Single Family Dwelling Calculation, Optional Method.
By: Dennis Bordeaux | Jul 18, 2016
If you are preparing to take an electrical licensing exam, you will need to know how to calculate the feeder or service load for a single family dwelling. Refer to section 220.82(A)&(B) in the 2014 National Electrical Code (NEC). Here is a sample problem. Use the optional calculation to find the load on the following single family dwelling:
2800 sq. ft.
14 kW range
3 kW water heater
5 kW clothes dryer
1.5 kW dishwasher
15 kW central heat
29 amp, 240 volt air conditioning
Step 1:
Multiply the sq. ft. area by 3 VA per sq. ft. 2800 sq. ft. x 3 VA =8,400 VA. VA = volt-amperes.
220.82(B)(1).
Step 2:
Add in 1500 VA for each 2-wire, 20-amp small-appliance branch circuit and the laundry circuit. 1500 VA x 3 = 4500 VA.
220.82(B)(2).
Step 3:
Add in the appliances at nameplate value.
220.82(B)(3).
Range 14,000 VA
Water heater 3,000 VA
Clothes dryer 5,000 VA
Dishwasher 1,500 VA
Step 4:
Add all appliance loads together.
220.82(B).
Total = 36,400 VA
Step 5:
Take the first 10kW at 100%. 10,000 VA
Take the remainder (26,400 VA) at 40%. 10,560 VA
220.82(B).
Step 6:
Add the two values from Step 5 together to find the general load: 10,000 VA + 10,560 VA = 20,560 VA.
220.82(B).
Step 7:
Compare the heating load to the AC load and take the larger of the two loads. 220.82(C).
AC load at 100%. 29 amps x 240 volts = 6,960 VA
Heat at 65%. 15,000 VA x .65 = 9,750 VA (largest load).
220.82(C)(4).
Step 8:
Add the general load to the largest of the AC or heating load.
General load = 20,560 VA
Heating load = 9,750 VA
Total = 30,310 VA
220.82(A)
Step 9:
Divide the load in VA by the voltage. 30,310 VA ÷ 240 volts = 126 amps.
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Question:
Does 220:82 apply only to single family or to any dwelling unit including dwelling unit in multi family residence?
Section 220.82 of the NEC is not intended for multi-family dwellings.
NEC 220.84 and T220.84 are intended for this purpose.
I encourage you to refer to the Tom Henry 2017 NEC Calculations Book, chapter 6 Optional Method – Multifamily Dwelling for a complete breakdown of the optional method, as it applies to multifamily dwellings.
Thank you for the question.
Jerry Durham
JADE Learning Instructor
you forgot the 25% increase for the largest motor
Greetings,
Do keep in mind, this is the “Optional” method for calculating an electrical service for a single family dwelling. This optional method does not take into account an increase for the largest motor being served, at least not in the traditional sense.
The optional method takes a broader approach to calculating the electrical service. While this method does look at the individal nameplate ratings for the loads, it gives a more generalized allowance for overall Volt-Amps expected to be consumed by the dwellings, as determined by T220.84 and T220.86. Within this general allowance, there is built-in cushion for inrush current that will be experienced due to motor loads.
Thank you for your comments.
JADE Learning Instructor
Jerry Durham
My customer has five HVAC systems at his property. Three are for the main house, the other two are for a music studio. How do I calculate the Air Conditioning electrical load assuming obviously that not all the equipment will be used at the same time. What is the reference number of the electrical code.??
Couple of questions on 220.82. It says 120/240V or 208Y/120V, 3-wire system,:
– Does this mean that the whether 120/240V or 208Y/120V, it automatically means a single phase system only because of the “3-wire” feeder mentioned (with the neutral wire counted)?
– Does it mean that we can’t use 220.82 when calculating 208Y/120V, 3-Ph load for single family dwelling unit?
Thank you.