Commercial Load Calculations
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Oct 1, 2004 12:00 PM, By Mike Holt, NEC Consultant
Knowing how to correctly size
loads in
commercial applications is an essential skill for electricians
Even if you work with stamped drawings, you`ll eventually need to do
commercial load calculations in the field or on a licensing exam. The NEC covers
commercial calculations in Art. 220, but other articles also apply. For example, you must know the definitions in Art. 100, be familiar with what Art. 210 says about continuous
loads, and understand the overcurrent protection requirements set forth in Art. 240.
Two items associated with this
type of
calculation repeatedly need clarification:
* Voltage
The voltage to use for your calculations depends on the system design voltage. Thus when you
calculate branch-circuit, feeder, and service
loads, you must use a nominal system voltage of 120V, 120/240V, 208Y/120V, 240V, 347V, 480Y/277V, 480V, 600Y/347V, or 600V unless otherwise specified (220.2) (Fig. 1 below).
* Rounding
Refer to 200.2(B) to end the rounding mystery. When the ampere
calculation exceeds a whole number by 0.5 or more, round up to the next whole number. If the extra is 0.49 or less, round down to the next whole number. For, example, round 29.5A up to 30A, but round 29.45A down to 29A.
Specific
loads. Art. 220 doesn`t cover all specific
loads. For example, you`ll find motors in Art. 430 and
air conditioners in Art. 440. To know if you should look in another Article, use the NEC index.
Fig. 1. Don`t make the mistake of using actual field measurements of system voltage in your calculations. Unless specified otherwise,
loads shall be computed using the nominal system voltage such as 120V, 120/240V, 208Y/120V, 240V, 347V, 480Y/277V, 480V, 600Y/347V or 600V.
Art. 220 has specific requirements for most
loads, including the following:
Dryers. Size the branch-circuit conductors and overcurrent protection device for
commercial dryers to the appliance nameplate rating.
Calculate the feeder demand
load for dryers at 100% of the appliance rating. If the dryers run continuously, you must size the conductor and protection device at 125% of the
load [210.19(A), 215.3, and 230.42]. Table 220.18 demand
factors don`t apply to
commercial dryers.
Let`s apply what we`ve just learned. What size branch-circuit conductor and overcurrent protection does the NEC require for a 7kW dryer rated 240V when the dryer is in a multi-family dwelling laundry
room (Fig. 2)?
I=P÷E
7,000W÷240V=29A
The ampacity of the conductor and overcurrent device must be at least 29A (240.4). Per Table 310.16, a 10 AWG conductor at 60°C is rated 30A. Therefore, you must use a 30A breaker with a 10 AWG conductor.
Fig. 2. When determining proper branch-circuit protection and conductor size for a
commercial clothes dryer, you must use a demand
load of 100%. The reduced demand
factors for multiple dryers (Table 220.18) don`t apply in a
commercial setting.
Electric heat [424.3(B)]. Size branch-circuit conductors and the overcurrent protection device for
electric heating to not less than 125% of the total heating
load, including blower motors.
Calculate the feeder/service demand load for electric heating equipment at 100% of the total heating
load.
Kitchen equipment. Size branch-circuit conductors and overcurrent protection for
commercial kitchen equipment per the appliance nameplate rating.
To determine the service demand
load for
commercial kitchen equipment that has thermostatic control or intermittent use, apply the demand
factors from Table 220.20 to the total connected
kitchen equipment
load. The feeder or service demand
load can`t be less than the sum of the two largest appliance
loads. The demand
factors of Table 220.20 don`t apply to space-heating, ventilating, or air-conditioning equipment.
Laundry equipment. Size these circuits to the appliance nameplate rating. You can assume a laundry circuit isn`t a continuous
load and that
commercial laundry circuits are rated 1,500VA — unless noted otherwise in the project drawings or exam question.
Lighting. The NEC requires a minimum
load per
square foot for general lighting, depending on the
type of occupancy [Table 220.3(A)]. For the guestrooms of hotels, motels, hospitals, and storage warehouses, you can apply the general lighting demand
factors of Table 220.11 to the general lighting
load.
Assume the general lighting
load for
commercial occupancies other than guestrooms of motels, hotels, hospitals, and storage warehouses is continuous.
Calculate it at 125% of the general lighting
load listed in Table 220.3(A).
Receptacles. You don`t do all receptacle
load calculations the same way. The NEC has separate requirements, depending on the application.
Multi-outlet receptacle assembly. For service calculations, consider every 5
feet (or less) of multi-outlet receptacle assembly to be 180VA. When you can reasonably expect a multi-outlet receptacle assembly to
power several appliances simultaneously, consider each foot (or less) as 180VA for service calculations. Normally, a multi-outlet receptacle assembly isn`t a continuous
load [220.3(B)(8)].
Receptacle VA
load. The minimum
load for each
commercial or industrial general-use receptacle outlet is 180VA per strap [220.3(B)(9)]. Normally, receptacles aren`t continuous
loads.
Number of receptacles permitted on a circuit. The maximum number of receptacle outlets permitted on a
commercial or industrial circuit depends on the circuit ampacity. To
calculate that number, divide the VA rating of the circuit by 180VA for each receptacle strap.
Fig. 3. The minimum
load for each
commercial general-use receptacle outlet is 180VA per strap. In this example, the 15A, 120V breaker could accommodate 1,800VA of
load (120V x 15A = 1,800VA). Therefore, you could install a total of 10 receptacles on this circuit.
Let`s work a sample problem. How many receptacle outlets are permitted on a 15A, 120V circuit (Fig. 3)?
Total circuit VA
load for a 15A circuit:
120V×15A=1,800VA
Number of receptacles per circuit:
1,800VA÷180VA=10 receptacles
Receptacle sizing. The NEC permits 15A circuits in
commercial and industrial occupancies, but some local codes require a minimum 20A rating (310.5).
Receptacle service demand
load. In other than dwelling units, you can add — to the lighting
loads — receptacle
loads computed at not more than 180VA per outlet per 220.3(B)(9). You can also add fixed multi-outlet assemblies computed per 220.3(B)(8). Both of these must adhere to the demand
factors given in Table 220.11 or in Table 220.13.
Bank and
office general lighting and receptacles.
Calculate the receptacle demand
load at 180VA for each receptacle strap [220.3(B)(9)] if the number of receptacles is known, or 1VA for each
square foot if the number of receptacles is unknown [Table 220.3(A) Note b].
Signs. The NEC requires each
commercial occupancy that`s accessible to pedestrians to have at least one 20A branch circuit for a sign [600.5(A)]. The
load for the required exterior signs or outline lighting must be at least 1,200VA [220.3(B)(6)]. A sign outlet is a continuous
load. You must size the feeder
load at 125% of the continuous
load [215.2(A)(1) and 230.42].
The following question will allow you to practice what we`ve just covered. What`s the demand
load for one
electric sign?
1,200VA×1.25=1,500VA
Neutral calculations. The neutral
load is the maximum unbalanced demand
load between the grounded (neutral) conductor and any one ungrounded (hot) conductor — as determined by the calculations in Art. 220, Part B. This means you don`t consider line-to-line
loads when sizing the grounded (neutral) conductor. What about
load reduction? That depends on certain
factors, which we`ll look at next.
Reduction over 200A. You can reduce the feeder/service net computed
load for 3-wire, single-phase or 4-wire, 3-phase systems that supply linear
loads for that portion of the unbalanced
load over 200A, by a multiplier of 70%.
To see how this would work for an actual
installation, determine the neutral demand
load for a balanced 400A, 3-wire, 120/240V feeder.
Total neutral
load for 400A service:
First 200A at 100%: 200A×1.00=200A
Remainder at 70%: 200A×0.70=140A
Total demand
load: 200A×140A=340A
Reduction not permitted. You can`t reduce the neutral demand
load for 3-wire, single-phase, 208Y/120V or 480Y/277V circuits that consist of two line wires and the common conductor (neutral) of a 4-wire, 3-phase wye system. This is because the common (neutral) conductor of a 3-wire circuit connected to a 4-wire, 3-phase wye system carries about the same current as the phase conductors [310.15(B)(4)(b)].
Fig. 4. Sizing the grounded (neutral) conductor can be tricky. Just remember that you can`t reduce the neutral demand
load for 3-wire, single-phase, 208Y/120V or 480Y/277V circuits that consist of two line wires and the common conductor (neutral) of a 4-wire, 3-phase system.
As proof of this theory, see the example in Fig. 4.
In addition, you can`t reduce the neutral demand
load for nonlinear
loads supplied from a 3-phase, 4-wire, wye-connected system, because they produce triplen harmonic currents that add on the neutral conductor. This situation can require the neutral conductor to be larger than the ungrounded conductor
load (220.22 FPN 2).
Knowing the correct way to do
commercial load calculations makes you more valuable because you can play a key role in the field design, inspection, and implementation process. It`s one more skill that helps you do the
job right the first time.>>>>>>>161688