How Many Fluorescent Lights on One Circuit | A Comprehensive Guide

The number of fluorescent lights that can be on a circuit depends on the circuit’s amperage and the wattage of the fixtures. For example, A 15-amp circuit can handle up to 12 amps, which means you can have up to 8 40-watt fluorescent fixtures on a single circuit.

This article will provide a nice and easy look at determining the number of fluorescent lights that can be safely installed on a single circuit. 

Calculating Total Fluorescent Wattage

Firstly the watts of all fluorescent tubes in a proposed group. Labeling on lamp ballasts gives exact specifications. For example, six common 4 ft 40W T12 bulbs would have:

6 bulbs x 40W each = 240W

This total wattage must stay under the maximum circuit allocation.

Determining Wire Run Distances

Long wire runs from the breaker panel to fluorescent fixtures can cause voltage drop issues. General guidelines are:

• Up to 100 ft run: Use standard 14 AWG cable
• 100 to 200 ft run: Go up to 12 AWG cable

Proper wire sizing is essential to efficiency and preventing hazards.

Allowing for Other Circuit Loads

Lighting circuits often also support other loads like exit signs, egress lights, and convenience receptacles. Built-in incandescent emergency lights can draw around 35W each. LED emergency lighting may use just 5W per fixture. These extras must be incorporated into total wattage calculations.

Considering Ballast Factor

Fluorescent light ballasts have a ballast factor (BF) that impacts capacity. Standard magnetic ballasts have a BF around 1.0. High-efficiency electronic ballasts can be 1.15 to 1.2 BF. Higher BF allows more tubes on existing wiring. Choose ballasts appropriately.

Common Lighting Circuit Configurations

Typical home fluorescent lighting circuits include:

• 15A lighting circuit: 10-12 x 4 ft T8/T12 tubes
• Multi-wire 20A lighting circuit: 20-24 x 4 ft tubes
• 15A continuous-use circuit: Only 6-8 x 4 ft tubes

Consult electrical codes for specifics.

Performing Load Calculations

Use this general formula to determine fluorescent fixture capacity per circuit:

1. Calculate the total wattage of bulbs (Section 3)
2. Derate wattage by 80% for continuous lighting loads per NEC
3. Ensure the total is below circuit capacity (Section 2)

For example:

12 x F32T8 40W bulbs = 12 x 40W = 480W 480W x 0.8 = 384W

Therefore, 12 bulbs work on a 15A lighting circuit. However, for safety, you should only put 80% load on a circuit, so the maximum number of fluorescent lights on a 15-amp circuit is 8.

Table of 40-Watt Fluorescent Lights per Circuit Amperage:

Circuit Amperage	Maximum Number Light
15	8
20	12
25	16
30	20

Managing Inrush Current

Inrush current is the initial spike when fluorescent tubes strike and rapidly heat filaments. It can be 2 to 5 times the normal operating current. Too much inrush can trip breakers. Solutions include:

• Spec breakers designed for fluorescent lighting
• Split startups across multiple circuits
• Use hybrid or programmed start ballasts

Considering Ambient Temperatures

Fluorescent fixtures generate significant heat. Ensure ambient temps don’t exceed ballast or tube specs over long periods. Excess heat accelerates lumen depreciation and shortens component life. Consider heat when locating fixtures.

Planning Adequate Ventilation

Dense fixture spacing can trap heat. Ensure housings and junction boxes dissipate heat properly. Validate temperatures inside enclosures stay within safety margins. Integrate cooling fans or external vents if needed for vent high-density fluorescent layouts.

Determining Minimum Light Levels

While circuit load calculations focus on maximum capacity, also check that the proposed fluorescent placement meets minimum lighting level guidelines. Assess lighting intensity output during design to satisfy illumination requirements per local codes.

Considering Emergency Backup

Integrate emergency backup and egress lighting based on fixture placement. Specify the capacity for these emergency fluorescents and pathway illumination in the circuit design. Typically backups add 5% to 10% to the total wattage.

Managing Voltage Drop

Excessive voltage drop along wire runs stresses components and reduces efficiency. Ensure drop stays under NEC guidelines:

• Feeders: 3% max
• Branch Circuits: 5% max

Perform voltage drop calculations as part of electrical design.

Getting Professional Assistance

For large or complex installations, consult certified electricians and lighting designers. Qualified pros can accurately assess site conditions and requirements, perform thorough load calculations, ensure NEC compliance, pull permits and oversee safe, long-lasting fluorescent lighting circuit integration.

FAQs – Frequently Asked Questions and Answers

1. What is the difference between a T8 and a T12 fluorescent lamp?

Answer: T8 and T12 are the two most common types of fluorescent lamps. T8 lamps are smaller and more energy-efficient than T12 lamps. However, T12 lamps are generally less expensive than T8 lamps.

2. What is the difference between a ballast and a starter?

Answer: A ballast is a device that regulates the current flow to a fluorescent lamp. The lamp must start and operate properly. A starter is a device that provides a momentary high-voltage surge to the lamp to help it start. 

3. What is the difference between a branch circuit and a feeder circuit?

Answer: A branch circuit is a circuit that supplies power to a specific area of a building, such as a room or a group of rooms. A feeder circuit is a circuit that supplies power to a group of branch circuits.

To Conclude

Loading fluorescent lighting circuits requires careful consideration of electrical codes, lighting needs, voltage drop, and wire sizing to ensure safe and reliable operation. By understanding these factors homeowners and electricians can make informed decisions, ensuring a well-lit and secure home environment.