Unit Load (NEC-2017)
The calculation of general-purpose branch circuits is determined by using a general unit load of 3 volt-amperes per square foot for one- and two-family dwelling units, which is derived from NEC Table 220.12.
The floor area is required to be calculated using the outside dimensions of the dwelling unit, apartment or other area that may be involved. Each floor level must be considered individually, then added together to determine the total floor area for the calculation.
The floor area to be included in the calculation is not required to include any open porches, garages, or any unused or unfinished spaces that are not adaptable for future use. The decision on whether or not these spaces must be included in the load calculation is determined by how the space is identified on the plans. (NEC 220.12, NEC-2017).
General Purpose Lighting
Amperes = (Floor area x 3 VA) / 120 Volts
Min. Number of Circuits = Amperes, Gen. Purpose Lighting / 15 or 20 Amp Circuits
Sample Branch Circuit Calculation
2100 sq. ft. x 3 VA/120 Volts
= 6300 VA / 120 Volts
= 52.5 Amperes
52.5 Amperes / 15 or 20 Amperes Circuits
= 4- 15 or 3- 20 Ampere Circuits
The calculation determined that 4- 15 ampere branch circuits are required or 3- 20 ampere branch circuits are required.
Ohm’s Law
Ohm’s law states that the electrical current through a conductor between two points is directly proportional to the voltage across two points. If any two values are known, the third can be found by use of the formula. For example, if the resistance and the voltage are known, the current can be determined by dividing the voltage by the resistance. This can be valuable in determining the amount of current that will flow in the circuit to properly size conductors as well as overcurrent devices.
Voltage = (E) or Pressure that pushes [Current x Resistance]
Resistance = (R) or Resistance in ohms [Voltage / Current]
Current = (I) or Amperes that flow [Voltage / Resistance]
Terms
Volt (V or E) — the unit of electrical pressure — is the pressure required to force one ampere through a resistance of one ohm; abbreviated as “E,” the first letter of the term electromotive force. While the symbols V or E are often used interchangeably, “E” is often used to represent voltage across a source (like a battery) and “V” is used to represent voltage across anything else.
Current Intensity (I) — unit of electrical current that will flow through one ohm under a pressure of one volt in one second; abbreviated as “I,” the first letter of the term intensity of current.
Ohm (R) — An ohm is the resistance through which one volt will force one ampere; abbreviated as “R,” the first letter of the term resistance. Resistance is measured in ohms.
Ampere (A or amp). An ampere is the unit used to measure electric current. Current is a count of the number of electrons flowing through a circuit. One amp is the amount of current produced by a force of one volt acting through the resistance of one ohm.
Watts (W) — the unit of measurement of the energy flowing in an electrical circuit at any given moment. It is also the amount of work being performed in the electrical circuit. Watts is the product of multiplying volts and amperes and is sometimes referred to as volt-amperes. One thousand volt-amperes are referred to as one kilovolt-ampere or one kVA.
Ohm ( Ω ). An ohm is the electrical resistance between two points of a conductor when a constant potential difference of one applied to these points produces a current of one ampere.