ECN Forum Forums General Topics of the Trade General Discussion Area # of conductors in conduit

Quote "The Inspector and Electrician looks at that conduit with 12 wires in where NEC says only 11 are allowed and says "NO!" The engineer looks at that conduit and says "Why does NEC say only 11 are allowed? Is it because of installation difficulty and risk of damage to the cables? Lets test the cables and see if any are damaged..." and so on."


Good Luck in your endeavours! Pull that with an inpector who was in the trade for 25 plus years and is showing up to your job at 4:30 on his 5th inspection of the day.


That is the attitude problem that seems to get us to where we are now. Rather than learn the codes and standards and learn from the seasoned veteran electricians, the engineer assumes superiority and questions the standards and practices by which they work.

The NEC is not a new document, it is the result of years and years of compiled knowledge. By no means perfect, but unless we are talking about a completely new technology and installation method or environment - it is pretty damn thorough and comprehensive.

As is relates to issues like conduit fill etc. The answer to the question "why this many or only this many" (aside from the reasions documented by others earlier in the post) is...because it has to be something.

It is a standardized number that is part of a greater set of standards. It may be 11 conductors's because if the NEC did not prescribe a limit, certain EC's would install 25, 30 or more.

I think you misunderstand my point; I'm not saying I'd have submitted a job with 12 conductors where only 11 are legal. But the contractor had already installed it and somehow ended up in court and hired me as a consultant to analyze it, I wouldn't just take their money, smile and say "Sorry, rip it out!", I'd actually analyze it and give my "expert witness" testemony of whether I felt the installation sitting there on the jobsite was safe or not.

I meant this as a broader example of the thought process I take to the job. NEC is not there to **** me up and drive up the cost of jobs, it's a compilation of methods and guidance required for a safe installation. I don't go around looking for ways to cheat NEC, I keep a tabbed and annotated copy on my desk and normally comply 100%. But there are times when I have to go above and beyond what's in the book to get the job designed right.

The hardest decisions are when I get to the jobsite and find NEC violations and have to decide whether it's acceptable or not. As I stated before, I'm not just the engineer, I'm also the AHJ and the program manager responsible for funding the job. This example of 12 wires in an 11-wire conduit is typical of the type of violations I find and have to make a decision on whether the installation is safe and we can proceed with testing or whether to cost my program a ton of money and do it strictly IAW code. Sometimes I sign off on it, sometimes I make them fix it. Either way, I'm accepting full liability as the responsible engineer. That's why I'm paid the big bucks!

PE EE morning exam has a few NEC questions- enough to force the poor compsci and circuit EEs to buy a copy of NEC to drag to the test. PE EE Power Afternoon exam has more in depth NEC questions. (5% of the test, IIRC) It also has fault analysis, power factor and a boatload of other difficult power questions. I'll see if I can dig up a sample exam to share here, I think you guys might enjoy trying your luck at it

PE EE, Afternoon Power Module (40 multiple-choice problems, 4 hours) http://ppi2pass.com/ppi/PPIInfo_pg_myppi-faqs-ee.html

General Power Engineering: approx. 15% of problems
Measurement, instrumentation, and statistics: 5%
Power metering
Instrument transformers
Transducers
Frequency responses of measurement devices
Data evaluation
Reliability

Special applications: 2%
Illumination design
Lightning and surge protection

Codes and standards: 8%
ANSI standards
NEC (code)
IEEE standards
NEMA standards
NESC (code)

Circuit analysis: approx. 28% of problems
Analysis: 15%
Short-circuit analysis
Wye-Delta transformation
Three-phase circuit analysis
Symmetrical components
Balanced and unbalanced systems
Per-unit analysis

Devices and power electronic circuits: 8%
Solid-state power device characteristics and ratings
Battery characteristics and ratings
Power supplies
Relays and switches
Power electronics

Electric and magnetic fields and applications: 5%
Transmission line models
Mechanical forces between components
Electromagentic fields, coupling, and interference
Electrostatics
Ferroresonance

Rotating Machines and Electromagnetic Devices: approx. 27% of problems
Rotating machines: 18%
Synchronous machines
Induction machines
DC machines
Machine constants and nameplate data
Equivalent circuits
Response times
Speed-torque characteristics
Speed control
Motor starting
Variable speed drives
Testing

Electromagnetic devices: 9%
Transformers
Reactors
Magnetic circuit theory
Testing

Transmission and Distribution: approx. 30% of problems
System analysis: 15%
Voltage drop and voltage regulation
Power factor correction
Parallel three-phase systems
Surge protection
Power quality
Fault current analysis
Grounding
Resistance grounding
Transformer connections
Models

Power system performance: 6%
Load flow
Models
Power system stability
Voltage profile
Computer control and monitoring

Protection: 9%
Overcurrent protection
Protective relaying
Protective devices
Coordination


- Historic pass rates for engineers who have graduated with at least a bachelors of science degree in engineer, passed the EIT/FE exam, and had 4+ years relavent experience and sat for the PE Electrical exam have been between 28-68% for different exams over the past 6 years. 2000 was a very bad year to have taken the PE, I think, lol...

[This message has been edited by SteveFehr (edited 02-11-2007).]

This post is intended to inject a "smile break" into the discussion:

If the PE exam has 40 questions, and 8% are based upon codes, how many code related questions are there?

If there are five categories of codes, and 3.2 questions, how many questions relate to the NEC?

I suppose it is fitting to ask engineers to answer .64 of a question- and do it in 1.2 minutes.



[This message has been edited by renosteinke (edited 02-11-2007).]

Well, seeing as there are two widely used Standards listed - ANSI and IEEE, I would bet that 2 of the 3.2 "Codes + Standards Multiple Guess Questions" would be related to ANSI and IEEE.

That leaves 1.2 "Codes + Standards Multiple Guess Questions" remaining.

Toss those into a Random Number Generator, and odds of getting the NEC Question would be 1:3 at best!

Maybe the remaining 1.2 Question would be a "50/50" between NEC and NESC, as NEMA is a Standard, and might be placed in Random Selection Cache with ANSI and IEEE!

My luck, I would draw an NESC Question, since I never deal with stuff on the Utility side of the Service Disconnect (other than Service Entrance Ducts and Feeders, Transformer Vaults and Pads, and Primary Feeder Ducts).

If I have the desire + time, I will place these 5 Codes + Standards to numeric results of a "0-4" RNG, just to see what comes out!
(not very likely to happen!)

LOL- The test changes every 6 months; I think they must average the tests to come up with those numbers. The practice exam I took had 5 questions that specifically referenced the NEC and 3 others where you pretty much needed NEC tables to answer it- I thus studied it VERY heavily (and was pretty active on this forum as part of my studying!). The PE exam afternoon session I took (Oct 06) only had 2 questions that directly referenced the NEC (easy relative to the rest of the test!) and one NESC question that I was absolutely clueless on and just had to guess- it was far more heavily slanted towards poco type power engineering No IEEE or ANSI questions on that test, either. I did use the NEC voltage drop equation and tables for several other questions, though.

40 questions isn't going to cover every possible nuance of power engineering- the questions they choose are representative. I mean, if you can delve through some of the more arcane aspects of 430 and pull out the right answer, you're probably going to find the derating tables without much effort


[This message has been edited by SteveFehr (edited 02-11-2007).]

"Sometimes I sign off on it, sometimes I make them fix it. Either way, I'm accepting full liability as the responsible engineer. That's why I'm paid the big bucks!"

Good description of what is involved with working as a PE, I spent many years working with large engineering groups, and they are usually a mix of a few PE's a much larger number of Electrical Systems Designers, and a constant changing number of EIT's.

One thing i noticed over the years, was many in the Drafting group, would have all the building code books handy, and they did reference all the codes, not just the electrical, if there was a design problem, they had the PE's on staff ready to assist.

IMO what is all comes down to is, keeping the lines of communication open on any project, If a problem is found with the design bring it to the engineers attention and work together to resolve it, everyone wins.

I have a solution to your derating problem...just put a junction box every 24"...lol

I guess that would be a "no"

I believe that engineering supervision means that a PE has stamped the plans. It doesn't mean that he has made any calculations, but he is approving the work of someone who doesn't have the PE. As Steve pointed out, the liability issue of stamping plans that aren't in compliance is huge. As far as being able to override a pertinent provision of the NEC, the NEC allows for this in 310.15.

NEC 310.15(A)(1) states:

"Tables or Engineering Supervision.

Ampacities for conductors shall be permitted to be determined by tables as provided in 310.15(B) or under engineering supervision, as provided in 310.15(C). "

310.15(C) states:

"NEC 310.15(C) Engineering Supervision

Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general formula: [FORMULA]"

In Wisconsin, the derating factors do not apply to residential branch circuits.

“ARTICLE 310 – CONDUCTORS FOR GENERAL WIRING

Comm 16.30 Ampacities for conductors rated 0 – 2000 volts [NEC310.15]. (1) ADJUSTMENT FACTORS [NEC 310.15 (B)(2). This is a department exception in addition to the exceptions specified in NEC 310.15(B)(2)(a):

Exception No. 6: The derating factors shown in Table 310.15(B)(2)(a) do not apply to branch circuits supplying an individual dwelling unit.”

It appears to me that Steve is on firm ground for applying the 310.15(C) formula. If he is trying to approve something that exceeds the 310.15(C) formula, I don't think he can legally do that.

As far as overriding the NEC absent a specific provision, I don't think that's true. Of course, if the local AHJ is willing to go with it, perhaps it is acceptable.

Sounds like something a bunch of lawyers would argue over!

By the way, I am also a PE in Electrical Engineering. The biggest thing you learn being a PE is not to sign something unless you can back it up. Caution is the order of the day.

Frank