Voltage Drop
Voltage drop is a measure of how much a circuit’s voltage fluctuates (or drops) once a
load is applied. Voltage drop can be calculated by comparing a voltage measurement
with no load on the circuit to a voltage measurement under full load.
The voltage drop calculation will be most accurate when no-load conditions are
compared to full load conditions. When using a digital multimeter to calculate voltage
drop, remove all loads from the circuit to take the no-load measurement. For the full
load measurement, use a space heater or some other appliance that will draw close to
15 amps.
Voltage Drop can also be measured with a SureTest® Circuit Analyzer. It uses a
patented technology to place a full load onto the circuit without tripping a breaker or
causing any interruption to equipment on the line. The SureTest compares the voltage
measurement at full load, with a measurement at no load and calculates the voltage
drop.
How Much Voltage Drop is Acceptable?
The National Electrical Code (NEC) recommends that the combined voltage drop of the
electrical system (branch circuit and feeders) not exceed 5% for optimum efficiency.
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It
is important to note that this is a recommendation and that local inspectors, or other
governing bodies, may use their own judgment on an acceptable level of voltage drop for
the electrical system.
For example, the Philadelphia Housing Development Corporation (PHDC) requires
contractors to calculate the voltage drop prior to installing blown insulation in existing
homes
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. If the voltage drop is 10 % or higher contractor must replace/repair the circuit
prior to proceeding with the insulation.
Prior to instituting this requirement, half a dozen smoldering fires resulted from the blown
insulation installations. In the 2,500 homes insulated during a two-year period after this
electrical integrity test was instituted, there were no fires reported. At least 15 other
municipalities have followed the PHDC's lead in requiring the load test as part of their
winterization programs.
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NEC code Articles {210-19(a) FPN No. 4} {215-2(d) FPN No. 2
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Kinney, Larry "Assessing the Integrity of Electrical Wiring" Home Energy Sept/Oct 1995: 5,6
% Voltage Drop = (V
(no-load)
V
(load)
)/ V
(no-load)
Voltage drop at a full
load can be easily
taken by simply
lugging the SureTest
into a receptacle.
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