Cable Ampacity

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Wedge

Member
This may be pure ignorance on my part. Never the less, I'll post the question because I am confident in the level of expertise and knowledge within this forum. I'm constantly impressed.

I recently received bid documents which specified the use of Houston Wire # HW254 "Diesel Locomotive Car Wiring (DLO) 2000 volt, 90 deg C, RHW-2 or RHH cable for underground feed via (3) 4" conduits from A/C drive to 1000 HP A/C motor.
A fellow competitor said this cable was specified beause it has greater ampacity capacity than THHN/THWN. Is there any truth to this?

The bid docs do not provide termination ratings for the A/C drive.
With respect to Article 310.10, 310.15 and 310.16, where is a provision to allow greater ampacity other than 310.15(C) or Annex B " which require engineering supervision".

Thanks for the insight.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered

thinfool

Senior Member
Location
Kentucky
Bid as spec'd if you want the job.

Bid as spec'd if you want the job.

According to 310.13 (table) the ampacity of RHW-2 is the same as THWN-2 and XHHW-2.

All have a 90c rating.

What ampacity is required for the actual load? Do you need to use the 90c rating to serve the load? It is not likely that the controller has 90c terminals.

Personally, I would bid as spec'd so that you can do the job for the money. If you get it, then deal with any pricing differential after you find out from the AHJ what he will accept.

true story: An EC had a job to install 3c-15 kv cables in 300' of concrete encased 3" or 4". He did as told even tho he knew that the conduit was too small for the overall size of the cable. They pulled about 50' of cable before it became hopelessly stuck. He was then paid a very healthy extra to dig/install a larger conduit on top of the original and pull and terminate the cable. In this case I believe it was a conflict between the engr and the EC that led to the action. Still, the EC did nothing illegal tho his ethics may be questionable.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The ampacity is not really any better, its just that the manufacturer only shows you the 90C free air ampacity. I have used it for large DC motors because it makes the terminations a lot easier and because the amarture voltage was 700 volts making standard building wire unsuitable. Not sure why it would be speced for an AC motor. It is costly and does not pull near as easy as THHN.
Don

 

Wedge

Member
Kingpb,
Thanks for the response.
I already have the cut sheets and your'e right, it is very similar to welding cable.
I also understand electrons flow around the outside of conductors, thus allowing more current, I just couldn't find a code reference allowing ampacities greater than table 310.16 with out engineering supervision for this specific cable or welding cable.
 

Wedge

Member
Thinfool,
The motor name plate is 1136 amp.
The 90c rating would be helpful although I can not figure it so, I do not have drive specs / terminal rating info. I still have to size 125% and bid to spec.
Interesting story, It's pretty common, although I do my best to disclose potetnial problems.
This job is a retrofit, they currently have a DC drive / motor and want to change it out to an A drive / motor. The existing 480 volt, 2000 amp switch remains and replace existing DC drive with new AC drive. I have an RFI regarding the existing switch. The existing drive is bussed and bottom fed from existing switch, the new drive allows only top or left side top 12" entry. The (e) switch can not be inspected due to 24/7 plant operation.
I was told, the designer basically copied a spec from another recent retrofit. It seems more of a general suggestion than detailed specifications.
Thank for your input.
 

Wedge

Member
Don,
I agree with you. It's very costly and other than terminating conductors, not a whole lot of value. I was told the designer copied a spec from a similar recent retrofit.
Thanks for the response
 

bob

Senior Member
Location
Alabama
Wedge said:
I recently received bid documents which specified the use of Houston Wire # HW254 "Diesel Locomotive Car Wiring (DLO) 2000 volt, 90 deg C, RHW-2 or RHH cable for underground feed via (3) 4" conduits from A/C drive to 1000 HP A/C motor.
A fellow competitor said this cable was specified beause it has greater ampacity capacity than THHN/THWN. Is there any truth to this?.

The ampacity of the cable shown on the cut sheet is close to the values in table 310.20 for cable in open air supported by a messenger. This of course is not what you have. The cut sheet the values are for a single conductors in open air at 40C. You need the cable rating in an underground duct with the
temp at either 75C or 90C. I suspect it will be close to the values in 310.16 for THHW or RHW2. RHH is not suitable for an underground duct installation.

Wedge said:
The bid docs do not provide termination ratings for the A/C drive. With respect to Article 310.10, 310.15 and 310.16, where is a provision to allow greater ampacity other than 310.15(C) or Annex B " which require engineering supervision".
I do not think the you will see any great increase in ampacity using 310.15C.
I believe you will need to use the 75C ratings. What size conductor does the specs call for?
 
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Wedge

Member
Bob,
Unfortunately they only spec the conductor type, the rest they leave up to the contractor.
The vague specs are a direct result of a big corp. cutting costs by eliminating all seniority with technical knowledge.
I will end up using the 75C rating and providing alternate options only after I can visually inspect the 2000 amp switch.
Thanks for your views.
 

paul

Senior Member
Location
Snohomish, WA
Wedge said:
Kingpb,
...I also understand electrons flow around the outside of conductors, thus allowing more current, I just couldn't find a code reference allowing ampacities greater than table 310.16 with out engineering supervision for this specific cable or welding cable.


Not quite correct. What you're talking of is called the 'skin effect.' As the frequency gets higher, the electrons move outwards and travel along the outside of the wire. Therefore, the more strands, the better the wire works with high frequency.
 

bob

Senior Member
Location
Alabama
I recently received bid documents which specified the use of Houston Wire # HW254 "Diesel Locomotive Car Wiring (DLO) 2000 volt, 90 deg C, RHW-2 or RHH cable for underground feed via (3) 4" conduits from A/C drive to 1000 HP A/C motor. The motor name plate is 1136 amp.
The circuit must equal the ampacity of the VFD. Assume the ampacity is
1.25 x 1136 FLA = 1420 and round up to 1500 amps. With only 3 conduits
how are you going to to handle it? 6 400 per phase = 1600 amps.
 

Wedge

Member
Bob,
This is why the 90C rating of the terminals "if applicable" would have an impact. 1136 amp X 1.25 = 1420, including derating for 4 to 6 current carrying conductors would be 250KCMIL at 90C. I will qualify the the use of the spec cable with 90C terminal rating, if 90C rating is not compliant than THWN may be substituted upon approval.
 

bob

Senior Member
Location
Alabama
Wedge
Thanks for the reply. It looked as if the 3 conduits might not be enough. Don't
forget the circuit must match the Drive rating and not the motor.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
You can use the 90 deg C rating for starting purposes and then derate from there. If the derated current value is less then the 75 Deg C current your in like flint. If it comes out to be equal or higher, it just means your limited to the 75 Deg C current, but derating has already been applied.
 
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