Special occupancies and equipment

The general rules of the code apply to every installation, but certain occupancies and types of equipment carry special risks or needs and have their own rules that modify or add to the general ones. Health care facilities protect patients who may be unconscious, connected to equipment and especially vulnerable to electric shock; swimming pools combine water, metal and bare skin; electric vehicle chargers, solar systems and battery storage add new sources and large continuous loads; fire pumps must keep running even at the cost of their own destruction; and IT rooms have their own rules for disconnects and cabling. This lesson surveys these special cases and the calculations electricians most often perform for them.

Relevant NEC articles include Article 517 (health care facilities, with NFPA 99), 680 (swimming pools, fountains and similar), 625 (electric vehicle power transfer systems), 690 (solar photovoltaic systems), 705 (interconnected power production sources), 706 (energy storage systems), 695 (fire pumps), 620 (elevators), 645 (information technology equipment) and 547 (agricultural buildings).

Health care facilities

Patients in hospitals may be connected to monitors, catheters and other devices that bypass the skin's natural resistance, so currents too small to be felt can be dangerous. Health care rules reduce shock risk, ensure reliable grounding and keep power available for life-sustaining equipment:

• NFPA 99 (Health Care Facilities Code) and Article 517 classify patient care spaces as Category 1 (critical care, where failure could cause serious injury or death), Category 2 (general care), Category 3 (basic care) and Category 4 (support).
• Redundant grounding: branch circuits serving patient care areas need two grounding paths, typically an insulated copper equipment grounding conductor plus a metallic wiring method (armored or metal-clad cable or metal raceway) qualified as a grounding path.
• Hospital-grade receptacles and specified numbers of receptacles at each bed location, with some on emergency power.
• Isolated power systems in wet procedure locations or anaesthetizing locations where required: an isolation transformer supplies ungrounded circuits, so a single fault to ground does not cause a shock or trip the circuit, and a line isolation monitor alarms when the potential fault current reaches about 5 mA.
• Essential electrical systems supplied by generators (Lesson 9) are divided into the life safety, critical and equipment branches, under NFPA 99 and Article 517 Part III.

Pools, spas and fountains

Water, wet skin and metal create high shock risk: a person in a pool is surrounded by conductive water, and small voltage gradients can cause paralysis and drowning. The rules include:

• Equipotential bonding: all metal parts of the pool structure, reinforcing steel, ladders, handrails, pump motors, metal fittings and the deck within a specified distance are bonded together with a solid copper conductor (typically 8 AWG, or 6 AWG in Canada) so that no voltage difference can exist between surfaces a swimmer can touch.
• Ground fault protection (GFCI or GFP) for underwater luminaires, pump motors and receptacles near the pool.
• Clearances: receptacles, overhead conductors and switching devices must be kept specified distances from the water's edge.
• Low-voltage underwater lighting through listed or certified transformers.

Article 680 contains these requirements, including equipotential bonding of the perimeter surface within 3 ft (680.26), receptacles at least 6 ft from the inside wall with GFCI protection within 20 ft (680.22), and GFCI protection for pool pump motors (680.21(C)) and underwater luminaires operating above the low-voltage contact limit.

Electric vehicle charging

EV chargers are large continuous loads: a Level 2 charger draws 32 to 80 A at 208 to 240 V for hours, and DC fast chargers draw tens to hundreds of kilowatts. Branch circuits and feeders must be sized at 125% of the charger's rating, and many chargers in a parking facility can quickly exceed the building's service capacity, so electric vehicle energy management systems (load management) that limit total charging current are permitted to reduce the calculated load.

Article 625 covers EV charging; 625.42 requires the overcurrent protection for feeders and branch circuits to be sized for continuous duty at no less than 125% of the equipment's maximum load, and allows an energy management system (Article 750) to limit the load used for service and feeder calculations. Ground-fault protection for personnel is required for many receptacles and charging equipment (625.54, 625.22).

Solar photovoltaic systems and energy storage

PV systems produce DC power from modules connected in series strings, converted to AC by inverters. Their rules differ from ordinary circuits because PV sources are current-limited (short-circuit current only slightly above operating current), their voltage rises in cold weather, and they cannot be turned off while the sun shines.

• Circuit current: PV source circuit current is the modules' short-circuit current multiplied by 1.25 (to allow for irradiance above standard test conditions), and conductors and overcurrent devices are then sized at a further 125% for continuous operation, giving 156% of short-circuit current.
• Maximum voltage: the string's open-circuit voltage increases as temperature falls, so it is corrected for the lowest expected ambient temperature and must stay within equipment ratings (1000 V or 1500 V DC typically).
• Rapid shutdown: on buildings, PV conductors must be reduced to safe voltages within a short time of shutdown to protect firefighters.
• Interconnection: where inverters connect to the load side of a service, the sum of the PV breaker and the main breaker is limited (the 120% rule) so that busbars are not overloaded.

Article 690 covers PV systems: 690.8 sets circuit currents (1.25 × Isc, then 125% for conductors and overcurrent devices), 690.7 maximum voltage with temperature correction, 690.12 rapid shutdown; 705.12 limits load-side interconnection (the 120% rule for busbars); and Article 706 covers energy storage systems.

Battery energy storage systems add hazards of high fault current, DC arcs and thermal runaway (DCE-340). Their rules cover disconnects, overcurrent protection, location, ventilation, signage and coordination with fire codes, which limit the energy capacity in a room and require separation and suppression.

Fire pumps

A fire pump must run when a fire threatens the building, even if it overheats or overloads, because the building matters more than the pump. The rules therefore reverse the usual priority of protecting equipment:

• Reliable supply: from a dedicated service, an on-site generator or a tap ahead of the main disconnect, with conductors routed or protected so a fire cannot easily destroy them.
• No overload protection that could stop the pump; overcurrent devices are sized to carry the motor's locked-rotor current indefinitely, so they open only for short circuits.
• Voltage drop limits so the motor can start and run: typically no more than 15% at the controller terminals while starting and 5% while running at 115% of full-load current.

Article 695 and NFPA 20 govern fire pumps; 695.4(B)(2) requires the supply overcurrent devices to be sized to carry the sum of the locked-rotor currents of the pump motor and the full-load currents of accessory loads indefinitely, and 695.7 sets the voltage drop limits.

Information technology rooms

IT equipment rooms contain dense cabling and equipment with large stored energy. Where an IT room meets specified conditions (separated by fire-rated construction, with a dedicated HVAC system and a disconnecting means at the exit), special rules permit cables under raised floors and require a disconnecting means that removes power from all electronic equipment and shuts down the room's HVAC, so that firefighters can make the room safe. Large data centers usually do not use these optional rules and instead follow the general wiring rules with fire-rated construction and the owner's standards (DCE-310, DCE-440).

Article 645 contains these optional rules for information technology equipment rooms, including the remote disconnecting means (645.10) and permitted cable types under raised floors (645.5).

Lesson 9 covers the emergency and standby power systems that many of these occupancies require, and Lesson 10 their fire alarm systems.