ELECTRICAL INSPECTION BULLETIN
(Effective 1991-11-01)
Rule 4-022
Ampacity of Neutral Conductor
CEC Rule 4-022
Nova Scotia Power has received an increasing number of requests to accept electrical installations incorporating a
neutral conductor having an ampacity less than the ampacity of the circuit phase conductors. CEC Rule 4-022
provides for the installation of reduced neutrals provided that certain criteria can be met. The practice of installing
reduced neutrals may realize significant initial cost savings; however, if not sized properly, may result in excessive
maintenance and shutdown costs.
Non-linear electrical loads such as may be encountered with electronic business machines, computers, electronic
ballast and the like, may present unexpected problems in the form of transformer failure, and overheating of neutral
conductors. The main concern is the harmonic currents which are generated by the diode charging capacitors
forming part of this type of equipment.
The C.E. Code is not specific on how to calculate unbalanced loads of this type. Rule 4-022 (2) (a) does, however,
note that no reduction may be made in the neutral for that portion of the load which consists of electric discharge
lighting. Until such time, specific rules are provided for non-linear loads, contractors, consultants, etc., shall be
responsible to ensure that the ampacity of the neutral is adequate to accommodate the load being supplied.
Nova Scotia Power Inspection Authority shall interpret Rule 4-022 as follows:
General:
Where reduced neutral are permitted, the minimum allowable size neutral conductor shall have sufficient ampacity
to carry the maximum unbalanced load (4-022(1) and (2)) and shall not be smaller than the size of the service
grounding conductor Rule 4-022(3)(b).
Single-Family Dwelling
Reduced neutral may be permitted for single-family dwellings where the service capacity is greater than 100 amp,
120/240 volts, single phase, 3-wire and the dwelling has a significant phase to phase (240V electric heat) load. The
minimum size neutral conductor shall have an ampacity not less than 100 amp (#3cu or #2AL).
Incandescent lighting, motors, heating loads are linear (constant impedance) in nature and harmonics are not
normally a concern.
Prepared by: David Conrad
Page 1 of 2
Approved by: David Conrad
Sept. 9, 1991
B-4-022
Minimum Ampacity of Neutral Conductor
Single Phase, 120/240 Volt Service
Service Ampacity
(Amps)
Non Electric
All Electric
0 – 100
No Reduction
No Reduction
101 - 200
No Reduction
Min. 100 A
201 - 400
By Special Permission
per Service
Min. 200 A
Commercial/Industrial/Institutional
Neutral conductors shall have an ampacity equal to or greater than that of the circuit phase conductors. Special
application may be made to the Inspection Department for the use of a reduced neutral in accordance with CEC Rule
4-022. Permission will be given for an individual service on a one-time basis only. The electrical contractor,
consulting engineer, etc., will be required to provide detailed load calculations noting the types of loads (linear, nonlinear) being supplied.
Customers having no phase-neutral load requirements who are required to provide a neutral conductor for utility
metering purposes only, shall be permitted to install a reduced neutral which shall not be smaller than the system
grounding conductor.
Neutral conductors associated with parallel service conductors and installed in separate raceways shall comply with
CEC Rule 10-204(3).
Reduced Neutrals Not Permitted:
Reduced neutrals shall not normally be permitted on a 3-phase, 4-wire, wye circuit where a major portion of the load
consists of electric-discharge lighting, data processing or similar electronic non-linear loads and where there is a
likelihood of harmonic currents present. For purposes of CEC Rule 4-004, the neutral conductor shall be considered
a current carrying conductor.
On a 3-phase, 4-wire, wye system where sub-services are provided via a 3-wire circuit consisting of 2 phase
conductors and a neutral, as may be found in most apartment buildings. For purposes of CEC Rule 4-004, the
neutral conductor shall be considered a current carrying conductor and no reduction in size will be permitted.
Page 2 of 2
Sept. 9, 1991
B-4-022