Chapter 9 tables seem to be obsessed with limiting conduit fill to 40% for 3 or more wires. The concerns being potential damage during installation, and heat generation from current flow. Is there a provision any place else in the NEC allowing greater fill for control wires. Modern insulation is so tough that the prospect of damage during installation has been greatly reduced, and there is no heat generated by conductors energized only momentarily.
Conduit Fill
- Thread starter Don S.
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Twoskinsoneman
Senior Member
- Location
- West Virginia, USA NEC: 2020
- Occupation
- Facility Senior Electrician
Don S. said:
310.15 (b) (2) (a)
Exception No. 1: Where conductors of different systems, as provided in 300.3, are installed in a common raceway or cable, the derating factors shown in Table 310.15(B)(2)(a) shall apply only to the number of power and lighting conductors (Articles 210 , 215, 220 , and 230 ).
Twoskinsoneman
Senior Member
- Location
- West Virginia, USA NEC: 2020
- Occupation
- Facility Senior Electrician
Don S. said:
Found this is the 2008 NEC Handbook. Thought it might be intersting
under 725.1
General Discussion of Remote-Control, Signaling, and Power-Limited Circuits
The wiring methods required by Chapters 1 through 4 of the Code apply to remote-control, signaling, and power-limited circuits, except as amended by Article 725 for specified conditions.
A remote-control, signaling, or power-limited circuit is the portion of the wiring system between the load side of the overcurrent device or the power-limited supply and all connected equipment. The circuit is categorized as Class 1, Class 2, or Class 3.
Class 1 circuits are not permitted to exceed 600 volts. In many cases, Class 1 circuits are extensions of power systems and are subject to the requirements of the power systems, except under the following conditions:
1. Conductors size 16 AWG and 18 AWG may be used if properly protected against overcurrent (see 725.43).
2. Where damage to the circuit would introduce a hazard, the circuit must be mechanically protected by a suitable means. [See 725.31(B).]
3. The adjustment factors of 310.15(B)(2) apply only if such conductors carry a continuous load. (See 725.51 for the exact requirements for adjustment factors affecting ampacity.)
Class 1 remote-control circuits are commonly used to operate motor controllers in conjunction with moving equipment or mechanical processes, elevators, conveyors, and other such equipment. Class 1 remote-control circuits may also be used as shunt trip circuits for circuit breakers. Class 1 signaling circuits often operate at 120 volts but are not limited to this value.
Conductors and equipment on the supply side of overcurrent protection, transformers, or current-limiting devices of Class 2 and Class 3 circuits must be installed according to the applicable requirements of Chapter 3. Load-side conductors and equipment must comply with Article 725. Class 2 and Class 3 conductors are required to be separated from and not occupy the same raceways, cable trays, cables, or enclosures as electric light, power, and Class 1 conductors, except as noted in 725.136.
Dry-cell batteries are considered Class 2 power supplies, provided the voltage is 30 volts or less and the capacity is equal to or less than that available from series-connected No. 6 carbon-zinc cells. (A No. 6 dry-cell battery is cylindrically shaped with nominal dimensions of 21/2 in. in diameter by 6 in. tall and weighs just over 2 lb. A No. 6 dry-cell battery is about 10 times the volume of the standard D-cell battery commonly used in flashlights.)
Circuits originating from thermocouples are categorized Class 2 circuits. Neither dry-cell batteries nor thermocouples are required to be listed.
- Location
- Illinois
- Occupation
- retired electrician
The Chapter 9 fill tables are only based on the physical damage issue, not the heat issue. The heat issue is covered in Article 310. As far as installing more that 40%, if the run has any length and a few bends, it is very difficult to install more than that. Also you are asking about control conductors which are normally smaller than power conductors and more easily damaged than the larger power conductors. Just the difference in length required when going around a 90, with a full raceway can result in a broken conductor. The conductors on the outside of the bundle going around an 90 require more length than those that are riding on the inside radius of the 90 and if the conduit is full and the run is long, the outside conductors cannot slide by the others to provide this length and if they can't slide, they may break.Don S. said:
- Location
- Massachusetts
First off, to be clear I agree with Don 100% and I would not try to exceed 40% even if the rules where removed.
That said in Chapter 9 both "Table 1" and "Note 4" shortly after it are worth a look.
That said in Chapter 9 both "Table 1" and "Note 4" shortly after it are worth a look.
Common sense, which is frequently in short supply, should prevent anyone from thinking they could get away with loading a run with four 90s way beyond 40% with small wire. But a few additional THHN #14s, say to 60%, in a moderately easy conduit run should not be a problem. Maybe I just answered my own question. Those writing the code probably figured that counting on common sense might be a source of big trouble.don_resqcapt19 said:
- Location
- Illinois
- Occupation
- retired electrician
Common sense can't be enforced by the inspector..only code rules can be enforced. How would you write an enforcable rule that would permit a higher fill for a "moderately easy conduit run"?Don S. said:
JohnJ0906
Senior Member
- Location
- Baltimore, MD
don_resqcapt19 said:
That's easy... and already done.
See Note 4 to Table 1, Chapter 9.
The 60% fill allowance for nipples has been around almost for ever. It should be expanded to cover more specific installations of greater distances. Can you believe Mike Holt bad mouthed me for showing up at his class in 1984 with a 1962 code book? He was just kidding......sort of.
- Location
- Illinois
- Occupation
- retired electrician
Proposals for the 2011 code are due no later than 5pm on 11/7/08. A proposal form is in the back of your code book. Write one for this application and submit it.Don S. said:
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