UFC 3 -520-01
June 10, 2002
NEC allows the use of #14 AWG wiring, use #12 AWG conductors wherever #14 AWG
wiring is authorized by the NEC to ensure a better overall design.
Do not exceed 5 percent combined voltage drop on feeders and branch
circuits if the transformer providing service is located within the facility. If the
transformer is located exterior to the facility, limit the combined voltage drop for service
conductors, feeders, and branch circuits to 5 percent. Individual voltage drop on branch
circuits should not exceed 3 percent. The NEC is generally concerned with ampacity
more than voltage drop and only addresses the a bove limits in NEC Articles
210.19(A)(1) (Fine Print Note [FPN] No. 4) and 215.2(A)(4) (FPN No. 2) (2002 Edition) .
Furthermore, bra nch circuits supplying sensitive circuits should be limited to less
voltage drop, usually 1 percent to 2 percent. IEEE 1100, Powering and Grounding
Sensitive Electronic Equipment, recommends a maximum voltage drop of 1 percent for
electronic installations . Paragraph 2 -6 provides information regarding the calculation of
Conductors can be placed in parallel for sizes #1/0 AWG and larger, provided
they are of the same length and size, and have the same type of insulation and
conductor material in accordance with NEC Article 310.4 (2002 Edition) . Arrange the
conductors and terminate them at each end in such a manner as to ensure equal
division of the total current between all of the parallel conductors. These requirements
apply to the parallel conductors in each phase to assure equal division of current within
that phase; it is not required for one phase to be the same as another phase, although
this is the preferred approach.
No more than three to six outlets per circuit should be used even if sizing in
accordance with the NEC indicates that more outlets can be installed on the circuit.
This is intended to accomplish the following:
Minimize the number and variety of sensitive equipment sharing a common circuit.
Minimize voltage drop.
Minimize the likelihood of interaction between circuits.
Allow flexibility for future load growth or equipment changes.
Provide receptacle branch circuits feeding predominantly nonlinear loads with
fully sized neutral conductors. The phase and neutral conductors should be labeled in a
manner that associates these conductors together for each circuit.
Locations, such as offices, data centers, and communications complexes,
that use computers, electronic equipment, and other potentially electrically sensitive
equipment should provide dedicated "computer" circuits off of branch panels for each
work location. If the equipment type and sensitivity warrants it, provide separate panel
boards fed from separate feeders back to the service entrance.