How to Determine the Number of Branch Circuits? – 3 Ways

Electrical Technology

How Many Branch Circuits Do You Need in a Dwelling Unit for Lighting Applications? 3-Methods with Examples

Before determining the required number of circuits and associated calculations, let’s define and differentiate between branch circuits, general-purpose lighting branch circuits, and individual branch circuits.

According to NEC Article 100 – Definitions:

Branch Circuit: Refers to the conductors between the overcurrent protection device and the final circuit element, such as an outlet. For example, the starting point of a branch circuit is the output wires from the breaker, and the endpoint is where those wires connect to the load, such as a light or receptacle.
General-Purpose Branch Circuit: Supplies power to more than one outlet or receptacle for general lighting purposes.
Individual Branch Circuit: Supplies power to a single specific piece of utilization equipment.

In this step-by-step tutorial, we’ll show how to determine the appropriate number of branch circuits for a dwelling unit using three methods, with solved examples based on NEC 2023.

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1: Outlets per Circuit Method

This method is straightforward and requires no complex calculations. It is commonly used by electricians to estimate the number of branch circuits and the outlets to be connected per circuit in a building.

The method assumes that 1A or 120W is the general-purpose load per outlet, while 1.5A or 180W is used for heavier loads, such as portable tools in a garage. For instance, a 15-amp lighting branch circuit is typically sufficient for a bedroom with 6 receptacles, two closet lights, and a ceiling fan with a light kit. If the load wattage is low, you can add more outlets to the circuit. Conversely, fewer outlets should be connected for higher-wattage loads.

In this method, just count the total number of required outlets and connect maximum of 10 outlets per circuit for low-wattage loads and 15 outlets per circuit for high-wattage loads.

Example:

Determine the required number of 15-amp lighting branch circuits for a dwelling unit with 90 general-purpose lighting and receptacle outlets (to be wired).

Solution:

If the anticipated load is high-wattage, connect 10 outlets per circuit:

Hence, you need a minimum of nine 15-amp general-purpose lighting circuits.

If the anticipated load is low-wattage, connect 15 outlets per circuit:

Based on the calculation, you need a minimum of six 15-amp general-purpose lighting circuits.

Related Post: How to Find the Number of Outlets on a Single Circuit Breaker?

2: Per Square Foot Method

This method is little bit complicated as calculation are involved. As we know from the previous ‘how-to tutorial’ on determining the number of lighting branch circuits in a dwelling unit, the minimum load requirement is 3 VA (volt-amperes) per square foot (3 VA/ft²), and it shall not be less than 33 VA/m² of occupied area, as specified in NEC Article 220.41.

For instance, a 600 ft² (55.8 m²) area typically requires one 15-amp branch circuit, while an 800 ft² (74.3 m²) area may require one 20-amp branch circuit. However, 20-amp circuits are less common for residential lighting applications.

Example:

How many 15-amp lighting branch circuits are needed for a house with a 3,000 ft² occupied area?

Solution:

Divide the total area by 600 ft² (the approximate area one 15-amp circuit can cover):

Based on the calculations, a minimum of five 15-amp general-purpose lighting branch circuits are needed for 3,000 ft² of covered area.

To be on the safe side, you may opt for more than five 15-amp general-purpose lighting branch circuits due to the numerous small-wattage electrical devices and appliances commonly found in homes today.

Related Post: How to Find the Number of Lights on a Single Circuit Breaker?

3: The 80% Rule Method

This method is based on the well-known 80% continuous load rule, detailed in NEC 210.19(A), 210.20(A), 215.2(A), 215.3, and 230.42(A). While primarily intended for commercial and industrial continuous loads (those lasting three hours or more, such as a water heater), applying this rule in residential settings enhances safety.

To use this method, sum the total wattage or amperage of the loads connected to the branch circuit. Limit the connected load to 80% of the circuit’s rated capacity.

For example, consider a 15-amp, 120V lighting branch circuit:

15A × 0.80 = 12A
12A × 120V = 1,440 W

Therefore, a maximum of 12A or 1,440W should be connected to this 15-amp, 120V circuit.

Similarly, for a 20A circuit:

20A × 80% = 16A
16A × 120V = 1,920 W

Thus, a maximum of 16A or 1,920W should be connected to a 20-amp, 120V lighting branch circuit.

Related Post: How to Determine the Number of Circuit Breakers in a Panel Board?

Good to Know:

NEC 210.11(A) mandates that the number of branch circuits must be sufficient for the total calculated load, determined by the circuit’s size or rating.
NEC 210.11(B) further stipulates that loads calculated by area (VA/ft² or VA/m²) must be distributed evenly across multi-outlet branch circuits supplied by the panelboard.
This load calculation encompasses all general-purpose 20-amp outlets and receptacles, as specified in NEC 210.11(B), 210.52(E) & (G), and 210.70.
Motors rated below 0.5 HP are included in the general lighting circuit calculation.
The number of circuits must always be adequate to handle the connected load. No circuit’s load may exceed the maximum specified in Article 220.11.
NEC 240.4(D) defines the maximum overcurrent protection (fuse or circuit breaker) ratings for conductors:
- 15-Amp for 14 AWG Copper or 12 AWG Aluminum
- 20-Amp for 12 AWG Copper or 10 AWG Aluminum
- 30-Amp for 10 AWG Copper