Hi all,

I am about to take my Journeyman electrical test on Saturday. I have been going over the NEC and trying to figure out these calculations. I really need help on this one I just cant figure out where some of the numbers came from or how they determined what to use. I am sure I am just over looking something.

Single-Family Dwelling Service Load Example

The dwelling has a floor area of 2000 ft 2, exclusive of an unfinished cellar not

adaptable for future use, unfinished attic, and open porches. Appliances are a 12-kW

range and a 5.5-kW, 240-V dryer. Assume range and dryer kW ratings equivalent to

kVA ratings in accordance with 220.54 and 220.55.

Calculated Load [see 220.40]

General Lighting Load: 2000 ft 2 at 3 VA per ft 2 = 6000 VA

Minimum Number of Branch Circuits Required [see 210.11(A)]

General Lighting Load: 6000 VA ÷ 120 V = 50 A

·

This requires three 15-A, 2-wire or two 20-A, 2-wire circuits.

·

Small Appliance Load: Two 2-wire, 20-A circuits [see 210.11(C)(1)]

·

Laundry Load: One 2-wire, 20-A circuit [see 210.11(C)(2)]

·

Bathroom Branch Circuit: One 2-wire, 20-A circuit (no additional load

calculation is required for this circuit) [see 210.11(C)(3)]

Minimum Size Feeder Required

[see 220.40]

·

General Lighting = 6000 VA

·

Small Appliance = 3,000 VA

·

Laundry = 1,500 VA

Total = 10500 VA

·

3000 VA at 100% = 3,000 VA (I dont get this one!)

·

9000 VA - 3000 VA = 6000 VA at 35% = 2625 VA ( or this one)

Net Load = 5625 VA

Range (see Table 220.55) = 8,000 VA

Dryer Load (see Table 220.54) = 5,500 VA

Net Calculated Load = 19125 VA

Net Calculated Load for 120/240-V, 3-wire, single-phase service or

feeder

19125 VA ÷ 240 V =

**79.7 A**

Sections 230.42(B) and 230.79 require service conductors and disconnecting means

rated not less than 100 amperes.

Calculation for Neutral for Feeder and Service

·

Lighting and Small Appliance Load = 5625 VA

·

Range: 8000 VA at 70% (see 220.61) = 5,600 VA

·

Dryer: 5500 VA at 70% (see 220.61) = 3,850 VA

Total = 15075 VA

Calculated Load for Neutral

15075 VA ÷ 240 V = **62.8 A**