We all have a mountain of electrical appliances around the house and many, if not all, of them, have some sort of motor running them. These may include furnaces, dishwashers, air conditioners, sump pumps, garbage disposals, and microwaves. According to the electrical code, each of these motorized gadgets needs a dedicated circuit just for their own use. Permanent heating appliances also have a fairly heavy electrical load, and most require their own dedicated circuits. Allowing these appliances to share a circuit with other devices can easily overload the circuit, since by nature they have a fairly heavy power draw, especially when they first startup. Older homes that have not had their wiring updated often have such appliances installed on circuits shared with other devices, and in these situations, it is quite common for circuit breakers to trip or fuses to blow. Here are some of the appliances that may require dedicated electrical circuits (check with local building codes for exact requirements):
So how is one to know what circuit size is required by each appliance? If you undersize a circuit feeding large central air conditioner, for example, you may find yourself with a situation in which your air conditioner circuit breaker trips whenever it is running at maximum power. Calculating the correct size for a dedicated appliance circuit involves calculating the maximum power demand that will be placed on a circuit, then choosing a circuit size that accommodates that demand, plus a margin for safety. Figuring the electrical requirements or demand of an appliance begins with an understanding of a simple relationship between amps, watts, and volts—the three key means of measuring electricity. A relationship principle known as Ohm's Law states that amperage (A) x volts (V) = watts (W). Using this simple relationship principle, you can calculate the available wattage of any given circuit size:
The simple A x V = W formula can be restated in a number of ways, such as W ÷ V = A, or W ÷ A = V. The Spruce / Michela ButtignolChoosing the correct size for a dedicated appliance circuit involves fairly simple arithmetic to make sure that the appliance's electrical demand is well within the capacity of the circuit. The load can be measured in either amp or watts, and it is fairly easy to calculate based on the information printed on the appliance motor specification label. Motors have a nameplate rating that is listed on the side of the motor. It lists the type, serial number, voltage, whether it is AC or DC, the RPM's, and, most importantly, the amperage rating. If you know the voltage and amperage rating, you can determine the wattage or total capacity needed for the safe operation of that motor. Heating appliances generally have their wattage ratings printed on the faceplate. For example, think of a simple hairdryer rated at 1,500 watts running on a 120-volt bathroom branch circuit. Using the W ÷ V = A variation of Ohm's law, you can calculate that 1,500 watts ÷ 120 volts = 12.5 amps. Your hair dryer running a maximum heat can draw 12.5 amps of power. But if you consider that a vent fan and bathroom light fixture might also be operating at the same time, you can see that a 15-amp bathroom circuit with a total capacity of 1,800 watts might be hard-pressed to handle such a load. Let's imagine that our sample bathroom has a vent fan that draws 120 watts of power, a light fixture that has three 60-watt bulbs (180 watts total), and an electrical outlet where that 1,500-watt hairdryer might be plugged in. All of these could easily be drawing power at the same time. The likely maximum load on that circuit could reach 1,800 watts, putting it right at the maximum that a 15-amp circuit (providing 1,800 watts) could handle. But if you put a single 100-watt lightbulb in the bathroom light fixture, you create a situation where a tripped circuit breaker is likely. Electricians usually calculate circuit load with a 20 percent safety margin, making sure that the maximum appliance and fixture load on the circuit is no more than 80 percent of the available amperage and wattage provided by the circuit. In our sample bathroom, a 20-amp circuit providing 2,400 watts of power can quite easily handle 1,800 watts of demand, with 25 percent safety margin. This is the reason why most electrical codes call for a 20-amp branch circuit to serve a bathroom. Kitchens are another location where 120-volt branch circuits serving outlets are virtually always 20-amp circuits. In modern homes, it is normally only general lighting circuits that are still wired as 15-amp circuits. Exactly the same principle is used to calculate the demand on a circuit serving a single appliance, such as a microwave oven, garbage disposal, or air conditioner. A large microwave oven with a built-in vent fan and light fixture can easily demand 1,200 to 1,500 watts of power, and an electrician wiring a dedicated circuit for this appliance would likely install a 20-amp circuit that provides 2,400 watts of available power. On the other hand, a large 1 hp garbage disposer drawing 7 amps (840 watts), can easily be served by a dedicated 15-amp circuit with 1,800 watts of available power. The same method of calculation can be used for any dedicated appliance circuit serving a single appliance. For example, a 240-volt electric water heater rated for 5,500 watts can be calculated in this way: A = 5,500 ÷ 240, or A = 22.9. But because the circuit requires a 20 percent safety margin, the circuit needs to provide at least 27.48 amps (120 percent of 22.9 = 27.48 amps). An electrician would install a 30-amp 240-volt circuit to serve such a water heater. Most electricians will slightly oversize the dedicated circuit size to allow for future changes. For example, if you have a fairly small 800-watt microwave oven, the electrician will normally install a 20-amp circuit even though a 15-amp circuit can easily handle this appliance. This is done so that the circuit will be able to handle future appliances that may be larger than the ones you have now.
220.14 Other Loads—All Occupancies Knowing how to perform load calculations in accordance with the National Electrical Code (NEC) plays a significant role in an electrician’s professional career. Before installing branch circuits, feeders or services on a job, loads must be calculated. Branch-circuit load calculation requirements are in Part II of Article 220. After calculating branch-circuit loads, conductor sizes and ratings for overcurrent protection must be determined. Results from calculations in Part II of Article 220 are used in conjunction with specifications from 210.19 to size branch-circuit conductors. Sizing of branch-circuit overcurrent protective devices must be done in accordance with 210.20 and Part II of Article 220. To size feeder (and service) conductors and overcurrent protection, loads must first be calculated in accordance with Part III or Part IV of Article 220. Last month’s column concluded by covering fixed multioutlet assemblies in 220.14(H). This month, the discussion continues with more requirements for general-use receptacles and outlets not used for general illumination. Load calculations for receptacle outlets are covered in 220.14(I), (J), and (K). A receptacle, as defined in Article 100, is a contact device installed at the outlet for the connection of an attachment plug. A single receptacle is a single contact device with no other contact device on the same yoke. A multiple receptacle is two or more contact devices on the same yoke (see Figure 1). Sometimes there is confusion pertaining to a single duplex receptacle on a branch circuit with no other devices. Although a duplex receptacle is installed and mounted by one strap or yoke, it is considered two receptacles. A branch circuit supplying only a duplex receptacle and no other device is not an individual branch circuit. An individual branch circuit, as defined in Article 100, is a branch circuit that supplies only one utilization equipment. Except for dwelling occupancies and, under certain conditions, banks and office buildings, the calculated load for receptacle outlets is 180 volt-amperes for each single or for each multiple receptacle on one yoke. The load calculation for a single receptacle is 180 volt-amperes. The load calculation for a duplex receptacle is 180 volt-amperes. The load for three receptacles on one yoke or strap is also calculated at 180 volt-amperes (see Figure 2). To calculate receptacles in accordance with 220.14(I), multiply the number of receptacles by 180 volt-amperes. For example, what is the calculated load for 30 15-ampere duplex receptacles in a retail store? Multiply the number of receptacles by 180 (30 × 180 = 5,400). The minimum calculated load for 30 15-ampere duplex receptacles in a retail store is 5,400 volt-amperes. The calculated load for 20-ampere receptacle outlets is no different than the calculated load for 15-ampere receptacle outlets. For example, what is the calculated load for 30 20-ampere duplex receptacles in a retail store? Although 20-ampere receptacles have a higher rating than 15-ampere receptacles, the calculated load is exactly the same. The minimum calculated load for 30 20-ampere duplex receptacles in a retail store is 5,400 volt-amperes (30 × 180 = 5,400) (see Figure 3). The calculated load is used to determine the maximum number of receptacles permitted on a branch circuit in all but dwelling occupancies. The ampere rating of the overcurrent protective device is what determines the maximum number of receptacles on a branch circuit. For example, the maximum number of receptacles on a 15-ampere breaker (or fuse), supplied by a nominal source voltage of 120, is 10. The calculation can be performed either by converting the ampacity rating to volt-amperes or by converting volt-amperes to amperes. Use Ohm’s Law to find amperes when volt-amperes and voltage are known (I = W ÷ E). Divide 180 by 120. The calculated load for one receptacle supplied by 120 volts is 1.5 amperes (180 ÷ 120 = 1.5). To find the maximum number of receptacles permitted on a 15-ampere breaker, divide the rating of the breaker by 1.5 amperes (15 ÷ 1.5 = 10). The maximum number of receptacles permitted on a 15-ampere, 120-volt breaker is 10 (see Figure 4). Because of provisions in Table 210.21(B)(3) and Table 210.24, 20-ampere receptacles are not permitted on a branch circuit having a rating of 15-amperes. Because of the higher rating on a 20-ampere breaker, more receptacles are permitted than on 15-ampere overcurrent devices. The calculated load per receptacle is the same, 1.5 amperes. To find the maximum number of receptacles permitted on a 20-ampere breaker, divide the rating of the breaker by 1.5 amperes (20 ÷ 1.5 = 13.3 = 13). The maximum number of receptacles permitted on a 20-ampere, 120-volt breaker is 13 (see Figure 5). In accordance with Tables 210.21(B)(3) and 210.24, these receptacles can be 15-ampere, 20-ampere or any combination thereof. Although a single receptacle and duplex receptacle do not share the exact same definition, they are counted the same in a load calculation. Unless specifically stated in 220.14(J) and (K), receptacle outlets shall be calculated at not less than 180 volt-amperes for each single or for each multiple receptacle on one yoke [220.14(I)]. For example, what is the branch-circuit load calculation for 30 15-ampere single receptacles in a retail store? The calculated load for 30 15-ampere single receptacles is the same as it would be for 30 15-ampere duplex receptacles, 5,400 volt-amperes (30 × 180 = 5,400) (see Figure 6). Some companies manufacture a single device containing four receptacles. Since there are four receptacles associated with this single piece of equipment, the load calculation is different. A single piece of equipment consisting of a multiple receptacle composed of four or more receptacles must be calculated at not less than 90 volt-amperes per receptacle [220.14(I)]. For example, what is the load calculation for a quad receptacle manufactured as a single device? Multiply the number of receptacles by 90 volt-amperes (4 × 90 = 360). Because there are four outlets in this single piece of equipment, the calculated load is 360 volt-amperes (see Figure 7). Two duplex receptacles in the same box and under one double-duplex receptacle cover plate, also has a calculated load of 360 volt-amperes. Not because it is one piece of equipment, but because the receptacle outlets are on two different yokes (2 × 180 = 360). Likewise, two single receptacles in the same box and under one cover must be calculated at 360 volt-amperes. The last sentence in 220.14(I) states that this load calculation provision does not apply to receptacles on small-appliance and laundry branch circuits in dwelling units. Receptacle outlets of 15- and 20-ampere ratings in dwellings are included in the general lighting-load calculations of 220.12. No additional load calculation is required for such outlets. Next month’s column continues the discussion of load calculations.
MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classes and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code and NFPA’s Electrical Reference. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by telephone at 615.333.3336, or via e-mail at . |
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