UFC 4-021-02NF
27 September 2006
change 1, 23 October 2006
9-6.11.1 To illustrate voltage drop calculations, consider the example of a 2 amp, 24
volt camera located 300 feet from the power supply. Figure 9-1 shows two attempts.
Figure 9-1. Attempts 1 and 2
Attempt 1--Try 14 AWG Cable. From chart, 1.01 voltage drop x 3 (for 300 feet) =
3.03 volts = 12.6% drop. No good.
Attempt 2--Try 12 AWG Cable. 0.64 voltage drop x 3 = 1.92 volts = 12.6 = 8%
drop. Good.
9-6.11.2 For exterior cameras such as parking lot surveillance cameras, higher wire
gauges may be required. In one example of a long distance run, 6 AWG was used to
reach the first camera. After that distance, 12 AWG was found to be adequate to power
the subsequent near-by cameras.
9-6.12
Harmonics. Harmonics in a power system are typically the odd multiples of
60 Hz such as 180 Hz and 300 Hz and are generated by switching power supplies such
systems, by electric welders, and by other rectifier type equipment. Harmonics in a
system are measured in total harmonic distortion (THD).
9-6.12.1 Harmonics in a power system can cause overheating of cables and
equipment along with false operations. NFPA 70 requires designs to consider
harmonics and IEEE 519 is a reference standard. When a neutral of a multiphase feed
has significant harmonics, it is to be oversized. UL and the IEEE both have methods for
de-rating standard transformers for harmonics.
9-6.12.2 Mitigation of harmonics involves either isolating the harmonic source from the
rest of the power system or in isolating sensitive equipment from the harmonics.
Methods of mitigation involve use of oversized/de-rated standard transformers or
harmonic K-rated transformers (K4 or K13 being common), use of oversized neutrals in
distribution systems (full size is adequate for feeds to individual equipment), use of input
line reactors or output filters (usually on motor drives), and use of surge suppressors at
panelboards, in wall receptacles, in power bars, or built into the input of ends loads,
such as a security panel.
9-6.12.3 To further reduce electrical noise, a copper equipment ground sized per
NFPA 70 (unless the cable is already shielded) and copper grounding electrode
conductors sized per NFPA 70 should be run in raceways in addition to bonding metallic
raceways and enclosures together.
9-6.13
Raceway. All conduit, wireway, and raceway shall meet the requirements of
NFPA 70.
9-6.13.1 Conduit runs shall have a maximum of three 90-degree bends or any
combination of bends not-to-exceed 270 degrees.
126
Previous Page
