26401 Load Calculations - Feeders and Services
What is the minimum ampacity for a feeder serving two (2) motors at 15HP, one (1) motor at 25HP, and one (1) motor at 40HP? All motors are rated at 480V, 3 Phase. 141 Amperes 153 Amperes 100 Amperes 128 Amperes
141 Amperes
What is the rating for a 250 kcmil THHN copper conductor in an ambient temperature of 101 Degrees Fahrenheit? 263.9 Amperes 278.4 Amperes 290 Amperes 232.05 Amperes
263.9 Amperes
In a three-wire, single-phase service, the conductor that carries only the unbalanced load between the two hot legs.
Neutral Conductor
Garage
A building or portion of a building in which one or more self-propelled vehicle scan be kept for use, sale, storage, rental, repair, exhibition, or demonstration purposes.
Feeder
All circuit conductors between the service equipment, the source of separately derived system, or other supply source and the final branch circuit overcurrent device.
Can be divided into three sections: Branch Circuits / Feeders / Services.
Basic Load Calculations
Using the 10' tap rule, what size copper THWN conductor is required for a tap from a 400A, 480V, 3Phase feeder to serve a 150A load? 1/0 2/0 No. 1 No. 2
No. 1
Load calculated at 2 circuit 1500VA.
Small Appliance Ciruit
Service
The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served
Service conductors
The conductors from the service point to the service disconnecting means.
True or False. The optional load calculation method is used if a school has HVAC equipment.
True
True or False. NEC Section 220.44 permits both general-purpose receptacles calculated at 180VA per outlet and fixed multi-outlet assemblies to be adjusted according to demand factors listed in NEC Tables 220.42 and 220.44.
True.
True or False. The receptacle loads in guest rooms in a motel are included in the general lighting load calculations.
True.
Voltage drop formula 2 VD = (2*L*K*I)/CM
VD = voltage drop L = load center or total length in feet K= constant (12.9 for copper conductors; 21.2 for aluminum conductors) I = current CM= area of the conductor in circular mils (from NEC Chapter 9, Table 8)
What is the net calculated current value that may be used to size a neutral conductor for a 240V residential service that has a net calculated general lighting and appliance load of 4,000VA, an electrical range with a nameplate VA rating of 8,000VA, and a clothes dryer with a nameplate VA rating of 6,000VA?
(14,000VA * 0.70) + 4,000VA = 13,800VA / 240V = 57.5 amperes
According to the 25′ tap rule, the tap conductors must be rated at no less than one-third the ampacity of the overcurrent device protecting the feeder. If the feeders supplying a continuous load are protected by a 125-ampere circuit breaker, what is the minimum ampacity of tap conductors connected to these feeders?
1/3 * 125 amp = 41.67
Show window loads should be calculated at _____ per linear foot of show window.
200VA
A 240-volt feeder supplying twelve 120-volt, 30A shore power receptacles in a marina requires a minimum ampacity of _____ amperes if the demand factor is 80%.
240v /2 = 120 volts Demand Factor: 0.8 12 * 30A * 120V = 43,200VA. 3,200VA * 0.8 = 34,560VA. 34,500 / 240 = 144 amperes.
What is the minimum general lighting load, in VA per square foot, required by the NEC for an office building? 3-1/2 4-1/2 2 3
3-1/2
Switchboard
A large single panel, frame, or assembly of panels on which are mounted on the face, back, or both, switches, overcurrent and other protective devices, buses, and usually instruments. Switchboards are generally accessible from the rears well as from the front and are not intended to be installed in cabinets.
Separately Derived System
An electrical source, other than a service, having no direct connection to circuit conductors of any other electrical source other than those established by grounding and bonding connections.
Tap conductors
As defined in NEC Article 240, a tap conductor is a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as describe din NEC Section 240.4
Operates without interruption for more than three hours consecutively.
Continuous Duty Load
In calculating feeder-circuit conductors, if noncontinuous loads equal 450 amperes and continuous loads equal 550 amperes, what is the total calculated feeder load that the conductors must handle? A. 1137.50 amperes B. 1100 amperes C. 950 amperes D. 1317.50 amperes
In order to calculate the total feeder load, you must add 100% of noncontinuous loads to 125% of continuous loads. 450 + (1.25 × 550) = 1137.50 amperes
Load calculated at 1 circuit 1500VA.
Laundry Ciruit
You must refer to NEC Table 310.15(B)(16) to answer this question. A single-phase feeder load is calculated at 100 amperes. If the conductor specifications call for using THWN copper conductors as feeders, what is the minimum size THWN conductor that must be installed? A. No. 1 AWG B. No. 2 AWG C. No. 3 AWG D. No. 4 AWG E. No. 6 AWG
No. 1 AWG
A kitchen equipment load is subject to the application of a _____ demand factor.
The kitchen equipment load is subject to a 65% demand factor.
Service point
The point of connection between the facilities of the serving utility and the premises wiring.
Underground service conductors
The underground conductors between the service point and the first point of connection to the service-entrance conductors in a terminal bus.
Service lateral
The underground conductors between the utility electric supply system and the service point.
What is the minimum demand required for the portion of a feeder that supplies 30' of show windows? 6,000 VA 5,400 VA 3,000 VA 2,700 VA
ft x va = va 30ft x 200VA = 6,000VA NEC 220.14(C)(2) At 200 Volt-amperes per linear 300 mm (1 ft) of show window
Refer to NEC Table 220.42 to verify your answer. In multi-family dwellings, a demand factor of _____ is applied to any excess general lighting and small appliance load over 120,000VA, after the first 3,000VA has been calculated at 100% and the next 117,000VA has been calculated at a demand factor of 35%.
25%
Service drop
The overhead conductors between the utility electric supply system and the service point.
What is the minimum general lighting load in VA per square foot, required by the NEC for a single family dwelling? 3 3-1/2 1 2
3
What is the minimum general lighting load, in VA per square foot, required by the NEC for a store? 3 2-1/2 5 2
3
Dwelling unit
A single unit providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking, and sanitation. A one-family dwelling consists solely of one dwelling unit. A two-family dwelling consists solely of two dwelling units. A multi-family dwelling is a building containing three or more dwelling units
Building
A structure that stands alone or that is separated from adjoining structures by firewalls.
Feeder conductor size is adjusted when? The load supplied is a continuous-duty load. More than three current-carrying conductors are in a raceway. The ambient temperature surrounding conductors exceeds temperature ratings. Voltage drop exceeds 3% for feeders or 5% for the combination of the feeder and branch circuit. All of the above
All of those above
When calculating the feeder load for a motor control center supplying several motors, which of the following statements is correct? Determine the sum of all motor loads plus 25% of the largest motor. Determine the sum of all motor loads plus 125% of the largest motor. Determine the sum of all motor loads. Determine the sum of all motor loads plus 30% of the largest motor.
Determine the sum of all motor loads plus 25% of the largest motor.
What is the general lighting load in VA for a house with an outside dimension of 2,500 sq ft, including a 400 sq ft garage?
Dwelling Unit General Lighting Load: 3VA per sq ft 2,500 - 400 = 2,100 * 3VA = 6,300VA
If a single-family dwelling has 1,350 sq ft of living area, two small-appliance circuits in the kitchen, and one laundry room receptacle, what is the calculated net lighting and small appliance load, including the laundry room circuit, rounded off to the nearest whole VA?
Dwelling Unit General Lighting Load: 3VA per sq ft Small Appliance Circuits: 1,500VA Laundry Room Circuit: 1,500VA The first 3,000VA is calculated at 100%, and the remainder is factored at 35%. (3VA × 1,350) + 4,500VA = 8,550VA 8,550VA - 3,000VA = 5,550VA × 0.35 = 1,942.5VA + 3,000VA = 4,942.5VA = 4,943VA
True or False. NEC Article 220, Part IV provides a separate method for computing farm loads.
False.
NEC Section _____ permits an alternative method for calculating loads in multi-family dwellings.
NEC Section 220.84
The 100' (30 m) Tap Rule
NEC Section 240.21(B)(4) lists specific conditions Limited to high-bay manufacturing buildings with wall heights over 35' (11 m). Only qualified persons can service and maintain the systems. Tap length limited to 25' (7.5 m) horizontally; total tap length cannot exceed 100'(30 m).
The load for a branch circuit installed for the supply of exterior signs or outline lighting must be computed at a minimum of _____.
NEC Section 220.14(F) requires a minimum of 1,200VA.
Service Equipment
The necessary equipment, usually consisting of a circuit breaker or switch and fuses, and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and means of cutoff of the supply.
Which of the following factors does not affect conductor ampacity adjustments? A. Ambient temperature B. Number of phases C. Continuous or noncontinuous load D. Number of conductors in a raceway
The number of phases is not listed as a factor that affects the allowable current-carrying capacity (ampacity) of a conductor, while the others listed do affect it.
When calculating the conductor size when two motors are to be connected to the same feeder circuit, the full load current of the largest motor must be multiplied by a factor of _____.
125% or 1.25
What is the ampacity for each of eight (8) No. 2 THWN copper current carrying conductors installed in a single conduit? 80.5 Amperes 91 Amperes 92 Amperes 57.5 Amperes
80.5 Amperes
Overhead service conductors
The overhead conductors between the service point and the first point of connection to the service-entrance conductors at the building or other structure.
Using the 25' tap rule, what is the minimum size copper THWN conductor required for a tap from a 400A, 480V, 3Phase feeder? 1/0 2/0 No. 1 No. 2
1/0
Voltage drop formula 1 VD = (2*L*R*I)/1,000
VD = voltage drop (in volts) L = load center or total length in feet R = conductor resistance per NEC Chapter 9, Table 8 I = current The number 1,000 represents 1,000' or less of conductor.
After calculating the first 3,000VA of the general lighting and small appliance load at 100% for a multi-family dwelling, what is the demand factor that may be applied to the next 117,000VA? A. 5% B. 25% C. 35% D. 50%
35% According to NEC Table 220.42, the first 3,000VA (out of 120,000VA) must be calculated at 100%, and the next 117,000VA (up to 120,000VA) has a demand factor of 35% applied to it.
