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Required Battery Capacity
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Total System Load
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Total Energy Required
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Derated Capacity
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Actual Runtime
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NEC Article 700 and NFPA 101 require emergency lighting to provide minimum 1 foot-candle (10.8 lux) illumination for 90 minutes minimum. Systems must automatically activate within 10 seconds of power failure and maintain illumination along egress paths.
Battery capacity is calculated by determining total load (watts), multiplying by required runtime (hours), dividing by system voltage, then applying derating factors for temperature and aging. The formula is: Capacity (Ah) = (Total Load × Runtime) / (Voltage × Derating Factors).
Self-contained units have individual batteries in each fixture, making installation easier but requiring more maintenance. Central battery systems use one large battery bank to power multiple fixtures, offering better monitoring and maintenance but higher upfront costs.
Cold temperatures reduce battery capacity significantly - typically 50% capacity loss at 0°C for lead-acid batteries. High temperatures accelerate aging and can cause thermal runaway. Temperature derating factors must be applied to ensure adequate capacity.
The aging factor accounts for battery capacity loss over time due to charge/discharge cycles and chemical degradation. New batteries may have 100% capacity, but after 3-5 years, capacity drops to 70-80%. Using 0.8 aging factor ensures reliable operation throughout battery life.
LED emergency lights consume significantly less power (typically 3-10W vs 20-40W for incandescent), allowing smaller battery systems and longer runtime. They also have longer lamp life (10+ years vs 1-2 years) and better light output stability over the discharge cycle.
Monthly 30-second functional tests and annual 90-minute duration tests are required. Central battery systems need quarterly battery inspections, annual capacity tests, and replacement every 5-10 years depending on type. Self-contained units typically need battery replacement every 3-5 years.
Only fixtures specifically designed as 'remote capable' can power additional remote heads. The main fixture's battery must be sized for the total load of the fixture plus all remote heads. Check manufacturer specifications for maximum remote head capacity and wiring distance limitations.