517.31 Requirements for the Essential Electrical System
- The number of transfer switches to be used shall be based on reliability and design. Each branch of the essential electrical system shall have one or more transfer switches.
- One transfer switch shall be permitted to serve one or more branches in a facility with a continuous load on the switch of 150 kVA (120 kW) or less. [99:22.214.171.124.1.4]
Informational Note No. 1: See NFPA 99-2018, Health Care Facilities Code, 126.96.36.199, Transfer Switches; 188.8.131.52.5, Automatic Transfer Switch Features; 184.108.40.206.5.15, Nonautomatic Transfer Switch Features; and 220.127.116.11.7, Nonautomatic Transfer Device Features.
Informational Note No. 2: See Informational Note Figure 517.31(a).
Informational Note No. 3: See Informational Note Figure 517.31(b).
(a) Raceways, cables, or enclosures of the life safety and critical branch shall be readily identified as a component of the essential electrical system (EES). Boxes and enclosures (including transfer switches, generators, and power panels) shall be field- or factory-marked and identified as a component of the EES. Raceways and cables shall be field- or factory-marked as a component of the EES at intervals not to exceed 7.6 m (25 ft).
(b) Conductors of the life safety branch or critical branch shall not enter the same raceways, boxes, or cabinets with each other or any other wiring system. It shall be permitted for the branch conductors to occupy common equipment, raceways, boxes, or cabinets of other circuits not part of the life safety branch and critical branch where such wiring complies with one of the following:
- Is in transfer equipment enclosures
- Is in exit or emergency luminaires supplied from two sources
- Is in a common junction box attached to exit or emergency luminaires supplied from two sources
- Is for two or more circuits supplied from the same branch and same transfer switch
(d) Where Category 2 (general care) locations are served from two separate transfer switches on the essential electrical system in accordance with 517.18(A), Exception No. 3, the Category 2 (general care) circuits from the two separate systems shall be kept independent of each other.
(e) Where Category 1 (critical care) locations are served from two separate transfer switches on the essential electrical system in accordance with 517.19(A), Exception No. 2, the critical care circuits from the two separate systems shall be kept independent of each other.
- Nonflexible metal raceways, Type MI cable, Type RTRC marked with the suffix -XW, or Schedule 80 PVC conduit. Nonmetallic raceways shall not be used for branch circuits that supply patient care areas.
- Where encased in not less than 50 mm (2 in.) of concrete, Schedule 40 PVC conduit, flexible nonmetallic or jacketed metallic raceways, or jacketed metallic cable assemblies listed for installation in concrete. Nonmetallic raceways shall not be used for branch circuits that supply patient care areas.
- Listed flexible metal raceways and listed metal sheathed cable assemblies in any of the following:
- Where used in listed prefabricated medical headwalls
- In listed office furnishings
- Where fished into existing walls or ceilings, not otherwise accessible and not subject to physical damage
- Where necessary for flexible connection to equipment
- For equipment that requires a flexible connection due to movement, vibration, or operation
- Luminaires installed in ceiling structures
- Flexible power cords of appliances or other utilization equipment connected to the emergency system.
- Cables for Class 2 or Class 3 systems permitted by Part VI of this article, with or without raceways.
Feeders shall be sized in accordance with 215.2 and Part III of Article 220. The alternate power source(s) required in 517.30 shall have the capacity and rating to meet the demand produced by the load at any given time.
Demand calculations for sizing of the alternate power source(s) shall be based on any of the following:
- Prudent demand factors and historical data
- Connected load
- Feeder calculation procedures described in Article 220
- Any combination of the above
Exception No. 1: Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exists on the transformer secondary.
Informational Note No. 1: The terms coordination and coordinated as used in this section do not cover the full range of overcurrent conditions.