Lighting Laws, Lamps & Luminaires

1. Key Lighting Definitions

Good lighting is essential in all building interiors — it helps work to be done efficiently and safely and plays an important part in creating pleasant, comfortable surroundings. Before studying lighting technology, we must define the fundamental quantities.

Definition

Luminous Intensity (I) — The illuminating power of a light source to radiate luminous flux in a particular direction. The earliest unit was candle power (based on a wax candle). The SI unit is the candela (cd).

Definition

Luminous Flux (F) — The total flow of light radiated from a source in all directions. The SI unit is the lumen (lm). One lumen is the light flux emitted within a unit solid angle (cone) from a point source of 1 candela.

Definition

Illuminance (E) — A measure of the light falling on (incident upon) a surface. The SI unit is the lux (lx). 1 lux = 1 lumen falling on an area of 1 m².

Definition

Luminance (L) — A measure of the brightness of a surface — the light reflected from or emitted by a surface towards the eye. Units vary: for diffusing surfaces (e.g. matt white paint) the unit is lumen per m²; for polished surfaces (e.g. reflectors) it is candela per m².

Definition

Lamp Efficacy — The ratio of luminous flux output (lumens) to electrical power consumed (watts). Measured in lumens per watt (lm/W). The higher the efficacy, the more efficiently the lamp converts electricity into light.

Definition

Colour Rendering — The ability of a light source to reproduce the natural colours of objects as they would appear in daylight. A lamp can only show a colour if it emits light at that colour's frequency.

Definition

Luminaire — The modern term for the complete light fitting — the equipment that supports and surrounds the lamp and may control the distribution of light. The 18th Edition IET Regulations (411.3.4) require all luminaire circuits in domestic premises to be protected by a 30 mA RCD.

Figure 1 — How the key lighting quantities relate to the lamp, the surface and the observer's eye.

2. Illumination Laws

The Inverse Square Law

Rays of light falling upon a surface from a distance d will illuminate that surface with a certain illuminance. If the distance is doubled, the same luminous flux spreads over four times the area — so the illuminance falls to one quarter. This relationship is the Inverse Square Law.

INVERSE SQUARE LAW E = I / d² (lux)

Where: E = illuminance (lx), I = luminous intensity (cd), d = distance from source to surface (m).

Figure 2 — The inverse square law: double the distance = ¼ the illuminance.

If a lamp's distance to a surface is doubled, its illuminance is reduced by a factor of 4 (not 2). If trebled, the illuminance falls to 1/9.

The Cosine Law

When a surface is tilted at an angle θ to the perpendicular rays of light, the same flux is spread over a larger area. The illuminance is reduced by the factor cos θ.

COSINE LAW E = (I × cos θ) / d² (lux)

Figure 3 — The cosine law: light at an angle θ produces less illuminance than light directly below.

Use the inverse square law (E = I/d²) for the point directly below the lamp. Use the cosine law (E = I cos θ / d²) for any point offset at an angle.

3. Worked Examples — Illumination Calculations

Example 1 — Inverse Square Law (Direct)

A lamp of luminous intensity 1000 cd is suspended 2 m above a laboratory bench. Calculate the illuminance directly below the lamp.

Formula: E = I / d²

Substitute: E = 1000 / (2)² = 1000 / 4

E = 250 lux

Example 2 — Cosine Law (Offset Point)

A street lantern has a 2000 cd light source suspended 4 m above the ground. Determine the illuminance (a) directly below the lamp and (b) at a point 3 m to one side.

(a) Directly below — Point A:

Formula: E = I / d²

Substitute: E = 2000 / (4)² = 2000 / 16

E_A = 125 lux

(b) At point B, 3 m to one side:

Step 1 — Find distance d (Pythagoras):
d = √(4² + 3²) = √(16 + 9) = √25 = 5 m

Step 2 — Find cos θ:
cos θ = adjacent / hypotenuse = 4 / 5 = 0.8

Step 3 — Apply cosine law:
E = (I × cos θ) / d² = (2000 × 0.8) / 5² = 1600 / 25

E_B = 64 lux

Example 3 — Finding Intensity and Distance

A discharge lamp is suspended 4 m above a bench. The illuminance directly below is 300 lx. Find (a) the luminous intensity and (b) the distance along the bench where the illuminance falls to 153.6 lx.

(a) Find I:

Rearrange: I = E × d²

Substitute: I = 300 × 4² = 300 × 16

I = 4800 cd

(b) Find horizontal distance x where E = 153.6 lx:

Use cosine law rearranged:
E = (I × cos θ) / d² where cos θ = h/d = 4/d

Substitute: 153.6 = (4800 × 4/d) / d² = 19200 / d³

Rearrange: d³ = 19200 / 153.6 = 125, so d = ∛125 = 5 m

Find x (Pythagoras): x = √(5² − 4²) = √(25 − 16) = √9

x = 3 m along the bench

Example 4 — Quick Practice

A lamp of 800 cd is mounted 5 m above a factory floor. What is the illuminance directly below?

E = 800 / 5² = 800 / 25

E = 32 lux

Example 5 — Cosine Law Practice

Using the lamp from Example 4 (800 cd, 5 m height), find the illuminance at a point 4 m horizontally from the base.

Find d: d = √(5² + 4²) = √(25 + 16) = √41 ≈ 6.40 m

Find cos θ: cos θ = 5 / 6.40 ≈ 0.781

Apply: E = (800 × 0.781) / 6.40² = 624.8 / 41

E ≈ 15.24 lux

Recommended Illuminance Levels (IES)

Task	Working Situation	Illuminance (lx)
Casual vision	Storage rooms, stairs, washrooms	100
Rough assembly	Workshops, garages	300
Reading / writing / drawing	Classrooms, offices	500
Fine assembly	Electronic component assembly	1000
Minute assembly	Watch making	3000

4. The Lumen Method

The inverse square law is suitable for outdoor schemes with no reflecting surfaces. For interior lighting, light reflected from walls and ceilings adds secondary illumination. The most widely used method for interior design is the Lumen Method.

LUMEN METHOD Number of luminaires = (E × A) / (F × UF × LLF)

Where:

E = required illuminance (lx) — from IES code or architect's specification

A = total working area (m²)

F = lumen output of each luminaire (lm) — from manufacturer's data

UF = Utilisation Factor (always < 1, typically 0.9) — accounts for light absorbed by surfaces

LLF = Light Loss Factor (typically 0.8–0.9) — accounts for dirt, ageing and lamp depreciation

The LLF combines four separate losses: Luminaire Dirt Depreciation (LDD), Room Dirt Depreciation (RDD), Lamp Failure Factor (LFF), and Lamp Lumen Depreciation (LLD). Always use the manufacturer's initial lamp lumens because the LLF already accounts for depreciation over time.

Example 6 — Lumen Method Calculation

An electronic workshop measuring 9 m × 8 m × 3 m requires illumination of 550 lx at bench level. Each luminaire uses one 1500 mm 65 W natural tube with an initial output of 3700 lumens. UF = 0.9, LLF = 0.8. How many luminaires are needed?

Formula: N = (E × A) / (F × UF × LLF)

Substitute: N = (550 × 9 × 8) / (3700 × 0.9 × 0.8)

Calculate: N = 39,600 / 2,664 = 14.86

15 luminaires required (always round up)

Example 7 — Lumen Method Practice

A classroom measures 12 m × 8 m and requires 500 lx. Each luminaire provides 5100 lumens (white tube). UF = 0.85, LLF = 0.8. How many luminaires?

N = (500 × 12 × 8) / (5100 × 0.85 × 0.8)

N = 48,000 / 3,468 = 13.84

14 luminaires required

Characteristics of a 1500 mm 65 W Bi-Pin Fluorescent Tube

Tube Colour	Initial Lumens	Design Lumens	Colour Rendering	Appearance
Artificial daylight	2600	2100	Excellent	Cool
De luxe natural	2900	2500	Very good	Intermediate
De luxe warm white	3500	3200	Good	Warm
Natural	3700	3400	Good	Intermediate
Daylight	4800	4450	Fair	Cool
Warm white	4950	4600	Fair	Warm
White	5100	4750	Fair	Warm

Initial lumens measured after 100 hours. Design lumens measured after 2000 hours. Rated life: 7500 hours. Efficacy: 30–70 lm/W.

5. Classification of Lamps

Lamps are broadly classified into two families based on how they create light:

Figure 4 — Family tree of electric lamp types.

GLS (General Lighting Service) Lamps

GLS lamps produce light by the heating effect of an electric current. A fine tungsten wire is coiled and coiled again (coiled-coil) to form the incandescent filament. This arrangement reduces filament cooling and increases light output by allowing a higher operating temperature. Most electricity is converted to heat, with only a small proportion producing light.

GLS Filament Lamp

Efficacy: 14 lm/W Life: 1000 hours Colour rendering: Fairly good Light colour: Warm white to yellow How it works: Heating a tungsten filament until white-hot (incandescence)

Incandescent

Tungsten Halogen Lamps & the Halogen Cycle

Standard GLS lamps suffer from tungsten evaporation — the filament gets thinner, the bulb wall darkens, and eventually the filament fails. The tungsten halogen lamp overcomes this with a clever regeneration cycle.

The glass envelope contains a trace of halogen gas (iodine, chlorine, bromine or fluorine). Evaporated tungsten combines with the halogen near the cooler bulb wall, forming a tungsten halide compound. This drifts back towards the hot filament where it breaks apart, redepositing the tungsten onto the filament and releasing the halogen for another cycle.

Figure 5 — The halogen regeneration cycle prevents bulb blackening and extends lamp life.

Tungsten Halogen Lamp

Efficacy: 20 lm/W Life: 2000 hours Colour rendering: Good Light colour: Very white, intense Caution: Never touch with bare hands — grease causes cracking

Incandescent — Halogen Cycle

Tungsten Halogen Dichroic Spot

Efficacy: 20 lm/W Life: 2000 hours Available as: 12 V bi-pin (20/35/50 W), 230 V GU10/GZ10 (20/35/50 W) Application: Commercial & domestic spotlighting

Incandescent — Dichroic

6. Discharge Lamps

Discharge lamps do not use an incandescent filament. Instead, light is produced by the excitation of a gas or metallic vapour contained within a glass envelope. A voltage applied to two electrodes sealed into the tube excites the gas and produces light directly. The colour depends on the gas or vapour used:

Gases

Neon → Red Argon → Green/Blue Hydrogen → Pink Helium → Ivory

Metallic Vapours

Mercury → Blue Sodium → Yellow Magnesium → Grass Green

Fluorescent Lamps

Fluorescent lamps are linear arc tubes, internally coated with fluorescent phosphor powder, containing low-pressure mercury vapour and argon gas.

Figure 6 — How a fluorescent lamp creates visible light through UV conversion.

Compact Fluorescent Lamps (CFLs)

CFLs are miniature fluorescent lamps designed to replace ordinary GLS lamps. Available in stick, spiral and double-D shapes to fit existing fittings. They use the same discharge principle but are much more compact and energy-efficient.

High-Pressure Mercury Vapour Lamp (MBF)

The discharge takes place in a quartz glass arc tube inside an outer bulb coated with fluorescent powder. Contains mercury vapour and argon gas. Takes 5–7 minutes to reach full brightness. If switched off, it cannot restrike until the pressure drops (another ~5 minutes).

Mercury Vapour (MBF)

Efficacy: 38–56 lm/W Life: 7500 hours Colour rendering: Fairly good (cold appearance) Applications: Street lighting, shopping centres, area floodlighting Warm-up: 5–7 minutes to full brightness

Discharge — Mercury

Low-Pressure Sodium Lamp (SOX)

The discharge takes place in a U-shaped arc tube of special sodium-resistant glass, encased in a clear glass outer bulb. Cannot start in sodium alone — neon gas starts the initial red discharge which heats the sodium over 6–11 minutes until the lamp turns bright yellow. Must be operated horizontally.

Low-Pressure Sodium (SOX)

Efficacy: 61–160 lm/W (very high) Life: 6000 hours Colour rendering: Very poor (yellow only) Position: Horizontal or within 20° of horizontal Applications: Street and motorway lighting

Discharge — Sodium LP

The SOX lamp has the highest efficacy of all discharge lamps because sodium light's wavelength is close to the peak sensitivity of the human eye. However, its colour rendering is very poor — everything looks yellow.

High-Pressure Sodium Lamp (SON)

The discharge occurs in a sintered aluminium oxide arc tube (the only material that can withstand sodium at high pressure). An electronic pulse igniter (2 kV+) starts the lamp. Takes ~5–7 minutes to reach full brightness. Produces a pleasant golden-white light.

High-Pressure Sodium (SON)

Efficacy: 100–120 lm/W Life: 6000 hours Colour rendering: Fair (warm, golden) Position: Universal Applications: Food halls, hotel receptions, factories, airports, car parks

Discharge — Sodium HP

Lamp Comparison Summary

Lamp Type	Efficacy (lm/W)	Rated Life (h)	Colour Rendering	Warm-up
GLS Filament	14	1000	Fairly good	Instant
Tungsten Halogen	20	2000	Good	Instant
Fluorescent Tube	30–70	7500	Fair to Excellent	Seconds
Mercury Vapour (MBF)	38–56	7500	Fairly good	5–7 min
LP Sodium (SOX)	61–160	6000	Very poor	6–11 min
HP Sodium (SON)	100–120	6000	Fair	5–7 min
LED	80–150+	25,000–50,000+	Good	Instant

7. LED Lamps

A light emitting diode (LED) is a very small semiconductor source of illumination. It is a P-N junction diode that emits photons of light when activated — a process called electroluminescence.

Figure 7 — LED principle: electrons crossing the P-N junction release photons of light. Colour temperature measured in Kelvin.

Advantages of LED lamps: Very low energy consumption, very long life (25,000–50,000+ hours), small size, greater robustness, instant start, available in Edison Screw, Bayonet Cap, Bi-Pin, GU10 and GX53 fittings.

Colour temperature: Measured in Kelvin (K). A 2,700K lamp gives a warm yellow/white light (similar to GLS). A 3,000K+ lamp gives a bright white light suitable for kitchens and clinical environments.

LED lamps and strips can exhibit flickering and stroboscopic effects similar to discharge lamps. Electrical designers must be aware of this.

8. Control Gear & Starting Circuits

Discharge lamps require additional control gear for safe and efficient operation. All discharge circuits are inductive and require power factor correction (usually a capacitor across the supply).

Switch-Start Fluorescent Lamp Circuit

The most common circuit, using a glow-type starter switch:

Quick-Start Fluorescent Lamp Circuit

Uses a small auto-transformer to heat the cathodes continuously. After a short pre-heating period, the mercury vapour ionises and the arc strikes. An earthed metal strip near the tube assists starting. No starter switch is needed. The choke limits current; a capacitor provides PF correction.

Advantage: Faster, more reliable starting than switch-start.

Semi-Resonant Start Circuit

A specially wound transformer replaces the choke. Primary and secondary windings are wound in opposition (180° out of phase), boosting the voltage across the tube. The electron cloud forms quickly and the arc strikes very easily — even at temperatures as low as −5°C.

Key advantage: No PF correction capacitor needed — the circuit is predominantly capacitive with a naturally high power factor.

Control Gear for Discharge Lamps

Each discharge lamp type requires specific control gear:

Lamp	Control Gear	Starting Method
Mercury Vapour (MBF)	Choke ballast + PF capacitor	Starting electrode near main electrode
LP Sodium (SOX)	Transformer ballast + PF capacitor	Leakage transformer applies ~2× mains voltage
HP Sodium (SON)	Choke ballast + ignitor + PF capacitor	Electronic pulse igniter (2 kV+)

Loading of Discharge Circuits

Discharge circuits must carry the total steady current (lamp + control gear). Where exact data is unavailable, the On-Site Guide states that the rating of final circuits for discharge lamps may be taken as:

ASSUMED DEMAND Assumed demand = rated lamp watts × 1.8

Example 8 — Discharge Lamp Loading

An 80 W fluorescent lamp luminaire: Assumed demand = 80 × 1.8 = 144 W

The functional switch rating should be twice the total steady current of the inductive circuit to withstand the excessive contact wear caused by the inductive load.

9. Stroboscopic Effect

All discharge lamps on a.c. circuits flicker because the arc is extinguished every half-cycle as the current passes through zero. This can cause rotating or reciprocating machinery to appear stationary or running at the wrong speed — an extremely dangerous effect called the stroboscopic effect (IET Regulation 559.9).

Figure 8 — The stroboscopic effect: discharge lamps flicker 100 times per second on a 50 Hz supply. The lead-lag circuit eliminates this.

10. Installation & Maintenance

Installation Considerations

Discharge lamps may have restrictions on their operating position. The luminaire must suit the environment — corrosive, outdoor, low-temperature or decorative. Vibration causes premature lamp failure. Control gear must be rigidly fixed; plasterboard or wooden panels can amplify choke noise. Self-contained luminaires must have an adjacent means of isolation in addition to the functional switch (Reg. 537.2). Control gear must be in non-combustible enclosures or positioned to allow heat dissipation (Reg. 559.4.1).

Maintenance — Spot vs Group Replacement

Spot Replacement

Replace individual lamps as they fail Best for: small installations (shops, offices, domestic) Disadvantage: repeated small disturbances

Group Replacement

Replace all lamps at the same scheduled time Based on manufacturer's rated life + operating hours Best for: big stores, major retail (done at night/weekends) Advantage: reduced labour costs and disruption

11. Power Consumption, kWh & Design Rating

What is a Kilowatt-Hour (kWh)?

Electricity is sold in units of energy, not power. The unit of electrical energy used for billing is the kilowatt-hour (kWh), often simply called a "unit" of electricity. One kWh is the energy consumed when a load of 1 kW operates for 1 hour.

ENERGY CONSUMED Energy (kWh) = Power (kW) × Time (hours)

Definition

Power (P) — The rate at which electrical energy is consumed or converted. Measured in watts (W) or kilowatts (kW). 1 kW = 1000 W.

Definition

Kilowatt-Hour (kWh) — A unit of energy equal to 1000 watts used for 1 hour. This is the "unit" shown on electricity bills. Cost = kWh × price per unit.

To convert watts to kilowatts, divide by 1000. Always convert to kW before calculating kWh.

Figure 9 — The relationship between power, time and energy (kWh).

Basic Power Consumption Calculations

Example 9 — Simple kWh Calculation

A 100 W GLS lamp is left on for 8 hours. How much energy does it consume?

Convert to kW: 100 W ÷ 1000 = 0.1 kW

Apply formula: Energy = 0.1 kW × 8 h

Energy = 0.8 kWh

Example 10 — Comparing GLS vs LED

A householder replaces a 60 W GLS lamp with a 7 W LED lamp that gives the same light output. Both are used for 5 hours per day for 365 days. Electricity costs 34p per kWh. What is the annual saving?

GLS lamp:

Annual energy: (60 ÷ 1000) × 5 × 365 = 0.06 × 1825 = 109.5 kWh

Annual cost: 109.5 × £0.34 = £37.23

LED lamp:

Annual energy: (7 ÷ 1000) × 5 × 365 = 0.007 × 1825 = 12.775 kWh

Annual cost: 12.775 × £0.34 = £4.34

Annual saving = £37.23 − £4.34 = £32.89 per lamp

Design Rating of Discharge Lamps — The 1.8 Multiplier

When calculating the electrical loading of discharge lamp circuits, the power consumed is not just the lamp wattage. The control gear (chokes, ballasts, transformers, ignitors) also consumes power. The On-Site Guide (Appendix 1) states:

DISCHARGE LAMP DESIGN RATING Assumed demand = Rated lamp wattage × 1.8

The ×1.8 multiplier accounts for the additional current drawn by the control gear (choke/ballast) and the low power factor of the inductive circuit. This must be used when sizing cables, protective devices and switches for discharge lighting circuits, unless more accurate manufacturer's data is available.

Figure 10 — The ×1.8 multiplier accounts for control gear losses and poor power factor.

Example 11 — Design Rating of a Single Discharge Lamp

A luminaire contains a 65 W fluorescent tube. What is its assumed demand for circuit design purposes?

Apply ×1.8: 65 W × 1.8

Assumed demand = 117 W

Example 12 — Design Current for a Discharge Lighting Circuit

A single-phase 230 V circuit supplies 12 fluorescent luminaires, each with a 58 W tube. Calculate the design current (I_b) of the circuit.

Step 1 — Total rated wattage: 12 × 58 = 696 W

Step 2 — Apply ×1.8 for discharge lamps: 696 × 1.8 = 1252.8 W

Step 3 — Design current: I_b = P / V = 1252.8 / 230

I_b = 5.45 A

Example 13 — Energy Cost with ×1.8 Multiplier

A workshop has 15 luminaires each containing an 80 W fluorescent tube. The workshop operates 8 hours/day, 5 days/week, 50 weeks/year. Electricity costs 34p per kWh. Calculate the annual energy cost using the ×1.8 multiplier.

Step 1 — Total rated wattage: 15 × 80 = 1200 W

Step 2 — Apply ×1.8 design rating: 1200 × 1.8 = 2160 W = 2.16 kW

Step 3 — Annual hours: 8 × 5 × 50 = 2000 hours

Step 4 — Annual energy: 2.16 × 2000 = 4320 kWh

Step 5 — Annual cost: 4320 × £0.34

Annual energy cost = £1,468.80

Example 14 — Comparing With and Without ×1.8

Using Example 13's workshop (15 × 80 W, 2000 h/year), compare the energy cost calculated without the ×1.8 multiplier vs with it.

Without ×1.8 (lamp wattage only):

Energy: (15 × 80 ÷ 1000) × 2000 = 1.2 × 2000 = 2400 kWh

Cost: 2400 × £0.34 = £816.00

With ×1.8 (design rating):

Energy: (15 × 80 × 1.8 ÷ 1000) × 2000 = 2.16 × 2000 = 4320 kWh

Cost: 4320 × £0.34 = £1,468.80

Difference = £652.80/year — ignoring the ×1.8 significantly underestimates the true energy cost

Example 15 — Mixed Lamp Types on a Circuit

A retail unit has 8 × 100 W GLS lamps and 6 × 70 W SON discharge lamps, all on a single-phase 230 V supply. Calculate the total design current.

GLS (no multiplier): 8 × 100 = 800 W

SON (×1.8): 6 × 70 × 1.8 = 756 W

Total assumed demand: 800 + 756 = 1556 W

Design current: I_b = 1556 / 230

I_b = 6.77 A

Note: The ×1.8 only applies to discharge lamps (fluorescent, sodium, mercury). GLS and LED lamps use their rated wattage directly.

Summary — When to Apply ×1.8

Lamp Type	Apply ×1.8?	Reason
GLS Filament	No	Purely resistive — no control gear
Tungsten Halogen	No	Purely resistive — no control gear
LED	No	Driver losses minimal — use rated wattage
Fluorescent Tube	Yes ×1.8	Choke/ballast + low power factor
CFL (with external ballast)	Yes ×1.8	Separate control gear
Mercury Vapour (MBF)	Yes ×1.8	Choke ballast + low PF
LP Sodium (SOX)	Yes ×1.8	Transformer ballast + low PF
HP Sodium (SON)	Yes ×1.8	Choke + ignitor + low PF

Diversity — On-Site Guide Table 1B

When designing an individual circuit (sizing cables, selecting MCBs), you use the full design current (I_b) with no diversity — the circuit must be capable of carrying its entire connected load.

Diversity is applied at the distribution board level when calculating the maximum demand of the whole installation. It recognises that not every lamp in a building will be switched on at the same time. The On-Site Guide Table 1B gives allowances for different premises types:

Diversity reduces the maximum demand at the distribution board — it does NOT reduce the individual circuit design current (I_b). Always size each circuit's cable and protective device for the full load.

Figure 11 — Diversity applies at the distribution board level, not at individual circuit level.

On-Site Guide Table 1B — Diversity Allowances for Lighting

Type of Premises	Diversity Allowance for Lighting	Applied to
Individual domestic dwelling	66%	Total connected lighting load
Small shops, stores, offices	90%	Total connected lighting load
Hotels, boarding houses, guest houses	75%	Total connected lighting load

These percentages are applied to the total lighting current demand when calculating the maximum demand contribution of lighting at the distribution board or main switch.

Example 16 — Maximum Demand with Diversity (Retail Unit)

Using the retail unit from Example 15 — 8 × 100 W GLS and 6 × 70 W SON on a single-phase 230 V supply. Calculate: (a) the individual circuit design current, and (b) the maximum demand contribution of the lighting at the distribution board.

(a) Circuit design current (NO diversity):

GLS: 8 × 100 = 800 W

SON (×1.8): 6 × 70 × 1.8 = 756 W

Total assumed demand: 800 + 756 = 1556 W

Design current: I_b = 1556 / 230 = 6.77 A

I_b = 6.77 A — use this to size cables and MCBs

(b) Maximum demand at DB (WITH diversity):

Premises type: Small shop → Table 1B diversity = 90%

Apply diversity: 6.77 × 0.90 = 6.09 A

Maximum demand contribution of lighting = 6.09 A — use this when calculating the total maximum demand at the DB and sizing the main switch/cable

Example 17 — Domestic Dwelling Maximum Demand

A domestic dwelling has three lighting circuits with the following connected loads: Circuit 1 = 800 W, Circuit 2 = 600 W, Circuit 3 = 500 W (all LED/GLS — no ×1.8 needed). Supply is single-phase 230 V. Calculate (a) each circuit's design current and (b) the total lighting maximum demand at the consumer unit.

(a) Individual circuit design currents (NO diversity):

Circuit 1: I_b = 800 / 230 = 3.48 A

Circuit 2: I_b = 600 / 230 = 2.61 A

Circuit 3: I_b = 500 / 230 = 2.17 A

Each circuit is sized individually for its full load.

(b) Maximum demand at consumer unit (WITH diversity):

Total lighting load: 800 + 600 + 500 = 1900 W

Total current: 1900 / 230 = 8.26 A

Premises type: Domestic dwelling → Table 1B diversity = 66%

Apply diversity: 8.26 × 0.66 = 5.45 A

Total lighting maximum demand = 5.45 A — this is the lighting contribution when calculating the overall maximum demand of the installation

Example 18 — Hotel with Discharge Lighting

A small hotel has a total connected lighting load of 40 × 36 W fluorescent luminaires on a single-phase 230 V supply. Calculate (a) the total connected load and (b) the lighting maximum demand at the DB.

(a) Total connected load:

Rated wattage: 40 × 36 = 1440 W

Apply ×1.8 (fluorescent = discharge): 1440 × 1.8 = 2592 W

Total current: 2592 / 230 = 11.27 A

(b) Maximum demand at DB (WITH diversity):

Premises type: Hotel → Table 1B diversity = 75%

Apply diversity: 11.27 × 0.75 = 8.45 A

Lighting maximum demand = 8.45 A

Note the order: first apply ×1.8 (for discharge lamps), then apply diversity (for maximum demand at the DB). These are two separate adjustments for different purposes.

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