In this part of course " EE-3: Basic Electrical design course – Level II " which explains the electrical design of Non-Dwelling Buildings as per NEC Code.
In Article " Non-Dwelling Buildings Load Calculations- Part One ", I introduced a List for ordinary Non-Dwelling Buildings Loads which was as follows:
- Lighting loads,
- Receptacles Loads,
- Kitchen Loads,
- Heating, Ventilation and air conditioning Loads (Non-Coincident Loads),
- Motor Loads,
- Other Loads.
Again, but for above Non-Dwelling Buildings Loads, I will explain the following points:
- Where and how to distribute each type of load in a dwelling unit as per NEC code?
- How to calculate its Demand load for feeder and service sizing calculations?
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Important!!!
All design Calculations for Non-dwelling Buildings will be as per
NEC standard calculation method but I will explain design calculations as per
NEC Optional calculation method only for the following Non-Dwelling buildings
as permitted by NEC, Part IV. Optional Feeder and Service Load Calculations:
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I explained the Design Calculation for first type of Non-Dwelling Building loads which is Lighting loads in Article.
Today, I will explain the second type of Non-Dwelling Building Loads which is Receptacle Loads.
Second: Receptacle Loads – Part One
1- Introduction
Before we go ahead in explaining the design calculation for Receptacles Loads in Non-Dwelling buildings, there are some parts explained previously in the design calculations of Receptacles Loads of Dwelling buildings and are applicable also in Non-dwelling buildings, these parts need to be reviewed from the following Articles:
2- Receptacle Branch circuit ratings and permissible loads
In no case shall the load exceed the branch-circuit ampere rating. The following are the permissible Receptacle Branch circuit ratings in dwelling buildings:
15- and 20-Ampere Branch Circuits shall be permitted to supply:
1- Only lighting units: this case explained before in previous article
2- Only utilization equipment: with condition that the combined load for all utilization equipment must not exceed the branch circuit rating.
3- Combination of both: in the case the permissible rating of the utilization equipment will depend on its type as follows:
- If it is not fastened-in-place, it can have a rating of up to 80 % of the branch circuit rating as in TABLE 210.21(B)(2).
- If it is fastened-in-place, other than luminaires, it shall not exceed 50 % of the branch-circuit ampere rating.
B) 30-Ampere Branch Circuits
A 30-ampere branch circuit shall be permitted to supply utilization equipment in any occupancy. A rating of any cord-and-plug-connected utilization equipment shall not exceed 80 percent of the branch-circuit ampere rating.
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Important!!!
A single receptacle installed on an individual
branch circuit shall have an ampere rating not less than that of the branch
circuit.
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C) 40- and 50-Ampere Branch Circuits
A 40- or 50-ampere branch circuit shall be permitted to supply cooking appliances that are fastened in place in any occupancy.
In other than dwelling units, such circuits shall be permitted to supply fixed lighting units with heavy-duty lampholders, infrared heating units, or other utilization equipment.
D) Branch Circuits Larger Than 50 Amperes
Branch circuits larger than 50 amperes shall supply only non-lighting outlet loads especially on industrial premises where conditions of maintenance and supervision ensure that only qualified persons service the equipment.
Example for using Multioutlet branch circuits greater than 50 amperes:
A common practice at industrial premises is to provide several single receptacles with ratings of 50 amperes or higher on a single branch circuit, to allow quick relocation of equipment for production or maintenance use, such as in the case of electric welders. Generally, only one piece of equipment at a time is supplied from this type of receptacle circuit. The type of receptacle used in this situation is generally a configuration known as a pin-and-sleeve receptacle, although the Code does not preclude the use of other configurations and designs. Pin-and-sleeve receptacles may or may not be horsepower rated.
3- Selecting Receptacle rating for a branch circuit
3.1 Receptacle rating, general
Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B) (3), or, where rated higher than 50 amperes, the receptacle rating shall not be less than the branch-circuit rating.
Exceptions for above rule are as follows:
- Receptacles for one or more cord-and-plug- connected arc welders shall be permitted to have ampere ratings not less than the minimum branch-circuit conductor ampacity permitted by 630.11(A) or (B), as applicable for arc welders.
- The ampere rating of a receptacle installed for electric discharge lighting shall be permitted to be based on 410.62(C).
3.2 Single Receptacle on an Individual Branch Circuit
A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit. For example, a single receptacle on a 20-ampere individual branch circuit must be rated at 20 amperes.
3.3 Receptacle supplying Total Cord-and-Plug-Connected Load
A receptacle shall not supply a total cord-and plug- connected load in excess of the maximum specified in Table 210.21(B)(2) mentioned above.
3.4 Range Receptacle Rating
The ampere rating of a range receptacle shall be permitted to be based on a single range demand load as specified in Table 220.55 in below.
4- Voltage ratings for Receptacle Branch circuit
In dwelling units the voltage rating for Receptacle Branch circuit shall not exceed the following:
A) 120 Volts between Conductors
Circuits not exceeding 120 volts, nominal, between conductors shall be permitted to supply the following:
- The terminals of lampholders applied within their voltage ratings Section 210.6(B)(1) allows lampholders to be used only within their voltage ratings.
- Auxiliary equipment of electric-discharge lamps Auxiliary equipment includes ballasts and starting devices for fluorescent and high-intensity-discharge (e.g., mercury vapor, metal halide, and sodium) lamps.
- Cord-and-plug-connected or permanently connected utilization equipment
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Important!!!
In guest rooms or
guest suites of hotels, motels, and similar occupancies, the voltage shall
not exceed 120 volts, nominal, between conductors that supply the terminals
of the following:
The term similar
occupancies refer to sleeping rooms in dormitories, fraternities, sororities,
nursing homes, and other such facilities.
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B) 208-volt or 240-volt circuit, nominal, between conductors
For Receptacle Branch circuit that supplies High-wattage cord-and-plug-connected loads, such as electric ranges and some window air conditioners.
Exceptions for (B) are as follows:
- For lampholders of infrared industrial heating appliances as provided in 422.14.
- For railway properties as described in 110.19.
C) 277 Volts to Ground
Circuits exceeding 120 volts, nominal, between conductors and not exceeding 277 volts, nominal, to ground shall be permitted to supply the following:
- Listed electric-discharge or listed LED type luminaires
- Listed incandescent luminaires, where supplied at 120 volts or less from the output of a step-down auto-transformer that is an integral component of the luminaire and the outer shell terminal is electrically connected to a grounded conductor of the branch circuit
- Luminaires equipped with mogul-base screw shell lampholders
- Lampholders, other than the screw shell type, applied within their voltage ratings
- Auxiliary equipment of electric-discharge lamps
- Cord-and-plug-connected or permanently connected utilization equipment. Typical examples of the cord-and- are through-the-wall heating and air-conditioning units and restaurant deep fat fryers that operate at 480 volts, 3 phase, from a grounded wye system.
The following image shows some examples of luminaires permitted to be connected to branch circuits.
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Important!!!
The requirements
in (C) describes the voltage as “volts, nominal, to ground,” whereas (A),
(B), (D), and (E) describe voltage as “volts, nominal, between conductors.”
Luminaires listed for and connected to a 480-volt source may be used in
applications permitted by 210.6(C), provided the 480-volt system is in fact a
grounded wye system that contains a grounded conductor (thus limiting the
system “voltage to ground” to the 277-volt level).
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Exceptions for (C) are as follows:
- For lampholders of infrared industrial heating appliances as provided in 422.14.
- For railway properties as described in 110.19.
D) 600 Volts between Conductors
Circuits exceeding 277 volts, nominal, to ground and not exceeding 600 volts, nominal, between conductors shall be permitted to supply the following:
(A) The auxiliary equipment of electric-discharge lamps mounted in permanently installed luminaires where the luminaires are mounted in accordance with one of the following:
- Not less than a height of 6.7 m (22 ft) on poles or similar structures for the illumination of outdoor areas such as highways, roads, bridges, athletic fields, or parking lots.
- Not less than a height of 5.5 m (18 ft) on other structures such as tunnels
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Important!!!
For luminaire
installations that are not on poles or in a tunnel, the branch circuit Voltage is
limited to 277 volts to ground.
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(B) Cord-and-plug-connected or permanently connected utilization equipment other than luminaires.
(C) Luminaires powered from direct-current systems where the luminaire contains a listed dc-rated ballast that provides isolation between the dc power source and the lamp circuit and protection from electric shock when changing lamps.
Exceptions for (D) are as follows:
- For lampholders of infrared industrial heating appliances as provided in 422.14.
- For railway properties as described in 110.19.
E) Over 600 Volts between Conductors. Circuits exceeding
600 volts, nominal, between conductors shall be permitted to supply utilization equipment in installations where conditions of maintenance and supervision ensure that only qualified persons service the installation.
5- The Maximum allowable number of receptacles on a branch circuit
As per NEC section 220.14(I), Receptacle outlets load shall be calculated at not less than:
- 180 volt-amperes for each single receptacle
- 180 volt-amperes for each multiple receptacle (duplex or triplex) on one yoke.
- 90 volt- amperes per receptacle for multiple receptacles (four or more).
But, if a receptacle is dedicated for a specific device, then the actual load is used and If this dedicated load is continuous, then the 125% overrate is appropriate.
To calculate the Maximum allowable number of receptacles on a branch circuit, make the following steps:
- Multiply the branch circuit voltage and amperage
- Then divide by 180 volt-amperes.
- The result = the Max. Allowable single, duplex or triplex receptacles or a combination of them on a branch circuit.
Example#1:
How many receptacles can be placed on a 120-volt, 20-amp circuit? How many can be placed on a 120-volt, 15-amp circuit?
Solution:
Determine the maximum circuit power (for 20-amp circuit) = 120 V × 20 A = 2400 VA
Determine the maximum circuit power (for 15-amp circuit) 120 V × 15 A = 1800 VA
Then divide the power by the load per receptacle
For 20-amp circuit:
Maximum allowable number of receptacles = 2400 VA / 180 VA = 13.3
For 15-amp circuit:
Maximum allowable number of receptacles = 1800 VA / 180 VA = 10
So,
A 120-volt, 20-amp circuit can supply 13 receptacles.
A 120-volt, 15-amp circuit can supply 10 receptacles.
6- The Minimum number of receptacle branch circuits for bank or office buildings
As per NEC section 220.14(J), if the number of receptacles is unknown so for bank and office buildings, we can calculate the receptacles load by multiplying the area in ft2 by the unit value (1 VA/ft2).
To get the required number of receptacle branch circuits for bank or office buildings make the following steps:
- Calculate the total receptacles load for the whole building as explained above.
- Calculate the total receptacle circuit load in VA by multiplying its voltage by its amperage as follows:
- For 15-A Branch circuits = 120 V × 15 A = 1800 VA
- For 20-A Branch circuits = 120 V × 20 A = 2400 VA
- Divide the total receptacle load for the whole building by the maximum load per circuit to determine the minimum number of circuits.
Example#2:
Determine the total receptacle load for an 80 ft × 120 ft office building? And determine the number of 15-amp circuits needed to supply the load. Noting that the number of receptacles is unknown.
Solution:
(a) The number of receptacles is unknown, so a receptacle load of 1 VA/ft2 can be calculated:
Area = 80 ft × 120 ft = 9600 ft2
Total Receptacle load = 1 VA/ft2 × 9600 ft2 = 9600 VA
(b) To determine the number of circuits required, first calculate the allowable load for a single circuit:
The allowable load for a single circuit = 120 V × 15 A = 1800 VA
Divide the total receptacle load by the maximum load per circuit to determine the minimum number of circuits:
Minimum number of 15-A receptacle Circuits = 9600 VA / 1800 VA = 5.33
This is the minimum number, so round up to six circuits.
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Important!!!
As per NEC section 220.14(J), limit using the unit
value (1 VA/ft2) for banks and office buildings but it can be used also (as
approximate) for other types of buildings including dwelling ones.
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In the next article, I will continue explaining Receptacles load calculations for Non-Dwelling Buildings. Please, keep following.
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