard and ensure the free end terminates in an attachment plug cap [550.10(B)]. The cap must be a 3-pole, 4-wire grounding type [550.10(B)]. The cord can’t be longer than 36.5 ft [550.10(D)], and it must be marked with amperage [550.10(E)].

Windows or vents cannot serve as the entrance point. The point of entrance for the feeder assembly must be the exterior wall, floor, or roof [550.10(F)]. From there, it can continue within the walls, if run (inside a continuous raceway) to the branch circuit panel [550.10(G)].

If you run a permanent feeder, you can run it to a mast weatherhead installation per Art. 230 or inside a raceway (metal raceway or nonmetallic conduit) [550.10(I)]. If you take the raceway approach, you must follow the manufacturer’s written instructions for sizing the feeder conductors and junction box [550.10(I)].

Each mobile home must have a single disconnecting means [550.11(A)]. You can combine the service disconnect with the branch circuit equipment, just as is commonly done in stick-built homes [550.11]. As with any other occupancy, this disconnect cannot be in a bathroom or clothes closet, and it must meet working space clearance requirements [550.11(A)].

Branch circuit requirements

The requirements for branch circuits appear after the requirements for the power supply. This sequence is especially logical for mobile homes because the panelboard with the power supply disconnect switch is also your branch circuit distribution panel.

Each branch circuit must have an overcurrent protection device (OCPD). It can be a breaker or a fuse. The OCPD’s purpose is to protect the conductors, so its rating must never exceed that of its circuit conductors [550.11(B)(1)]. Additionally, the OCPD can’t exceed:

• 150% of the rating of a single appliance rated 13.3A or more (if supplied by a single branch circuit).

• The OCPD size marked on the air conditioner (or other motor-operated appliance).

For circuit breakers used with 240V loads (e.g., air conditioner), ensure you protect the load with a 2-pole common or companion trip or use breakers with identified handle ties [550.11(C)]. You’ll also need a metal nameplate that complies with 550.11(D).

How many branch circuits do you need in a mobile home? Critics say the NEC’s minimum requirements for residential branch circuits and receptacles are woefully behind the times. For example, consider the 6-ft rule. The NEC adopted it long before the plethora of mobile devices, kitchen appliances, and home electronics we have today, and it does not meet the needs of most people.

At some point, the NEC may address this shortcoming by also requiring some minimum number of receptacles per type of room. In the meantime, people are using gang receptacle adapters, extension cords, and other devices that the 6-ft rule formerly made unnecessary (that’s the reason it was first put into the Code).

So, yes, plan things out to meet the NEC requirements specified in 550.12 (circuits) and 550.13 (receptacles). But consider adding more circuits and receptacles based on the normal number of loads for that type of room. For upscale units, such as those typically purchased by wealthy retirees, installing receptacles in a manner considerate of their actual needs can be a selling point.

In certain locations, the receptacles must be GFCI-protected. These locations are outdoors, in compartments accessible from outside the unit, and in bathrooms [550.13(B)].

Luminaires and appliances

The luminaire and appliance requirements are in 550.14 and 550.15. Some key points:

• Use listed luminaires.

• Don’t mix metallic and nonmetallic wiring methods.

• Bond all metallic objects; e.g., “ground” metallic faceplates.

• Provide a flexible cable at least 3 ft long to the range.

Periodically cleaning behind the range is part of normal home maintenance. In the typical installation, the range is wedged between two sections of countertop and you nearly stand on your head to unplug it so you can slide the range those last 5 in. Here’s another example of where considering real conditions of use means exceeding the NEC (by installing a longer cable).

Grounding

The way subsection 550.16 states its requirements could lead to tragic mistakes. So that you don’t make them, review the Art. 100 definitions of grounding and bonding. You’re normally going to make one grounding connection; all the others will be bonding connections. Don’t, for example, run the “equipment grounding” connection of the range to a ground rod. Don’t forget to bond the utilities together [550.16(B)(3)].

Testing

After you finish installing the branch circuits, luminaires, and appliances, you must conduct these tests:

• Dielectric strength test [550.17(A)].

• Continuity test [550.17(B)(1)].

• Operational test [550.17(B)(2)].

• Polarity checks [550.17(B)(3)].

Park power

Mobile home park power requirements, in Part III of Art. 550, address wiring outside the individual units. These requirements are in three categories:

1. Distribution within the park.

2. Service equipment.

3. Feeders.

The secondary distribution within the park is always going to be 120/240V [550.30]. If the park service exceeds 240V, then you must treat transformers and secondary panels as services.

When you calculate the park’s system load, assume the larger of two values per mobile home lot:

1. 16kVA.

2. The load for the largest typical mobile home each lot will accept, as determined by the calculations detailed in 550.18.

You can apply demand factors per Table 550.31.

The detailed requirements for service equipment are in 550.32. In this subsection, the NEC distinguishes between a mobile home and a manufactured home. This is one of those “unless otherwise indicated” situations mentioned in 550.2. These two types of structures differ in many ways, so there are bound to be differences in requirements.

For example:

• You can’t locate the service in or on a mobile home [550.32(A)].

• You can locate the service in or on a manufactured home [550.32(B)].

The last subsection in Art. 550 deals with feeders to mobile home (and manufactured home) lots. They have to be rated at least 100A [550.33(B)] and have a capacity at least that of the supplied loads. You can size the feeder per the requirements for 120/240V single-phase dwelling feeders in 310.15(B)(7).

Perspective

For all types of mobile homes the NEC requirements are a special case of the Chapter 2 requirements for residential structures. Much of what you’d do in a site-built stick home applies to these, as well.

But you’re dealing with a type of structure that imposes additional limitations on what’s safe. Thus, we have the special requirements in Part II. When you put many such units into a mobile home park, power distribution for that park becomes an issue due to several factors (the most visible often being high unit density); thus, the requirements of Part III.    

Lamendola is an electrical consultant located in Merriam, Kan. He can be reached at [email protected].

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

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