Technical Article

National Electrical Code Basics: Computing Branch Circuits, Feeders and Service for a One-Family Dwelling

July 01, 2022 by Lorenzo Mari

Learn how to calculate branch circuits, feeders, and service in a one-family dwelling.

A set of wires from the serving utility’s line enter a house at the service equipment. The service equipment contains the main overcurrent protection (circuit breakers or fuses) and switches to disconnect from the utility. Wires run to outlets through feeders and branch circuits to use the power inside the house.

 

Image used courtesy of Pixabay

 

Typically, a small residence’s cabinet contains the main and the branch-circuit overcurrent protection.

In larger dwellings, the branch circuits may originate in separate panels close to the load centers. When so, the wires between the main overcurrent protection and the panel are the feeders. The following example will show how to calculate the number of branch circuits, wire sizes per branch circuit, feeders, service entrance, and the overcurrent protection for all wiring in a one-family dwelling.

 

Example

Compute the branch circuits, feeders, service-entrance conductors, and wire protection for a 192 m², two-story, one-family dwelling.

Conditions:

  • Single-dwelling.
  • Supply system: 120/240V, 3-wire, single-phase, from service-drop wires.
  • No voltage drop calculation. The NEC® does not rule the voltage drop, although informative notes affirm that a total of 5% VD on feeders and branch circuits to the farthest outlet is reasonable. Yet, this is not mandatory.
  • Wiring through Electrical Metallic Tubing.
  • Section 250-118 allows EMT as equipment grounding conductor (EGC). For instance, there are no additional EGCs.
  • Minimum wire size: N° 12 AWG.
  • Minimum ampere rating: 20A.
  • Insulation: TW.
  • Wire material: copper.
  • VA = W.
  • No short-circuit analysis.
  • No extra load.
  • Open porches.
  • Conductor ampacity per Table 310.16.
  • Maximum number of conductors in EMT per Table C.1.
  • Grounding Electrode Conductor per Table 250.66.

The main panel contains the service equipment disconnecting means and the overcurrent protection. The main panel is in the laundry area, a readily accessible location nearby the entrance of the service conductors and close to the outdoor meter (See figure 1).

 

Figure 1. Location of service equipment. Image used courtesy of Lorenzo Mari

 

The main panel supplies panels A, B, and C – positioned close to the load centers – as well as the clothes dryer and the range.

Panel A, close to the kitchen, covers an area of 31 m², Panel B, located at the entrance, covers 88 m², and Panel C involves 73 m² on the second floor (see figure 2).

 

Figure 2. Panel distribution. Image used courtesy of Lorenzo Mari

 

Figure 3 shows a typical internal wiring diagram for panels A, B, and C.

 

Figure 3. Typical internal wiring diagram for panels A, B, and C. Image used courtesy of Lorenzo Mari

 

Tables 1, 3, 5, and 7 display the branch circuits for panels A, B, C, and the Main Panel, and tables 2, 4, 6, and 8 show their load analyses.

One single-pole breaker protects one 120 V branch circuit, and one 2-pole breaker protects one 240 V branch circuit.

These panels are used downstream from the service equipment. Then, their neutrals must be insulated from the cabinet.

 

PANEL A
Branch circuit Item Load (VA) Rating (A) Poles Voltage (V) Wire size (AWG) EMT (Inches) Remarks
1 Lighting (31m²) 1 023 20 1 120 2 x N° 12 ½”  
2,4 A/C 1 587 20 2 240 2 x N° 12 ½”  
3,5 Small appliances 3 000 20 2 2 x 120 2 x N° 12 ½” Duplex receptacles
6 Dishwasher 1 200 20 1 120 2 x N° 12 ½”  
7 Laundry 1 500 20 1 120 2 x N° 12 ½”  
8 Refrigerator 600 20 1 120 2 x N° 12 ½”  
9,11 Water heater 3 000 20 2 120/240 2 x N° 10 ½”  
10 Bathroom 0 20 1 120 2 x N° 12 ½” No additional load calculation required
12 Mangle (machine) 1 600 20 1 120 2 x N° 12 ½” Home version
Table 1. Branch circuits for Panel A. Image used courtesy of Lorenzo Mari

 

PANEL A - LOAD ANALYSIS
Topic Gross computed load (VA) Demand factor (%) Net computed load (VA) Net computed load Neutral (VA)
Lighting and general-purpose receptacles 1 023     1 023
Small appliance circuits 3 000     1 500
Laundry circuit 1 500     1 500
Bathroom circuit No additional load calculation required      
Gross general lighting load 5 523     4 023
First 3 000 VA   100 3 000 3 000
Remaining VA   35 883 358
Net general lighting load     3 883 3 358
Fixed appliances:        
Dishwasher 1 200      
Mangle 1 600      
Water heater 3 000      
Refrigerator 600      
Gross fixed appliance load 6 400      
Net fixed appliance load   75 (four appliances) 4 800 4 800
Air conditioning 1 587 100 1 587 0
25% of air conditioning 397   397 0
TOTAL LOAD     10 667 8 158
Table 2. Load analysis for Panel A. Image used courtesy of Lorenzo Mari

 

Feeder A

Phase conductor:

I = P/V = 10 667 VA/240 V = 44.4 A.

Continuous load: 44.4 A x 1.25 = 55.5 A.

Conductors (Table 310.16): 2 x N° 4 AWG, TW, copper.

Conductor protection (Table 240.6(A)): 60 A 2-pole circuit breaker (located in Main Panel).

Neutral conductor: 

I = P/V = 8 158 VA/240 V = 34 A.

Conductor (Table 310.16): 1 x N° 8 AWG, TW, copper. 

EMT (Table C.1): 1”.

 

PANEL B
Branch circuit Item Load (VA) Rating (A) Poles Voltage (V) Wire size (AWG) EMT (Inches) Remarks
1 Lighting 1 (50 m²) 1 650 20 1 120 2 x N° 12 ½”  
2,4 A/C 1 587 20 2 240 2 x N° 12 ½”  
3 Lighting 2 (38 m²) 1 254 20 1 120 2 x N° 12 ½”  
5 Receptacles 1 3 000 20 2 2 x 120 2 x N° 12 ½” Duplex receptacles
6 Receptacles 2 3 000 20 2 2 x 120 2 x N° 12 ½” Duplex receptacles
Table 3. Branch circuits for Panel B. Image used courtesy of Lorenzo Mari

 

PANEL B - LOAD ANALYSIS
Topic Gross computed load (VA) Demand factor (%) Net computed load (VA) Neutral load (VA)
Lighting and general-purpose receptacles (88 m²) 2 904     2 904
Receptacle circuits 6 000     3 000
Gross general lighting load 8 904     5 904
First 3 000 VA   100 3 000 3 000
Remaining VA   35 2 066 1 016
Net general lighting load     5 066 4 016
Air conditioning 1 587 100 1 587 0
25% of air conditioning 397   397 0
TOTAL LOAD     7 050 4 016
Table 4. Load analysis for Panel B. Image used courtesy of Lorenzo Mari

 

Feeder B

I = P/V = 7 050 VA/240 V = 29.4 A.

Continuous load: 29.4 A x 1.25 = 36.8 A.

Conductors: 2 x N° 8 AWG, TW, copper.

Conductor protection: 40 A 2-pole circuit breaker (located in Main Panel).

Neutral conductor:

I = P/V = 4 016 VA/240 V = 16.7 A.

Conductor: 1 x N° 12 AWG, TW, copper.

EMT: ¾”.

 

PANEL C
Branch circuit Item Load (VA) Rating (A) Poles Voltage (V) Wire size (AWG) EMT (Inches)
1 Lighting 1 (45 m²) 1 485 20 1 120 2 x N° 12 ½”
2,4 A/C 1 1 587 20 2 240 2 x N° 12 ½”
3 Lighting 2 (28 m²) 924 20 1 120 2 x N° 12 ½”
5 Receptacles 1 1 500 20 1 120 2 x N° 12 ½”
6 Receptacles 2 1 500 20 1 120 2 x N° 12 ½”
7,9 A/C 2 1 587 20 2 240 2 x N° 12 ½”
8 Spare            
10 Spare            
Table 5. Branch circuits for Panel C. Image used courtesy of Lorenzo Mari

 

PANEL C - LOAD ANALYSIS
Topic Gross computed load (VA) Demand factor (%) Net computed load (VA) Neutral load (VA)
Lighting and general-purpose receptacles (73 m²) 2 409     2 409
Receptacle circuits 3 000     3 000
Gross general lighting load 5 409     5 409
First 3 000 VA   100 3 000 3 000
Remaining VA   35 843 843
Net general lighting load     3 843 3 843
Air conditioning 3 174 100 3 174 0
25% of largest air conditioning 397   397 0
TOTAL LOAD     7 414 3 843
Table 6. Load analysis for Panel C. Image used courtesy of Lorenzo Mari

 

Feeder C

I = P/V = 7 414 VA/240 V = 30.9 A.

Continuous load: 30.9 A x 1.25 = 38.6 A.

Phase conductors: 2 x N° 8 AWG, TW, copper.

Conductor protection: 40 A 2-pole circuit breaker (located in Main Panel).

Neutral conductor: 

I = P/V = 3 843 VA/240 V = 16 A.

Conductor: 1 x N° 12 AWG, TW, copper.

EMT: ¾”.

 

MAIN PANEL (SERVICE EQUIPMENT)
Branch circuit Item Load (VA) Rating (A) Poles Voltage (V)

Wire Size

(AWG)

EMT (Inches)
1,3 Panel A 10 667 60 2 120/240 2 x N° 4 + 1 x N° 8 1”
2,4 Panel B 7 050 40 2 120/240 2 x N° 8 + 1 x N° 12 ¾”
5,7 Panel C 7 414 40 2 120/240 2 x N° 8 + 1 x N° 12 ¾”
6,8 Clothes dryer 5 000 30 2 120/240 3 x N° 10 ½”
9,11 Range 8 000 50 2 120/240 3 x N° 6 ¾”
10 Spare            
12 Spare            
Table 7. Feeders and branch circuits for Main Panel. Image used courtesy of Lorenzo Mari

 

SERVICE - LOAD ANALYSIS
Topic Gross computed load (VA) Demand factor (%) Net computed load (VA) Neutral load (VA)
Lighting and general-purpose receptacles (192 m²) 6 336     6 336

Receptacle circuits

2 x 1500 VA + 3 x 3 000 VA

12 000     7 500
Gross general lighting load 18 336     13 836
First 3 000 VA   100 3 000 3 000
Remaining VA   35 5 368 3 793
Net general lighting load     8 368 6 793
Appliance load Panel A 6 400 75 4 800 4 800
Clothes dryer 5 000 100 5 000 3 500 (220.61(B))
Air conditioning (4 units) 6 348 100 6 348 0
25% of largest air conditioning 397   397 0
Range 12 000 Table 220.55 8 000 5 600 (220.61(B))
TOTAL LOAD     32 913 20 693

 

Service-entrance Conductors

I = P/V = 32 913 VA/240 V = 137.1 A.

Continuous load: 137.1 A x 1.25 = 171.3 A.

Phase conductors: 2 x N° 4/0 AWG, TW, copper.

Main overcurrent protection and disconnecting means: 175 A 2-pole circuit breaker.

Neutral conductor: 

I = P/V = 20 693 VA/240 V = 86.2 A.

Conductor: 1 x N° 2 AWG, TW, copper.

EMT: 2”.

Grounding Electrode Conductor according to Table 250.66: 1 x N° 2 copper.

Figure 4 shows the internal wiring diagram for the main panel (Service equipment). This panel is breaker-type service equipment containing one main breaker and additional breakers to protect the feeders and individual branch circuits.

This panel is listed as service equipment. For instance, it has a neutral bonded to the cabinet and connected to the grounding electrode through a Grounding Electrode Conductor.

 

Figure 4. Internal wiring diagram for the main panel (service equipment). Image used courtesy of Lorenzo Mari

 

Conclusion

This article presented branch-circuit, feeders, service, and protection calculations for a one-family dwelling, following the minimum requirements by the NEC®.

Although not required by the NEC®, it is advisable to use wires and other equipment larger than the minimum to allow for future loads.

Supplementary essential studies in the electrical distribution system are voltage drop, short-circuit, and coordination.

 

Feature image used courtesy of Pixabay