PART XII Reference + General Applications
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Lighting Design

Lumen method · point method · IES · fixtures · controls · IES Handbook · NEC 410

Lighting is one third of NCEES "General Applications." §31 covers compliance — how much wattage is allowed. This section covers design — how many fixtures of what type at what spacing produce the right footcandles where the work happens. Two methods: lumen method for general illumination of large flat spaces, point method for task lighting and outdoor design.

The Vocabulary — Photometric Quantities

QuantitySymbolUnitWhat it physically is
Luminous fluxΦLumen (lm)Total light output of a source — like a faucet's gallons-per-minute
Luminous intensityICandela (cd) = lm/srLight flowing in a particular direction — direction-specific
IlluminanceEFootcandle (fc) = lm/ft² · Lux (lx) = lm/m²Light arriving at a surface — what you measure with a light meter
LuminanceLcd/m² (nit)Light leaving a surface toward the eye — what creates brightness perception
Efficacyηvlm/WLumens out per watt in. LED: 100–200. Fluorescent: 70–100. Incandescent: 10–17.
Color rendering indexCRI / Ra0–100How accurately a source renders color vs reference. ≥ 80 typical office. ≥ 90 retail/galleries.
Color temperatureCCTKelvin (K)2700 K warm yellow · 3500 K neutral · 4000 K cool · 5000 K daylight · 6500 K bluish

Quick conversions: 1 fc ≈ 10.76 lx (often rounded to 1 fc ≈ 10 lx for sanity-check math). Workplane is the imaginary plane where the task lives — typically 30 inches above the floor (desk height).

The Lumen Method — General Illumination

The lumen method answers: how many fixtures do I need to deliver target average footcandles across a room? It assumes the space is roughly rectangular and the lighting is generally uniform. Cornerstone equation:

Lumen method — fixture count
N = (E · A) / (Φ · CU · LLF)
N = number of fixtures · E = target footcandles · A = room area (ft²) · Φ = lumens per fixture · CU = coefficient of utilization · LLF = light loss factor
TermWhat it capturesTypical values
E — illuminance targetRequired footcandles for the task (IES Handbook Lighting Recommendations)Office task: 30–50 fc · Conference: 30 fc · Corridor: 5–10 fc · Warehouse: 10–30 fc · Retail: 30–75 fc · Operating room: 200–500 fc
A — room areaFloor area in ft²Just the area of the room
Φ — luminous flux per fixtureInitial lumen output from manufacturer photometric data (.IES file)2×4 LED troffer ≈ 4,000 lm · Hi-bay LED ≈ 30,000 lm · Wall sconce ≈ 800 lm
CU — coefficient of utilizationFraction of fixture lumens that actually reach the workplane (the rest is absorbed by walls / ceiling / floor)Open ceiling: 0.50–0.65 · Suspended grid w/ light walls: 0.65–0.80 · Recessed troffer in dark room: 0.40–0.55
LLF — light loss factorCombined depreciation: lumen depreciation (LLD) × dirt depreciation (LDD) × ballast/driver factorOffice (clean): 0.80 · Industrial (dirty): 0.65 · Outdoor: 0.70

Coefficient of Utilization — Driven by Room Cavity Ratio

CU is read from the manufacturer's photometric data table for a given fixture. Inputs are the room cavity ratio (RCR) and the reflectances of ceiling / walls / floor. Typical IES reflectance shorthand: 80/50/20 means 80% ceiling, 50% walls, 20% floor.

Room cavity ratio
RCR = 5 · hcav · (L + W) / (L · W)
hcav = ceiling-to-workplane height (ft). L, W = room length and width. Lower RCR (= taller, longer rooms) → higher CU.

The Point Method — Task and Outdoor Lighting

The point method answers: at this specific point on the workplane, what illuminance does each fixture contribute? Use it for non-uniform layouts (task lights, sports field, parking lot, façade lighting) where average illuminance is meaningless. Two laws govern:

Inverse-square law (perpendicular)
E = I / d²
E in fc when I in candela and d in feet. Light fall-off is quadratic with distance.
Cosine law (off-axis)
Eh = (Iθ / d²) · cos θ = (Iθ · h) / d³
θ is the angle from vertical. h is mounting height above workplane. d is the slant distance from fixture to point. Iθ is the candela in direction θ — read from the photometric polar diagram.
Point method geometry fixture at height h · target point P at horizontal distance r · slant d workplane (30 in AFF) FIXTURE I (cd) at angle θ h (mounting height) d r θ P Eh at P = (Iθ · cos θ) / d² = (Iθ · h) / d³ Photometric polar diagram candela vs angle from nadir · read I(θ) directly 0° 90° 270° 180° 2k cd 4k cd 6k cd θ=30° I(30°)≈3,200 cd Read I(θ) from the polar plot at the angle to your target — feed it into the cosine law.

Source Types Today

TypeEfficacy (lm/W)LifeCRIStatus
LED100–20050,000–100,000 hr80–95Default for new construction. Effectively all of new and retrofit since ~ 2018.
T8 / T5 fluorescent80–10020,000–40,000 hr75–85Legacy. ASHRAE 90.1-2022 LPDs require LED for most spaces.
Metal halide / HPS70–100 / 80–14015,000–24,000 hr65 / 25Legacy outdoor and high-bay. Replaced by LED.
Compact fluorescent (CFL)50–7010,000 hr80Phased out — most jurisdictions ban sales as of 2024.
Halogen / incandescent10–251,000–4,000 hr100Banned for general lighting in US since 2023 (DOE rule).

Lighting Controls — ASHRAE 90.1 / IECC Mandates

Modern energy codes require lighting controls. The exam tests both the technology and where each is mandatory.

ControlWhat it doesWhere required
Manual on / auto offOccupant turns lights on; sensor turns them off after timeoutMost enclosed spaces ≥ 300 sq ft (ASHRAE 90.1-2022 §9.4.1.1)
Occupancy sensorPIR / ultrasonic / dual-tech detects motion; turns lights on/off automaticallyRestrooms, corridors, conference rooms, offices, classrooms
Vacancy sensorSame hardware as occupancy but does not auto-on (occupant must manually turn on)Spaces where automatic on is undesirable — private offices preferred by some jurisdictions
Daylight harvestingPhotosensor dims/switches lights based on daylight contributionSpaces with windows or skylights ≥ 250 sq ft of effective aperture
Time-of-day schedulingBuilding automation system sweeps off after hoursRequired as override on top of manual / occupancy in most commercial
Multi-level / continuous dimming0–10 V or DALI dimming; 50% bi-level switching minimumMost enclosed spaces; matches ASHRAE LPD assumed dimming credit
Plug load controlRelay-controlled receptacles auto-off on schedule or vacancy≥ 50% of receptacles in offices, copy rooms, conference, classrooms (per IECC C405.10)

Worked Example 1 — Lumen Method for an Office

Example 01 · Lumen methodOpen office · 60 ft × 40 ft · 9 ft ceiling · target 35 fc · 80/50/20 reflectance · 2x4 LED troffers (4,000 lm each, 35 W)
  1. Geometry. Area A = 60 × 40 = 2,400 ft². Ceiling height 9 ft − workplane 2.5 ft = hcav = 6.5 ft.
  2. Room cavity ratio.
    RCR = 5 · 6.5 · (60 + 40) / (60 · 40) = 5 · 6.5 · 100 / 2,400 = 1.35
  3. CU from photometric data. For 80/50/20 reflectance and RCR ≈ 1.5, manufacturer table for a recessed 2x4 LED troffer with medium distribution gives CU ≈ 0.78.
  4. LLF. LED has very low LLD (~ 0.95 over 50,000 hr). Office is clean, LDD ≈ 0.94. Driver factor ~ 1.0. LLF = 0.95 × 0.94 × 1.0 ≈ 0.89.
  5. Solve for fixture count.
    N = (E · A) / (Φ · CU · LLF)
    N = (35 · 2,400) / (4,000 · 0.78 · 0.89)
    N = 84,000 / 2,776 = 30.3 fixtures → 30 fixtures
  6. Layout check. 30 fixtures in 60 × 40 = a 6×5 grid (10 ft × 8 ft spacing) or 5×6 grid (12 ft × 6.7 ft). Manufacturer SC ratio (spacing-to-mounting-height) typically 1.2–1.4. With hcav = 6.5 ft → max spacing = 1.3 × 6.5 ≈ 8.5 ft. Use the 6×5 grid (8 ft spacing along short axis).
  7. Connected lighting load. 30 × 35 W = 1,050 W for 2,400 ft² = 0.44 W/ft². ASHRAE 90.1-2022 LPD for open office: 0.61 W/ft² (whole-building method varies). Comfortably under LPD limit.
  8. Verify. Initial fc (no LLF) = 30 × 4,000 × 0.78 / 2,400 = 39 fc. Maintained fc = 39 × 0.89 = 35 fc. Target met.

Worked Example 2 — Point Method for a Parking Lot

Example 02 · Point methodParking lot pole · 25 ft mounting · LED area light 20,000 lm · check fc at a point 30 ft horizontally from the pole base.
  1. Geometry. h = 25 ft. r = 30 ft. Slant distance d = √(h² + r²) = √(625 + 900) = √1,525 = 39.05 ft.
  2. Angle from vertical.
    θ = arctan(r / h) = arctan(30 / 25) = arctan(1.2) ≈ 50.2°
  3. Read I(θ) from polar plot. For a Type III (medium) distribution area light, the polar shows ~ 5,000 cd at 50°. (Manufacturers publish "Type II / III / IV / V" street-light distributions per IES TM-15.)
  4. Apply cosine law.
    Eh = I(θ) · cos θ / d² = 5,000 · cos(50.2°) / 39.05²
    = 5,000 · 0.640 / 1,525
    = 3,200 / 1,525
    = 2.10 fc initial
  5. Apply LLF. Outdoor LED LLF ≈ 0.70 (lumen depreciation + dirt + ambient temp). Emaintained = 2.10 × 0.70 = 1.47 fc.
  6. Compare to IES target. IES RP-20-14 (parking lots) recommends 0.5–2.0 fc minimum, 4:1 max-to-min uniformity. 1.47 fc passes the minimum but a single fixture cannot deliver good uniformity across the lot — multiple poles overlap to keep ratio acceptable.
  7. Add a second pole. Place pole #2 at 60 ft from the first. The point in question (30 ft from #1) is now also 30 ft from #2. By symmetry, second pole adds another ~ 1.47 fc. Total at this point ≈ 2.94 fc. The point method scales linearly with fixture count — sum the contributions.
i
Why the cosine law matters here
If you'd used the inverse-square law alone (E = I / d²), you'd get 5,000 / 1,525 = 3.28 fc. That's the illuminance on a surface perpendicular to d. The pavement is horizontal — it receives only the cos θ projection. The 0.64 factor (not 1.0) is precisely why parking lots use Type III (high-angle) distributions: they push lumens out at θ ≈ 60°, where cos θ is low but the candela are high enough to compensate.
What you can do after this section
  1. Convert between footcandles, lux, candela, and lumens; explain the difference between illuminance and luminance.
  2. Apply the lumen method (N = E·A / (Φ·CU·LLF)) to size general illumination for any room.
  3. Apply the inverse-square + cosine laws to compute illuminance from a point source at any geometry.
  4. Read a polar photometric diagram and pick the right luminaire distribution for the application.
  5. Identify required ASHRAE 90.1 / IECC controls (occupancy, daylight, scheduling, plug load) by space type.
  6. Pair this section with §31 (Energy Codes) for full design + compliance — design counts fixtures, §31 verifies LPD.

Drill — Quick Self-Check

Drill 1 · units

1 footcandle = ___ lux (approximate)?

Drill 2 · LLF

Three components of light loss factor?

Drill 3 · efficacy

Typical LED efficacy in lm/W?

Drill 4 · point method

Inverse-square law E = I / d² gives illuminance on a surface ___?

Drill 5 · controls

Difference between an occupancy sensor and a vacancy sensor?

If You See THIS, Think THAT

If you see…Think / use…
"Lumens"Total light output of a source.
"Candela"Light intensity in a particular direction.
"Footcandle"Light arriving at a surface (lm/ft²). 1 fc ≈ 10.76 lx.
"CU"Coefficient of utilization — what fraction of fixture lumens reach the workplane.
"RCR"Room cavity ratio = 5·hcav(L+W)/(L·W). Drives CU lookup.
"LLF"Light loss factor — multiply by initial illuminance for maintained.
"80/50/20"Reflectances: 80% ceiling / 50% walls / 20% floor.
"Type II / III / IV / V"IES outdoor distribution classes (street light beam shapes).
"Cosine law" / "cos θ"Project light onto a tilted surface — multiply by cos of angle from normal.
"CRI" / "Ra"Color rendering 0–100. ≥ 80 office, ≥ 90 retail / gallery.
"CCT 3500K"Color temperature — neutral white.
"DALI" / "0–10 V"Standard dimming protocols — DALI digital, 0–10 V analog.
"Daylight harvesting"Photosensor dims electric light when daylight is sufficient.
"NEC 410"Code article for luminaires (mounting, support, voltage limits).
"IES Handbook"Reference for recommended illuminance per task.
"BUG rating"Backlight / Uplight / Glare — outdoor light pollution + spill control.
"LM-79 / LM-80"IES standards for LED testing (LM-79 photometric, LM-80 lumen maintenance).
Also see
§31Energy
Energy Codes
ASHRAE 90.1 / IECC LPD compliance — the design's energy verification.
§03Loads
Load Analysis
Lighting feeds into the connected and demand load calculations.
§04Branch
Branch Circuits
Branch circuit sizing for lighting per NEC 210, NEC 410.
§28Docs
Reading Drawings & Specs
Lighting fixture schedule, photometric layout drawings on E-sheets.