How to Determine the Number of Lighting Branch Circuits for Dwelling and Non-Dwelling Units – NEC 2023

According to NEC Article 100 – Definitions, a branch circuit refers to the conductors between the overcurrent protection device and the final circuit element, such as an outlet. A general-purpose branch circuit supplies more than one outlet or receptacle for general lighting purposes. Furthermore, an individual branch circuit is defined as a branch circuit that supplies power to only one specific piece of utilization equipment.

In the following step by step guide, we will show how to know the number of general-purpose lighting branch circuits in a dwelling and non-dwelling building with solved example based on NEC-2023.

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Lighting Branch Circuits in a Dwelling Unit

To determine the number of lighting branch circuits required in the dwelling and non-dwelling units, the National Electrical Code (NEC) 2023 provides specific guidelines in Article 220 and associated tables for branch circuits and lighting load for specified occupancies.

Article 220.41 specifies that the minimum load requirement for a dwelling unit is 3 VA (Volt-Amperes) per square foot (3 VA/ft²), which shall not be less than 33 VA/m² of occupied area.

Based on these values, the minimum requirements are as follows this equals to:

• One 15-Amp lighting branch circuit for every 600 ft².
• One 20-Amp lighting branch circuit for every 800 ft².

Since 20-Amp branch circuits are uncommon for lighting in residential units, we typically use 15-Amp lighting branch circuits.

Example 1

The following formulas can be used to determine the minimum number of 15-Amp lighting branch circuits required for a residence:

Suppose the total occupied area of a single-family unit with a basement is 3,000 ft² (278.7 m²). To find the total amperes required:

Amperes = (3VA × ft²) / 120V

(3VA × 3000 ft²) / 120V = 75-Amp

Now, Divide the calculated amperes by the rated amperes of the branch circuit (15 Amps).

Min. number of 15A lighting branch circuits = Amperes ÷ 15A

= 75A ÷ 15A

= 5

Alternatively, you may divide the total area in ft² by 600 ft² to get the number of 15A lightning branch circuits, and hence, will get the same answer i.e. 5.

Alternatively, you can divide the total area in ft² by 600 ft² to get the number of 15-Amp lighting branch circuits. The result will  be same as above

3000 ft² ÷ 600 ft² = 5

Thus, the total number of required lighting branch circuits for a dwelling unit with a total occupied area of 3,000 ft² is 5.

Lighting Branch Circuits For Non-dwelling Unit

The general lighting load for non-dwelling units (where the VA/ft² or VA/m² differs from that of a dwelling unit) with floor area determined by 220.5(C) can be calculated using Table 220.42(A). If the total load exceeds 3 kVA, a demand factor specified in Table 220.45 applies to the total branch circuit load for illumination. However, this is not applicable when determining the number of branch circuits for general lighting.

Example 1: Office Space

Given:

• • Area = 3,000 ft²
  • Lighting load = 1.3 VA/ft² (from NEC 2023 Table 220.41(A) for office buildings)
  • Voltage = 120V
  • Circuit Breaker = 15A

Steps:

1. Total Lighting Load:

3,000 ft² × 1.3 VA / ft² = 3,900 VA

2. Circuit Capacity:

Maximum load per circuit: 15A × 120V × 0.8 = 1,440 VA

3. Number of Branch Circuits:

Divide total load by capacity:

3,900 VA /1,440 VA = 2.7 circuits

Which is round up to 3 circuits.

Example 2: Retail Store

Given:

• • Area = 2,500 ft²
  • Lighting load = 1.9 VA/ft² (from Table 220.41(A) for retail spaces)
  • Voltage = 277V (common in commercial buildings)
  • Circuit Breaker = 20A

Steps:

1. Total Lighting Load:

2,500 ft² × 1.9 VA/ft² = 4,750 VA

2. Circuit Capacity:

Maximum load per circuit:

20A × 277V × 0.8 = 4,432 VA

3. Number of Circuits:

Divide total load by circuit’s capacity:

4,750 VA / 4,432 VA/circuit = 1 circuit.

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