Protection of Conductors
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The designations "National Electrical Code” and “NEC” refer to the
National Electrical Code®, which is a registered
trademark of the
National Fire Protection Association.
Top 50 NEC Rules
Protection of Conductors
By Mike Holt
Except as permitted by (A) through (G), conductors must
be protected against overcurrent in accordance with their ampacity
ampacity adjustment, as specified in 310.15.
(A) Power Loss Hazard. Conductor overload protection is not
required, but short-circuit protection is required where the
interruption of the circuit would create a hazard, such as in a
material-handling electromagnet circuit or fire pump circuit.
(B) Overcurrent Protection Not Over 800A. The next higher
standard rating overcurrent device (above the ampacity of the
conductors being protected) is permitted, provided all of the following
conditions are met:
This "next size up rule" doesn't apply to feeder tap conductors
[240.21(B)], or secondary transformer conductors [240.21(C)].
- The conductors do not supply multi-outlet receptacle branch
- The ampacity of a conductor, after ampacity adjustment and/or
correction, doesn't correspond with the standard rating of a fuse or
circuit breaker in 240.6(A).
- The protection device rating doesn't exceed 800A.
(C) Overcurrent Protection Over 800A. If the circuit's
overcurrent protection device exceeds 800A, the conductor ampacity,
after ampacity adjustment and/or correction, must have a rating not
than the rating of the overcurrent device.
(D) Small Conductors. Unless specifically permitted in
240.4(E) or (G), overcurrent protection must not exceed 15A for 14AWG,
20A for 12 AWG, and 30A for 10 AWG copper, or 15A for 12 AWG and 25A
10 AWG aluminum, after ampacity adjustment and/or correction.
(E) Tap Conductors. Tap conductors must be protected against
overcurrent as follows:
- Household Ranges and Cooking Appliances and Other Loads --
210.19(A)(3) and (4)
- Fixture Wire -- 240.5(B)(2)
- Location in Circuit -- 240.21
- Reduction in Ampacity Size of Busway -- 368.17(B)
- Feeder or Branch Circuits (busway taps) -- 368.17(C)
- Single Motor Taps -- 430.53(D)
(F) Transformer Secondary Conductors. The primary overcurrent
protection device sized in accordance with 450.3(B) can protect the
secondary conductors of a 2-wire system or a 3-wire, 3-phase,
delta/delta connected system, provided the primary protection device
does not exceed the value determined by multiplying the secondary
conductor ampacity by the secondary-to-primary transformer voltage
(G) Overcurrent for Specific Applications. Overcurrent
protection for specific equipment and conductors must comply with that
referenced in Table 240.4(G).
Editor's note: This information was extracted from Mike Holt's
the National Electrical Code.
- Air-conditioning and refrigeration equipment and circuit conductors
must be protected against overcurrent in accordance with 440.22.
Typically, the branch-circuit conductor and protection size is marked
the equipment nameplate [440.4(A)].
- Motor circuit conductors must be protected against short circuits
and ground faults in accordance with 430.52 and 430.62 [430.51].
- Motor control circuit conductors must be sized and protected in
accordance with 430.72.
- Remote-control, signaling, and power-limited circuit conductors
be protected against overcurrent according to 725.23 and
Anticipate, prevent and troubleshoot motors, electrical and equipment
maintenance with fast, accurate non-contact temperature measurements
with Fluke infrared thermometers. www.fluke.com/codewatch_temp
What's Wrong Here?
By Joe Tedesco
Think you know how this installation violates the
NEC? Visit EC&M's Web
site to see the answer.
Hint: Disturbed by this mix and match arrangement.
By Mike Holt
Q. Is liquidtight flexible metal conduit (LFMC)
permitted under the raised floor of a computer room? If yes, must it be
Web site to see the answer.
By Steven Owen
Q. What is the minimum size NM cable required
an electric space-heating branch circuit installed 3 inches above a
heated ceiling (with no thermal insulation) in a dwelling unit? The
branch circuit will supply a calculated load of 12A.
A) 14 AWG NM
B) 12 AWG NM
C) 10 AWG NM
D) 12 AWG Type TC
Web site for the answer and explanation.
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