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Load Calculations - Branch and Feeder Circuits (Chapter 1) Ampacity - The maximum current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating Branch circuit - circuit conductors between the final overcurrent device protecting the circuit and the outlet(s) continuous load - load where the maximum current is expected to continue for three hours or more device - unit of an electrical system, other than a conductor, that carries or controls electric energy as its principal function individual branch circuit - branch circuit that supplies only one piece of utilization equipment outlet - point on the wiring system at which current is taken to supply utilization equipment overcurrent - any current in excess of the rated current of equipment or the ampacity of a conductor. it may result from overload, short circuit, or ground fault. receptacle - contact device installed at an outlet for connection as a single contact device. a single receptacle is a single contact device with no other contact device on the same yoke. a multiple receptacle is a single device containing two (duplex) or more receptacles. receptacle outlet - outlet where one or more receptacles are installed. utilization equipment - equipment that utilizes electric energy for electronic, chemical, heating, lighting, electromechanical, or similar purposes.
what is the maximum continuous load that may be connected to a 20A, 120V fuse? - 20A x 80% or .80 = 16A (continuous loads are calculated at 125% of the maximum current rating of the load. the reciprocal of 125% is equivalent to 80%) The voltage drop for a 208V, 1 ott circuit with a load of 28A, using No. 8 stranded copper conductors at the circuit length of 160' is....... - VD (Voltage Drop) = (2xLxRxI) / 1000, L- load center or total length in feet, R- conductor resistance (NEC chapter 9, table 8), I- current. (2x160ftx.776x80A)/1000 = 6.97V The ampacity of a 120V, 1 ott load rated at 800VA is..... - 800VA / 120V = 6.67A Appliance - utilization equipment, generally other than industrial, normally built in standardized sizes or types, that is installed or connected as a unit to perform one or more functions such as clothes washing, air conditioning, food mixing, deep frying, or other purposes Appliance branch circuits - Branch circuits supplying energy to one or more outlets to which appliances are to be connected. such circuits are to have no permanently connected lighting fixtures that are not part of an appliance Demand factors - ratio of the maximum demands of a system, or part of a system, to the total connected load of a system or the part of the system under consideration General-purpose branch circuit - Branch circuits that supply two or more receptacles or outlets for lighting and appliances what is the calculated load, in amps, for a 120V circuit supplying 8 recessed fluorescent fixtures operating continuously? (Each fluorescent ballast is rated at 0.5A.) - 8 fixtures x 0.5A = 4A, 125% of 4A, 4Ax 1.25 = 5A What is the maximum number of continuous duty duplex receptacles that can be connected to 20A, 120V circuit in a commercial building? - total circuit capacity = 20A x 120V = 2400VA, rating per receptacle = 180VA x 125% = 225VA, total loads that may be connected = (circuit capacity)/(rating per receptacle) = 2400VA/225VA = 10.7 (round down) = 10 receptacles
What is the maximum continuous load in VA that may be connected to a 20A, 120V sign circuit? - 20A x 120V = 2400VA, 2400VA x .80 = 1920VA Per NEC section 220.14(E), outlets for heavy duty lamp holders are each calculated at minimum of..... - Look at NEC section 220.14(E), 600VA What is the demand load for 4 units of commercial kitchen equipment with a total VA rating fo 36000VA? - 36000VA x .80 = 28,800VA What is the motor full-load current for a 5hp, 208V, 1 ott - look at NEC table 430.248, NEC table 430.250. 30.8A Which size copper branch circuit conductors would be required to supply a nonmotor- generator arc welder with a nameplate primary current of 100A and a duty cycle of 50%? - look at NEC Table 630.11(A): 0.71x100A = 71A, look for 60C (140F) column of NEC Table 310.16: 71A requires No. 3 AWG General lighting load for a store is..... - 1.9VA sq ft (NEC 220.12 Table (retail)) General lighting load (expressed as a unit load per square foot) for a warehouse is.... - 1.2VA sq ft (NEC 220.12 Table (Warehouse)). Continuous load is defined by the NEC as a load whose maximum current is expected to continue for at least - 3 hours What is the capacity, in amperes, for a 20A circuit breaker supplying a continuous load? - 20A x .80 = 16A Branch circuit conductors supplying continuous duty loads are calculated at..... - 125% of the rated load
What is the current-carrying ampacity for each of 10 No. 10 THHN copper conductors installed in a single conduit? - NEC section 310.16 table, No. 10 copper = 40A, NEC section 310.15(C)(1) table, # of conductors for 10-20 = 50%, 40Ax50% = 20A The voltage drop between the point of service and the farthest outlet in a branch circuit should not exceed..... - 3% The voltage drop for a 208V, 1 ott circuit with a load of 26.5A, using No. Stranded Copper conductors at at circuit length of 145' is...... - Use VD = (2xLxRxI) / 1,000, VD = (2x145x.778x26.5) / 1000 = 5.98V The voltage drop for a 480V, 3 ott circuit with a load of 26.5A, using No. 8 stranded copper conductors at a circuit length of 145' is....... - Using 3 phase Voltage Drop formula, VD=((√3)xLxRxI)/1000, VD= (1.732x145x.778x26.5)/1000 = 5.18V Using the resistance formula, the voltage drop for an 18A, 208V, 3 ott load with a total circuit length of 105' using No. 10 solid copper conductors is..... - 3.96V Load calculations for circuits supplying lighting units with ballasts are based upon...... - the ampere ratings of the ballasts What is the calculated load, in amps for a 120V circuit supplying four 150W recessed incandescent fixtures and six recessed fluorescent fixtures operating continuously? (Each fluorescent ballast is rated at 0.65A) - 4x150VA = 600VA, 600VA/120V= 5A, 5A x 1.25= 6.25A, 6x.65A= 3.9A, 3.9A x 1.25 = 4.875A, 4.875A + 6.25A = 11.125A what is the maximum number of general purpose noncontinuous duty duplex receptacles that can be connected to a 20A, 120V circuit in a commercial building? what is the NEC required load rating for each receptacle? - 13 receptacles; 180VA what is the total load, in VA, for two small appliance branch circuits in a single-family dwelling? - 1 small appliance = 1500VA (NEC 220.52), 2x1500VA= 3,000VA
