Mobile Homes, RV Parks and Marinas (NEC 550, 551 and 555): Service Location, Site Receptacles and the Load Math

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.

Which article covers what

Installation	Article	What the exam usually asks
Mobile and manufactured homes, mobile home parks	550	Service location and four-wire feeder; the home and park load calculations are in Article 120
Recreational vehicles and RV parks	551	Site receptacle requirements and GFCI protection; RV park demand factors are in Article 120
Marinas, boatyards, docks and piers	555	Ground-fault protection, shore power receptacles, wiring height above water

Mobile homes: Article 550

• Service location. For a mobile home, the service equipment is generally installed separately, not in or on the home, in a readily accessible outdoor location within sight of the home, which means in view and not more than 50 ft away. Older editions also had a fixed 30 ft limit from the exterior wall, which the 2023 NEC removed. A manufactured home on a permanent foundation can carry its own service when the NEC conditions are met.
• Four-wire feeder. The feeder to the home has two ungrounded conductors, a grounded (neutral) conductor and an equipment grounding conductor.
• Neutral kept separate. Inside the home’s panelboard, the neutral is insulated from the enclosure and the grounding conductors. Bonding them there is a classic wrong answer (NEC 550.18(A)(1)).
• Service size. Mobile home service equipment generally has to be rated at least 100 A at 120/240 V.
• Park feeders. Mobile home park services and feeders use a minimum load per lot and a demand factor table, which the 2026 NEC moved to Article 120. The calculation for one individual mobile home is a separate method, shown in worked example 1. Do not use one in place of the other.

RV parks: Article 551

• Receptacle mix. For standard powered RV sites, every site needs a 20 A, 125 V receptacle. At least 70% of sites also need a 30 A, 125 V receptacle. At least 40% of new sites and 20% of existing sites need a 50 A, 125/250 V receptacle, and every site with a 50 A receptacle must also have a 30 A receptacle. Outdoor site receptacles also follow the wet-location rules. Check the tent-site provisions and exceptions in your 2026 book.
• GFCI. The 125 V, single-phase, 15 and 20 A receptacles on the site supply equipment need GFCI protection.
• Park load. For a site serving one RV, use the NEC calculated load assigned to its highest-rated receptacle, not the sum of all receptacle ratings. For a park feeder or service, apply the demand factor based on the number of sites served. Sites set up to serve more than one RV follow a separate rule, so check Article 120 before using this shortcut. The 2026 NEC moved this calculation from Article 551 to Article 120.

Site type	Load counted per site
Site with a 50 A receptacle	12,000 VA
Site with 20 A and 30 A receptacles	3,600 VA
Site with a 20 A receptacle only	2,400 VA
Dedicated tent site	600 VA

Marinas and docks: Article 555

• Ground-fault protection. Feeders and branch circuits supplying docking facilities generally need ground-fault protection of equipment set to trip at no more than 100 mA, subject to the NEC exceptions (NEC 555.35). Shore power receptacles need their own individual ground-fault protection set to trip at no more than 30 mA, and other receptacles on the dock have their own personnel protection rules. Keep the 100 mA feeder and branch-circuit value and the 30 mA shore power value apart. Before the system is put into service, check the 2026 Article 555 rules on coordinating, testing and documenting the ground-fault protection. Leakage current in water is a shock and drowning hazard, which is why these limits are tight.
• Height above water. Electrical connections are kept above a reference level set from the high water line, with separate rules for floating docks (NEC 555.16(B) and 555.30).
• Warning signs. Marinas post signs warning of shock hazard in the water.
• Demand factors. Shore power receptacles have their own demand factor table for marina feeders and services.

Worked example 1: a mobile home load

A mobile home measures 14 ft by 70 ft on its outside dimensions. For this simplified example, it has two 20 A small-appliance circuits, one laundry circuit, a 5,000 W electric resistance heater and a 4,500 W water heater. Assume the air-conditioning load is smaller than the heating load, so only the heating load is included. Assume no range, dryer, other fixed appliances or motor loads that would need an extra allowance. The heater is resistance only, with no blower motor. The method below follows the NEC calculation for an individual mobile home, which the 2026 NEC moved into Article 120. It is not the general dwelling service calculation and not the park calculation. This method counts lighting at 3 VA per square foot. The 2026 general dwelling service calculation uses 2 VA per square foot, so do not mix the two methods.

1. Lighting: 14 × 70 = 980 sq ft × 3 VA = 2,940 VA.
2. Small appliance and laundry: 3 × 1,500 = 4,500 VA.
3. Subtotal: 2,940 + 4,500 = 7,440 VA. First 3,000 VA at 100% = 3,000 VA. The remaining 4,440 VA at 35% = 1,554 VA. Result: 4,554 VA.
4. Add the full 5,000 VA heating load and the 4,500 VA water heater. With no motor load in this example: 4,554 + 5,000 + 4,500 = 14,054 VA.
5. Calculated load current at 240 V: 14,054 ÷ 240 = 58.6 A. This is the calculated load, not the service rating; the service equipment still has to meet the 100 A minimum above.

Check: 14,054 ÷ 240 = 58.558 A, which rounds to 58.6 A.

Worked example 2: an RV park feeder

A new RV park has 20 powered sites: 8 sites with 20 A, 30 A and 50 A receptacles, and 12 sites with 20 A and 30 A receptacles. That mix meets the receptacle minimums above (40% of sites have 50 A and every site has 30 A). The feeder is 120/240 V single-phase and supplies only these sites. Count each site by its largest receptacle. For 20 sites, the RV park demand factor table in Article 120 gives 45% on the row for 19 to 21 sites; the percentage changes with the number of sites.

1. Connected load: 8 × 12,000 = 96,000 VA, plus 12 × 3,600 = 43,200 VA, total 139,200 VA.
2. Demand load: 139,200 × 0.45 = 62,640 VA.
3. Current at 120/240 V single-phase: 62,640 ÷ 240 = 261 A.

Check: 261 × 240 = 62,640, and 62,640 ÷ 0.45 = 139,200. Both match. This covers the site loads only; park lighting, buildings and other loads are added separately.

Check yourself

1. Can the service equipment for a mobile home be mounted on the home? Generally not. Mobile home service equipment is installed separately in a readily accessible outdoor location within sight of the home. A manufactured home on a permanent foundation may have the service equipment mounted on it when the NEC conditions for that are met.
2. Are neutral and grounding conductors bonded in the mobile home’s panel? No. They are kept separate.
3. What load is counted for an RV site with a 50 A receptacle? 12,000 VA.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Articles 120, 550, 551 and 555