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CEM: Lighting Systems

CEM · Work through your exam, one topic at a time.

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Lighting Systems

This domain tests lighting metrics, technology comparisons, code limits, and controls. Questions often ask you to calculate lighting energy savings, check an LPD limit, or distinguish lamp lumens from delivered lumens.

Metrics and technology

  • Lumens measure light output; lux is lumens per square meter; footcandles are lumens per square foot.
  • Efficacy is lumens per watt; always compare luminaire efficacy, not just lamp efficacy.
  • Lighting power density (LPD) is connected lighting input watts per square foot.
  • CRI describes color rendering; CCT describes color appearance in kelvin; UGR describes glare.
  • Ballast factor and driver losses affect input power; lumen depreciation and dirt depreciation reduce delivered light over time.
  • Controls include occupancy/vacancy sensors, daylight dimming, scheduling, and personal tuning. Savings depend on space use and commissioning.

Worked example

A plant replaces 300 high-pressure sodium fixtures that draw 460 W each including ballast with LED fixtures that draw 160 W each. Operating hours are 4,000 per year and electricity costs $0.12/kWh. Demand reduction = 300 × (460 − 160) = 300 × 300 = 90,000 W = 90 kW. Annual energy savings = 90 kW × 4,000 h = 360,000 kWh. Cost savings = 360,000 × $0.12 = $43,200 per year. The reasoning: use input wattage including ballast or driver, multiply by annual operating hours, and then apply the energy rate. If the utility has a demand charge, add the coincident demand savings separately.

Code and design checks

An LPD check uses the same floor area as the code allowance and includes all permanently installed lighting. A design can meet LPD while failing illuminance or glare criteria, so lighting quality and code compliance are separate tests. Controls credits may allow additional connected power in some code versions, but only where the adopted code permits it.

Traps that cost marks

  • Using lamp wattage instead of fixture input wattage.
  • Comparing lamp lumens with luminaire delivered lumens.
  • Ignoring ballast factor, driver losses, or controls standby power.
  • Assuming controls savings are always additive.
  • Using the wrong area for LPD, such as gross building area instead of the code-defined space.
  • Forgetting that LED retrofit savings may be reduced by increased hours or higher illuminance.