Cable Size and Code Requirements

[tc7]

We are setting up a temporary winch system which will utilize a generator to drive several 480V-3phase motors.
I have determined that the connecting cable size should be size 1 or 1/0 based on start-up currents of ~134 amps (I'm told this start-up current will last for 7-8 seconds and normal load current will settle to ~20 amps).

Our contractor insists on using size 4 wire (stranded,Class THWN), stating that the Free Air ampacity of size 4 wire is 125 amps and within the working limits of this application.

So I see in NEC Table 310-17 it would seem this contractor may be correct if we do not install the cable inside conduit, but rather just laying cable on the ground and unprotected. Will I be violating any other Code requirement for temporary installations?
Advice/thoughts on any of the above gladly rec'd.
Thanks,
Tom

 

[busbar]

Use of 310-17 as described would be a misapplication. Note wording in titles. NEC Table 310-17. ”Allowable Ampacities of Single-Insulated Conductors …in Free Air…” versus NEC Table 310-16. ”…Not More than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried)…”

 

[advidana]

In free air means on poles or over head. that means the conductors must be separated by a few inches or more in free air.

Also used the motor normal name plate full load current not the starting current. If this current is 20 amps then yes you should be ok with these cables laid on the ground without overheating the wires but will not be safe for traffic and people unless physically protected somehow, a different issue.

Another consideration is if the motor are far away from the generator then larger size conductors will be needed to prevent a large voltage drop. have the electrical conractor provide you with voltage drop calcs of the conductors used with a voltage drop of less than 3 percent. he should know how to do the calcs since it should be a requirement to past the electrical licensing exam.

 

[HCBFlash]

If the 134Amps is what you and your contractor are using to size the wires it's apparent that the "contractor" doesn't know much about electrical motors.

The motor size and nameplate data is what'll determine the overcuurent and overload protection, which will in turn determine the minimum conductor sizing (ampacity).

Conductor sizing per NEC Art 310 is good to use. Get it, read it. Also refer to Art 300.50.

Of course if you find out that the code ALLOWS smaller wires , that doesn't make it a good idea. -maybe the run is very far and voltage drop is the real concern (but that'll help reduce the startup surge). maybe the physical durability and availability are factors.

"Temporary" is the biggest excuse out there for lousy installations. Heck, the NEC doesn't even apply to stuff temporarily left laying on the ground. Of course OSHA does. What you need to think about is the possible problems if that cable is hit by a conductive (metal) tool or other object, or snagged, or somehow compressed or smashed. If somebody just trips over it and gets hurt you may have a bigger problem than you're thinking of right now.

remember: An opinion is only as good as the one who gives it!

 

[skiier]

If you are within "code" distance- to allow for vdrop - then all you need to feed those 20A monsters is #10 awg wire. Use cordsets rated for the amperage, voltage, and type of use. I would suggest any "SOW" type cable rated for hard usage and outdoors. Use receptacles where possible as cordsets aren't considered permanent. And do not forget to ground things safely. I know your system is "temporary" but no one can object to a "safe" code installation. If you are outside cover things up and use properly rated enclosures suitable for the install. People don't die "temporarily" so let's avoid problems. In fact I would tend to think that on average more people die as a result of "temporary" wiring than any other type of wiring.
Oh, and by the way if you have to discuss wiring methods with your "professional" contractor I would suggest that you hire another contractor.

 

[mc5w]

You can also claim that this is a varying duty application that falls under table 430.22(E). This requires 200% of full load current conductors including the ability of motor controllers, disconnects, and circuit breakers to operate resistance heaters.

In this application you might need 50 HP contactors to run your 15 HP motors to get reasonable contactor life.

National Electrical Code is a bare minimum that does not necessarily cover demanding applications such as yours. What this contractor want to do is skimp on wire and conduit size and stick the extra money in his pocket. Buy yourself some 1/0 wire and some 2.5 inch conduit. Conduit size is based on schedule 80 PVC conduit, Chapter 9 Fine Print #2 jamming criteria, wire bending space in conduit bodies, and general experience that an oversize conduit saves enough on wire pulling labor to pay for the cost. If you do not have a place to hook up a wire pulling winch or a forklift truck then an oversize conduit is the only thing that will work.

Mike Cole, mc5w@earthlink.net

 

[skiier]

20A! The load is only 20A! "Several motors" to me only means 2 or 3 motors. With each motor drawing only 20A I am guessing you only need a #4 or a #3 AWG wire size.
The CEC in Canada tells me that to calculate the feeder conductors for these motors I only need 125% of the FLA of the largest motor plus the FLA of all the motors in the group. So 20(1+1+1.25)= 65A. Most people would say use a #4 conductor but there is 5% rule that we use here as well that allows me to use a #6.
Does no one else read the code book? Why would anyone run a 2" conduit and 1/0 wire for these loads - unless they were say, over 500' from the generator.
Get another contractor who is licensed. Any licensed Electrician worth his salt should be very familiar with load calculations. And no, most engineers don't know the code as well as an experienced electrician.

 

[ScottyUK]

"And no, most engineers don't know the code as well as an experienced electrician."

Wow, what a statement!!

I don't think that applies in the UK, except on the domestic and light commercial level where relatively generic cable sizing is (wrongly) applied according to application. Most of the time this results in an acceptable installation. Bigger installations are invariably designed by an engineer purely because of the complexity of the fault level and protection coordination calculations, load flow studies, etc. Most electricians I know would suffer a brain meltdown at the prospect! Are things so different over 'the pond'?




----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[rbulsara]

Firstly, THWN conductor are not listed for exposed use as described (layinn on ground) and in direct sunlight, they have to be installed in raceways. Cable tray is acceptable indoors.

For outdoor/indoor temporary use as described, one can use cable listed for hard exposed usage, such as 'welding' cables or extra flexible cables, which do have quite a higher rating in free air than THWN in raceways. You still need to meet OSHA as pointed out by earlier posts.

 

[skiier]

Sorry Scotty but I didn't mean to offend. I was just making an "observed" statement. Yes things are different on this side of the pond.
Most Engineers in North America have little exposure to the electrical code until they begin to work and design. While the trades are exposed to it every day right from the day they enter the trade.
Canadian tradesman are different than any other tradesman I have seen in other countries. We multi task different trades and we don't have the mindset that something is not our "job". In the States there is a real "us and them" mentality. A very unproductive attitude I might add.
I have held several positions as "engineering" support where it was my responsibility to ensure that installations and designs were up to "code". This includes load calculations AND fault calculations. I am just a tech/electrician who saw a need to fill these positions because engineering lacked these skills.
It still surprises me when I see machinery that has unsafe wiring. The biggest beef being the misuse of the tap rule.

 

[skiier]

And no Scotty, my calculations were never double checked by an Engineer. This was assumed to be my area of experience and no one questioned it.

 

[ScottyUK]

Hi skiier,

Thanks for the reply. I had no idea that tradesmen in Canada were able to do what in the UK is normally a design engineer's job. What is the typical circuit size that this sort of practice takes place on? Most of my work is on utility-level equipment: our organisation tends to apply the same design approach for the largest equipment on site - 2.5MVA, 60kA fault @ 415V being the largest of the LV plant - to the smallest jobs, so in my opinion we tend to over-design the smaller installations. I think the average UK electricians are well capable of doing gentle installation design up to light commercial level, but the big stuff is damned unforgiving of errors and from an accountability point of view it is better that a professional engineer at least reviews the design.

Apologies to everyone else for dragging the thread off-topic!

----------------------------------

If we learn from our mistakes,
I'm getting a great education!