O.T. (U.S.)NEC question

Ken Moore · Mar 23, 2007

I'm mostly a controls guy, know a little about general wiring, but not a whole lot. Usually I always run each motor's leads in a separate conduit. However, I have an application where the money savings would be tremendous if I could run all the leads in one conduit.

I have to install 4 exhaust ventilation fans on the roof peak of a two story, open bay type building. The motors are 7.5 H.P. 460V 3 phase, each motor will be connected to a MCC bucket, so each will have it's own breaker, starter, overloads, etc...

I browsed through the 2006 NEC, and couldn't find anything that prohibited it, but I may have missed it. If someone can point me to the correct article of the code, I would appreciate it.

rsdoran · Mar 23, 2007

There is nothing to prohibit it since all the wires will be rated and run at same voltage, just make sure the wire is rated for the voltage and to run in conduit.

The problems usually occur when trying to run dc or signals with this voltage i.e. the induction.

Jimmie_Ohio · Mar 23, 2007

Besides referring to the appropriate ampacity table in the NEC for wires run in conduit, be sure to follow the conduit sizing tables and percentage fill data. (Sorry, I don't have them handy)

Ken Moore · Mar 23, 2007

What you guys are stating is exactly what I found in the NEC, I just have to size correctly, nothing prohibits it. The electrical contractor that quoted the job, tried to convince me that it was against the NEC. When I asked for an explicit reference, I got a "it's against the code" as an answer.

Paul T · Mar 23, 2007

And finally, make sure that you follow the de-rating requirements for multiple conductors in one conduit. 4 ckts 3Ph, 12 current-carrying conductors is in the 10-20 range and the conductor must be derated to 50% of normal ampacity. See NEC 310.15 for the ampacity tables and de-rating req'ts. You are also supposed to derate for temperatures that are above normal; if these are ventilation fans, might be something to consider too.

theDave2 · Mar 23, 2007

http://forums.mikeholt.com/ ask there. Also consider any derating since you'll have more than 3 current carrying conductors in the same raceway / bundle. See 310.15(B)(2)(a) for some rules.

allscott · Mar 23, 2007

Ken Moore said:
What you guys are stating is exactly what I found in the NEC, I just have to size correctly, nothing prohibits it. The electrical contractor that quoted the job, tried to convince me that it was against the NEC. When I asked for an explicit reference, I got a "it's against the code" as an answer.

Your contractor must be looking for work, I've seen this done HUNDREDS of times (U.S and Canada).

CaseyK · Mar 23, 2007

Ken Moore said:
When I asked for an explicit reference, I got a "it's against the code" as an answer.

He just didn't say what code!

It's "HIS CODE".

The one that says he came make a ton more money with 4 conduits, then 1 that is slightly larger!!!

HOWEVER-------

If he thinks that it is against the NEC, then he shouldn't touch anything else in the plant, except the door knob on the way out!

rsdoran · Mar 23, 2007

Ken Moore said:
What you guys are stating is exactly what I found in the NEC, I just have to size correctly, nothing prohibits it. The electrical contractor that quoted the job, tried to convince me that it was against the NEC. When I asked for an explicit reference, I got a "it's against the code" as an answer.

Ken, my suggestion is obtain the NEC Handbook. It is hard covered, has graphics, and explanations of the code.

310.... whatever just deals with ampacity for the most part.

As I mentioned before there is nothing to prohibit motor wiring to be in the same conduit. Technically there is not much to prohibit any combo of wiring BUT as I mentioned there may be problems when if you do.

This is what I mean when refering to the Handbook: http://www.nfpa.org/catalog/product.asp?pid=70HB05&order_src=A291

darrenj · Mar 23, 2007

the only time i use seperate conduits for motor wires is if they had drives on them..i have had weird readings when there are 2 or 3 motor's in a conduit..

Off the top of my head i think a 7.5hp motor will draw around 10 amps at 480..so size your wire to number 12 and you are good to go (Unless your going a good distance..rule of thumb is 100') that means if you have 4 motors sharing the one conduit you would have 12 wires plus a ground=13 wires..so depending on your wire type will depend on the size of the conduit.

rsdoran · Mar 23, 2007

darrenj said:
the only time i use seperate conduits for motor wires is if they had drives on them..i have had weird readings when there are 2 or 3 motor's in a conduit..

Off the top of my head i think a 7.5hp motor will draw around 10 amps at 480..so size your wire to number 12 and you are good to go (Unless your going a good distance..rule of thumb is 100') that means if you have 4 motors sharing the one conduit you would have 12 wires plus a ground=13 wires..so depending on your wire type will depend on the size of the conduit.

That may be a prefence, 10a @ 480vac and 100 feet could be 14ga.

Rule of Thumb:
At 1800 rpm, a motor develops a 3 lb.ft. per hp
At 1200 rpm, a motor develops a 4.5 lb.ft. per hp
At 575 volts, a 3-phase motor draws 1 amp per hp
At 460 volts, a 3-phase motor draws 1.25 amp per hp
At 230 volts a 3-phase motor draws 2.5 amp per hp
At 230 volts, a single-phase motor draws 5 amp per hp
At 115 volts, a single-phase motor draws 10 amp per hp

darrenj · Mar 23, 2007

you are right Ron but 99% of the specs i work with say #12 min so thats what i always go with..it also gives me a little wiggle room if i am a little off on distance and ampacity..

Thanks for the little table..i figured out 480v amps the "old" calculator way...good to know the average is 1.25

d

rsdoran · Mar 23, 2007

Thats funny, the last place I worked the lead electrician would not use #12, dont ask me why, dont know.

leitmotif · Mar 23, 2007

I completely agree that there is no NEC reason to not run all 4 circuits in the same conduit.
ASSUMING
all four loads (ventilation fans werent they ??) are non vital or not "emergency related" ie stairwell pressurization fans etc.
HOWEVER
if one conductor goes bad (overheats etc etc) then it may damage the others thus taking all four out of service.
OR a forklift may hit the conduit knocking out all four motors. Then there is the woe of the electrician who has to go into a junction box where he HAS locked out the one circuit but has to handle the other hot circuits conductors (because they all have to go thru a common box at the other end).

What is the cost if you lose all four fans?

I have never liked running multi wire circuits in one conduit. Yes I have done it, worked with a whole bunch of em. Still prefer to run each one separate.

A few bucks saved on the install may cost a whole bunch more when it comes to the reliability or servicing of these circuits.

Dan Bentler

controlled · Mar 23, 2007

leitmotif said:
I completely agree that there is no NEC reason to not run all 4 circuits in the same conduit.
ASSUMING
all four loads (ventilation fans werent they ??) are non vital or not "emergency related" ie stairwell pressurization fans etc.
HOWEVER
if one conductor goes bad (overheats etc etc) then it may damage the others thus taking all four out of service.
OR a forklift may hit the conduit knocking out all four motors. Then there is the woe of the electrician who has to go into a junction box where he HAS locked out the one circuit but has to handle the other hot circuits conductors (because they all have to go thru a common box at the other end).

What is the cost if you lose all four fans?

I have never liked running multi wire circuits in one conduit. Yes I have done it, worked with a whole bunch of em. Still prefer to run each one separate.

A few bucks saved on the install may cost a whole bunch more when it comes to the reliability or servicing of these circuits.

Dan Bentler

I agree that it can be a pain dealing with live conductors in a box after you have locked out the circuit you want to work with. But having multiple motor feeds in one conduit is no different than having different lighting circuits in one conduit. The price would rise very quickly if we had to run a seperate conduit for every circuit in a plant. As long as all the circuits are coming from the same source, I do not see a problem with it.

O.T. (U.S.)NEC question

Ken Moore

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rsdoran

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Jimmie_Ohio

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Ken Moore

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Paul T

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