Building Design and Construction Handbook - Merritt - Ventech!

15.42 SECTION FIFTEEN
ers for recording current consumed are furnished by the utility company. Unless
otherwise permitted by the utility company, meters must be located near the point
of service entrance. Sometimes, the utility company permits one or more tenant
meter rooms at other locations in the cellar of an apartment house to suit economical
building wiring design. Tenant meter closets on the upper floors, opening on public
halls, also may be permitted. The most common form of tenant meters used is the
three-wire type, consisting of two phase wires and the neutral, taken from a 208/
120-V three-phase four-wire service.
The service switch and metering equipment may be combined in one unit, or
the switch may be connected with conduit to a separate meter trough. For individual
metering, the detachable-socket-type meter with prongs that fit into the jaws of the
meter-mounting trough generally is used.
15.9.4 Switchboards and Panelboards
For low-capacity loads, wiring may be taken directly to a panelboard. For larger
loads, wiring may be brought first to a switchboard and then to panelboards. This
equipment is described in Art. 15.7. Branch circuits extend from panelboards to
the various loads.
15.9.5 Sample Calculations for Apartment-Building Riser
A diagram of a light and power riser for a nine-story apartment building is shown
in Fig. 15.10. Calculation of required wires and conduits may be carried out with
the aid of tables in the NFPA ‘‘National Electrical Code Handbook.’’
1. Typical Meter Branch to Apartment Panel. Note that the meters are threewire
type, and three apartments are connected to the same neutral. Under balanced
conditions, when each of the three identical apartments is under full load, no current
will flow in the neutral. But at maximum unbalance, current in the neutral may be
twice the current in the phase wire for any one apartment. The neutral wire must
be sized for this maximum current, though the usual practice is to compute voltage
drops for the balanced condition.
Assume that the apartment area is 900 ft 2 . The one-way run from meter to
apartment panel (apartment A) is 110 ft.
2
Apartment lighting load � 900 � 3 W/ft � 2700 W
Apartment appliance load � 3000
Total � 5700 W
The electric service is three-phase four-wire 208/120 V. Thus, the voltage between
phase wires is 208, and between one phase and neutral 120 V. Assume a
90% power factor. From Eq. (15.25):
5700
Current per phase � � 26.4 A
2 � 120 � 0.9
The local electrical code requires the minimum size of apartment feeder to be
No. 8 wire. The allowable current in No. 8 RH wire is 45 A; so this wire would
be adequate for the current, but it still must be checked for voltage drop. The neutral
must be sized for the maximum unbalance, under which condition the current in
the neutral will be 2 � 26.4 � 52.8 A. This will require No. 6 wire.