How to Find the Number of Circuit Breakers in a Load Center & Distribution Board Based on the Calculated Load for Home?
In the USA and Canada, the common supply voltage to the residential buildings and homes is 120V & 240V based on the NEC and CEC. This single phase supply (actually a split phase system) has three wires (Hot 1, Hot 2 and a Neutral) from the distribution transformer to the meter box and main service panel i.e. main switch breaker.
The available voltage levels in a single phase 120V/240V load center and panel box installed in the home are as follow:
- Voltage between Hot 1 (any one of Black or Red) and Neutral (White) = 120V Single Phase
- Voltage between Hot 1 (Black) and Hot 2 (Red) = 240V Single Phase (Split phase voltage)
- Voltage between Neutral (Black) and Ground (Green or Bare Conductor) = 0V.
Note: The NEC wiring color codes are used for the above statement.
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The following figure shows the basic configuration of 120V/240V single phase supply connected to a 24 breaker space or load circuits in the load center, breaker box or panelboard. This also shows that 120V single phase is available between Black and White (any of Hot 1 or Hot 2) and White (Neutral) while the 240V split phase is available via Black & Red (Hot 1 & Hot 2). The required number of wires and cables depends on the load circuit and device configuration i.e. some of the 240V circuit doesn’t need a Neutral wire while others do.
In a load center, a 120V single pole circuit breaker occupies a single position while a 240V double pole breaker acquires two positions in the load center.
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Determining the Number of Circuit Breakers / Circuits in the Panelboard
Note 1: The following example is based on VA (Volt x Amperes) which is known as apparent power. You may use the active or real power (in Watts) which is equal to Apparent power x power factor or VA x PF as power factor in residential buildings are almost unity (1). In this case, the apparent power in VA is equal to real power in W “Watts”.
Note 2: It is recommended to check the previous example for sizing a load center and distribution board as we have used the same terminology for this example.
Determine the suitable number of breakers and circuits in a load center or distribution board for an 1500 ft2 (139.35 m2) home floor plan having the following load circuits: Consider the power factor to be unity “1”.
- General lighting = 4.5 kVA
- Air conditioner = 6 kVA
- Electric range = 8.4 kVA
- Electric heater = 7.2 kVA
- Clothes dryer = 3.6 kVA
- Dishwasher = 1.2 kVA
- Garbage disposal = 0.96 kVA
- Two Small appliances circuits in the kitchen for refrigerator, blinder, etc.
- 20% expansion & Future load etc.
General Lighting Circuits:
Lighting circuits are generally 120V having 15A or 20A. To find the number of breakers and space in the load center, we have to calculate the load current by using the basic power formula as follows:
I = P / V
Putting the values:
I = 4500 VA / 120V
I = 37.5 A
These are the total amperage needed for the lighting circuits. Either 15A or 20A circuit breaker can be used with proper sized wire and suitable switches & plugs. Mostly, 15A breakers are used for the lighting points, this way, we will divide the total lighting amperes by 15A to determine the number of lighting circuits.
37.5 A / 15A = 2.5A (rounded up to 3).
It means, we will use 3 numbers of lighting circuit breakers.
In the given example, there are two small appliances load circuits i.e. 2 Nos of 120V, 20A circuit breaker.
There is only 1 number of 20A, 120V circuits needed for a clothes dryer in the given example.
Large Appliances circuits:
We mean the large appliances which operated on two poles 240V circuit breakers. All the air-conditioner, electric range/stove, electric heater, clothes dryer etc. are operated on 240V circuit and hence considered individually. Keep in mind that the 240V double pole breaker occupies two spaces (position) in the breaker box. Now let’s find the required number of circuit breakers in the load center (based on Amps) for large appliances by dividing the kVA rating of the device by 240V.
Using the basic current formula as above
I = P / V
I = 6 kVA / 240V
I = 25A.
Generally, the ampere rating of a circuit breaker should be selected at 125% (factor of 1.25) for the continuous load i.e. electric heaters and air conditioner etc. In case of air-conditioner,
125% x 25A = 31.25 A.
This way, the next and nearest available circuit breaker like 35A should be used for a 6kVA air-conditioner. Keep in mind the same (125%) is applicable to the wire size i.e. both 35A wire and circuit breaker should be used for 31.25 Amp load specified for Air-conditioner any other continuous and simultaneous load.
Electric Heater Circuit
Same like above calculation, the amps rating of the 7.2 kVA electric heater is as follow:
I = 7.2 kVA / 240V = 30A
Multiplying the safety factor of 125%.
1.25 x 30A = 37.5 A
As the ampacity of the circuit breaker should not exceed the current rating of the conductor used for it. A 40A breaker along a 40A wire size (the next & nearest standard size) must be used for an electric heater circuit rated for 7200 VA.
Electric Range Circuit:
Using the above method of basic calculations, It is a 35A, 240V circuit (8.4 kVA / 240V). Now with safety factor
1.25 x 35A = 43.75A
The next and nearest available & recommended size of circuit breaker and wire size is 50A for 8400VA, 240V electric stove.
Clothes Dryer Circuit
The clothes dryer is 15A, 240V (3.6kVA / 240V). Multiplying the safety factor:
125% x 15A = 18.75A
The recommended wire size and circuit breaker is 20A, 240V double pole.
Garbage Disposal Circuit
Bothe garbage disposal and dishwasher are 120V circuits. The garbage disposal is 10A, 120V circuit (1.2kVA / 120V). With safety factor:
125% x 10A = 12.5A
The next available & suitable wire size and circuit breaker is 15A, 120V single pole.
The dishwasher is 8A, 120V circuit (0.96kVA / 120V). With safety factor:
125% x 8A = 10A.
The proper size of single pole, 120V, 10A circuit breaker is needed with the same wire size.
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Total Number of Spaces for circuit breakers in the breaker box:
Now, we may calculate the total number of required spaces for circuit breakers in the load center as shown in the table below.
|Circuit||Qty||Circuit Breaker||Voltage||Space Required|
Based on the calculation for the above example, a minimum of 16 spaces of load center is required. Due to the future expansion and 20% extra load points, a 20-space panel box is the suitable option to be on the safe side as shown in the following figure.
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