Notes:
1. 240.4(B)
Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of the
conductors being protected) shall be permitted to be used, provided all of the
following conditions are met: (1) The conductors being protected are not part of
a multioutlet branch circuit supplying receptacles for cord and- plug-connected
portable loads. Also see Table 210.24 and Section 210.23.
240.4(C) Devices Rated Over 800
Amperes. Where the overcurrent device is
rated over 800 amperes, the ampacity of the conductors it protects shall be
equal to or greater than the rating of the overcurrent device defined in 240.6.
2. 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 as a current carrying conductor.
(b) In a 3-wire circuit consisting of two phase wires and the neutral of a
4-wire, 3-phase, wye-connected system, a common conductor carries approximately
the same current as the line-to-neutral load currents of the other conductors
and shall be counted as a current carrying conductor.
(c) On a 4-wire, 3-phase wye circuit where the major portion (over 50 per cent)
of the load consists of nonlinear loads such as electric discharge lighting and
computer loads, harmonic currents are present in the neutral conductor and the
neutral shall be considered a current carrying conductor.
(d) Each current-carrying conductor of a paralleled set of conductors shall be
counted as a current carrying conductor.
3. Termination provisions
of equipment for circuits rated 100 amperes or less, or marked for 14 AWG
through 1 AWG conductors, shall be used only for one of the following:
(1) Conductors rated 60°C (140°F).
(2) Conductors with higher temperature ratings, provided the ampacity of such
conductors is determined based on the 60°C (140°F) ampacity of the conductor
size used.
(3) Conductors with higher temperature ratings if the equipment is listed and
identified for use with such conductors.
(4) For motors marked with design letters B, C, or D, conductors having an
insulation rating of 75°C (167°F) or higher shall be permitted to be used,
provided the ampacity of such conductors does not exceed the 75°C (167°F)
ampacity.
Termination provisions of equipment for circuits rated over 100
amperes, or marked for conductors larger than 1 AWG, shall be used only for one
of the following:
(1) Conductors rated 75°C (167°F)
(2) Conductors with higher temperature ratings, provided the ampacity of such
conductors does not exceed the 75°C (167°F) ampacity of the conductor size used,
or up to their ampacity if the equipment is listed and identified for use with
such conductors.
Author's Note: In general most terminals are rated for 75
degrees C. Conductors with 90 degree C insulations can be used on these
terminals provided they are not used at an ampacity higher than the 75 degree C.
ampacity. However, the 90 degree insulation ampacity can be used for
derating purposes. For example a No. 12 AWG copper 90 degree
C. THHN rated at 30 amperes can be used in a raceway with up to 9 current
carrying conductors on a 20 ampere circuit breaker since the derated
ampacity is 0.7 x 30 = 21 amperes.
75 degree terminations may be stamped as AL7CU.
4. Conductors in Parallel.
Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger,
comprising each phase, polarity, neutral, or grounded circuit conductor, shall
be permitted to be connectedin parallel (electrically joined at both ends).
5. Where parallel conductors are run in separate raceways or
cables, the raceways or cables shall have the same physical characteristics.
Where conductors are in separate raceways or cables, the same number of
conductors shall be used in each raceway or cable. Conductors of one phase,
polarity, neutral, or grounded circuit conductor shall not be required to have
the same physical characteristics as those of another phase, polarity, neutral,
or grounded circuit conductor to achieve balance.
6.
Where conductors are run in parallel in
multiple raceways or cables as permitted in NEC Section 310.4, the equipment
grounding conductors, where used, shall be run in parallel in each raceway or
cable.
The size of the equipment grounding conductor shall be based on 1 or 2 below:
1.
(This calculator uses this method.) Each parallel equipment grounding
conductor shall be sized on the basis of the ampere rating of the
overcurrent device protecting the circuit conductors in the raceway or cable in
accordance with Table 250.122.
2. Where ground-fault protection of equipment is installed, each
parallel equipment grounding conductor in a multiconductor cable shall be
permitted to be sized in accordance with Table 250.122 on the basis of the trip
rating of the ground-fault protection where conditions given in NEC Section
250.122 are met.
Example
Given:
Find the combination of 350 kcmil copper XHHW-2 conductors, BARE equipment
grounding conductor, and size of conduit at 40 per cent fill to supply a 1000
ampere load. Voltage drop is limited to 2 per cent. Terminals are
rated at 75 degree C.
The circuit is 480/277 volts three phase in PVC schedule 80 conduit 100 feet
long. The majority of the load is nonlinear.
Using the T310.16 Parallel Calculator: Since the majority of the load
is nonlinear there are a minimum of 4 current carrying conductors in each
raceway, A-ph, B-ph, C-ph, and the neutral. 4ea 350 kcmil are
permitted on a 1000 ampere circuit breaker. The equipment ground is 2/0
AWG. The voltage drop for 100 feet is 0.3 per cent.
Using the raceway fill calculator the size of conduit is: 3 inch
schedule 80 PVC.
The installation would consist of 4 each 3 inch raceways with 4 each 350
kcmil conductors and a bare 2/0 equipment grounding conductor in each
raceway. Each raceway would have A-phase, B-phase, C-phase, and a
Neutral 350 kcmil XHHW-2 copper conductor and a 2/0 AWG bare copper
equipment grounding conductor. Copyright Notice
|