ViSAGE VIS127, Руководство пользователя, Страница: 28 / 28

Sizing the
Circuit
Breakers
Using the
Spec Sticker
Using the
Watts/Volts Method
Considering the
Power Factor
2 A / 0.7 = 2.8571 A
Using the
Volt Amps Method
360 VA / 120 V = 3 A
Selecting the
Circuit Breaker
22 A * 1.25 = 27.5 A
Calculating the total current drawn by the fixtures connected to a particular circuit
is not complicated if the installer has the right information at hand and knows how
to interpret it.
With the fixture's current draw information, the installer can calculate and select the
right circuit breaker size (rating) to which they can connect a group of fixtures.
Our fixtures come with a sticker that indicates the current they consume in a
circuit at the specified voltage. This greatly simplifies calculating the total
current drawn.
For instance, if the sticker on the fixture indicates, "0.1 A @ 115 VAC, 60 Hz" and the
installer is connecting 12 of them on the same 115 VAC circuit, to determine the total
current required by the fixtures it would be enough to do this simple calculation:
0.1 A x 12 = 1.2 A
Some installers may prefer to determine the current drawn by the fixture by dividing
its power consumption, indicated in watts (W), by the voltage (V) on the circuit. As
an example, assuming that a certain fixture consumes 240 W and it is connected to
a 120 VAC circuit, the current it draws would be:
240W / 120 V
2A
The above method is accurate only with fixtures whose power factor (PF) is equal,
or very close, to "1." Otherwise, the calculated current may be too low with respect
to the actual current drawn by the fixture.
In fact, as the PF decreases, the difference between the current calculated using
the watts/volts method and the actual current increases.
Therefore, for fixtures with a PF below "0.9," the installer must always consider the
fixture's PF when using the watts figure to calculate the current it draws.
For the above example, if the published fixture's PF were "0.7," the resulting drawn
current would be as follows:
This is approximately equal )( to 2.86 A, 2.9 A, or even 3 A, depending on the
installer's desire for accuracy. In other words, the actual current ended up being
close to 50% higher than originally calculated.
If the fixture's sticker indicates the power consumption in "volt amps" (VA), the
calculation of the drawn current is simply the result of dividing the amount in VA by
the voltage on the circuit (V). For a fixture with a consumption of 360 VA, the
calculation would be as follows:
Note that when the power consumption is in VA, the fixture's PF is never part of the
current draw calculation.
The National Electric Code (NEC) determines that circuit breakers should handle
80% of their rated capacity for continuous loads (those being on for three or more
hours) and 100% for intermittent loads. For safety reasons, We recommends
assuming that all loads are continuous.
After calculating the total current the fixtures connected to a particular circuit will
draw, the installer must consider the 80% rule indicated above. For a total current
of 22 A, the calculation is as follows:
The installer should use a 30 A CB because the immediately lower CB rating, 25 A,
would not be enough for this load.
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