Chapter 3 Wiring Methods and Materials

ARTICLE 310 -
CONDUCTORS FOR GENERAL WIRING
310.15
Table 310.15(B)(2)(c) Ambient Temperature Adjustment for
Conduits Exposed to Sunlight On or Above Rooftops
Temperature Adder
Distance Above Roof to Bottom of
Conduit °C of
0- 13 mm (VZ in.) 33 60
Above 13 mm (VZ in.) - 90 mm (3Yz in.) 22 40
Above 90 mm (3Yz in.) - 300 mm (12 in.) 17 30
Above 300 mm (12 in.) - 900 mm 14 25
(36 in.)
main disconnect and the panelboard
The feeder conductors
to a dwelling unit shall not be required to have an allowable
ampacity rating greater than their service-entrance conductors.
The grounded conductor shall be permitted to be
smaller than the ungrounded conductors, provided the requirements
of 215.2, 220.61, and 230.42 are met.
Table 310.15(B)(6) Conductor Types and Sizes for
120/240-Volt, 3·Wire, Single-Phase Dwelling Services and
Feeders. Conductor Types RHH, RHW, RHW·2, THHN,
THHW, THW, THW-2, THWN, THWN-2, XHHW,
XHHW·2, SE, USE, USE-2
Conductor (AWG or kcmil)
(3) Bare or Covered Conductors. Where bare or covered
conductorsare_ with insulated conductors,
(4) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit
shall not be required to be counted when applying the provisions
of 31O.l5(B)(2)(a).
(b) In a 3-wire circuit consisting of two phase •••
and the neutral of a 4-wire, 3-phase, wyeconnected
system, a common conductor carries approximately
the same current as the line-to-neutralload currents
of the other conductors and shall be counted when applying
the provisions of 310.15(B)(2)(a).
(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic currents
are present in the neutral conductor; the neutral•••
... shall therefore be considered a current-carrying conductor.
(5) Grounding or Bonding Conductor. A grounding or
bonding conductor shall not be counted when applying the
provisions of 310.15(B)(2)(a).
(6) 120/240-Volt, 3·Wire, Single-Phase Dwelling Services
and Feeders. For individual dwelling units of onefamily,
two-family, and multifamily dwellings, conductors,
as listed in Table 31O.15(B)(6), shall be permitted as
1201240-volt, 3-wire, single-phase service-entrance conductors,
service-lateral conductors, and feeder conductors
that serve as the main power feeder to each dwelling unit
and are installed in raceway or cable with or without an
equipment grounding conductor. For application of this section,
the main power feeder shall be the feeder between the
Aluminum or
Service or Feeder
Copper-Clad
Rating (Amperes) Copper Aluminum
100 4 2
110 3 1
125 2 1/0
150 1 2/0
175 1/0 3/0
200 2/0 4/0
225 3/0 250
250 4/0 300
300 250 350
350 350 500
400 400 600
(C) Engineering Supervision. Under engineering supervision,
conductor ampacities shall be permitted to be calculated
by means of the following general formula:
1= TC-(TA+D.1D)
RDC(l+YC)RCA
where:
TC =conductor temperature in degrees Celsius (0e)
TA = ambient temperature in degrees Celsius (OC)
I1TD = dielectric loss temperature rise
RDC = dc resistance of conductor at temperature TC
YC =component ac resistance resulting from skin
effect and proximity effect
RCA = effective thermal resistance between conductor
and surrounding ambient
FPN: See Annex B for examples of formula applications.
2008 Edition NATIONAL ELECTRICAL CODE 70-147