Unit Feeder/Service Conductor Calculations
How do you size conductors for residential services and feeders?
By Mike Holt, Extracted from my Electrical Calculations textbook.
Most electrical licensing exams (e.g., journeyman, master electrician, contractor) require you to calculate residential loads and feeders. To do this, you might use the standard method contained in Article 220, Part II. Usually, you can use the optional method in Article 220, Part III. In either case, you are free to exceed the NEC requirements-these are minimum requirements, not design specifications.
Though the standard method involves more steps, many people use it exclusively to avoid using the wrong method. However, most residential construction qualifies for the optional method. So, its prudent to understand both methods. The standard method is where well start-it requires six sets of calculations:
General lighting and receptacles, small-appliance, and laundry
The NEC recognizes these circuits will not all be on (loaded) simultaneously. Thus, you may apply a demand factor to the total connected load [220.16]. To determine the feeder demand load, refer to Table 220.11 and follow these steps:
Example: What is the general lighting, small-appliance and laundry demand load for a 2,700 sq ft dwelling unit? (See Figure 9-13).
Graphics are not included in this Newsletter.
(a) 8,100 VA (b) 12,600 VA (c) 2,700 VA (d) 6,360 VA
Air-conditioning versus heat
Because air-conditioning and heating loads are not on simultaneously, you may omit the smaller of the two loads [220.21]. Calculate each of these at 100% [220.15].
Example: What is the service demand load for 5-hp, 230V A/C versus three 3 kW baseboard heaters? (see Figure 9-14).
(a) 6,400 VA (b) 3,000W (c) 8,050 VA (d) 9,000W
Per 220.17, you can use a 75% demand factor when four or more fastened in place appliances (e.g., dishwasher, waste disposal) are on the same feeder.
Example: What is the demand load for a waste disposal (940 VA), dishwasher (1,250 VA), and water heater (4,500 VA)?
(a) 5,018 VA (b) 6,690 VA (c) 8,363 VA (d) 6,272 VA
Per 220.18, the feeder or service demand load for electric clothes dryers in a dwelling unit shall not be less than 5,000W. Exception: if the nameplate rating exceeds 5,000W, use that rating as the load. You can omit this calculation if the unit has no electric dryer provision-however, its common to provide both gas and electric sources. If you see gas on the plans, verify electric will be omitted-do not just assume.
Example: What is the service and feeder demand load for a 4 kW dryer? (see Figure 9-16).
(a) 4,000W (b) 3,000W (c) 5,000W (d) 5,500W
For household cooking appliances rated over 13/4 kW, you can use the demand factors in 220.19, Table and Notes 1, 2, and 3.
Example: What is the service and feeder demand load for a 13.6 kW range in a dwelling unit?
(a) 8.8 kW (b) 8 kW (c) 9.2 kW (d) 6 kW
Feeder and Service Conductor Size.
400A and Less: For 3-wire, 120/240V, 1Ø systems, size the feeder or service conductors to Table 310.15(B)(6). For all others, use Table 310.16. Size the grounded (neutral) conductor to the maximum unbalanced load [220.22] per Table 310.16.
Over 400A: Size the ungrounded and grounded (neutral) conductors per Table 310.16.
What size THHN feeder or service
conductor (120/240V, 1Ø) does the NEC require for a 225A service
(a) 1/0 AWG (b) 2/0 AWG (c) 3/0 AWG (d) 4/0 AWG
Answer: (c) 3/0 AWG
You can use the easier optional method ( in 220.30) only when the total connected load is served by a single 3-wire, 120/240V or 208Y/120V set of service or feeder conductors with an ampacity of 100A or greater. Because this condition describes the typical residential service, the optional method is likely to apply. Using it can simplify the design process and save you time because you have so many fewer sets of calculations.
The calculated load shall not be less than 100% for the first 10 kW, plus 40% of the remainder of the following loads:
Include the largest of the following:
Sizing Service/Feeder Conductors
Now that weve seen how to determine residential loads, lets size the service/feeder conductors.
400A and Less. Size the ungrounded conductors per Table 310.15(B)(6) for 120/240V, 1Ø systems up to 400A. You must size the grounded (neutral) conductor to carry the unbalanced load per Table 310.16.
Over 400A. Size the ungrounded and grounded (neutral) conductors per Table 310.16.
An example will help illustrate how to do these. What size service conductor does a 1,500 sq ft dwelling unit need, if it contains the following loads?
(a) 6 AWG (b) 4 AWG (c) 3 AWG (d) 2 AWG
Answer: (c) 3 AWG
First 10,000 VA at 100% = 10,000 VA x 1.00 = 10,000 VA
Remainder VA at 40% = 21,600 VA x 0.40 = 8,640 VA
Demand load = 18,640 VA
Note: The feeder/service conductor is sized to 110A, 3 AWG.
Now that weve walked through the process of calculating residential feeders, you can see that doing so is fairly easy. You need to calculate the loads, then the feeder size. The NEC provides the requirements in Articles 220 and 230. Doing these calculations correctly can save you money during design and construction, while providing safe homes for the families that occupy them.