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High Voltage ductbank Derating #90901 12/17/04 10:03 AM
Joined: Mar 2003
Posts: 849
Y
Yoopersup Offline OP
Member
13800 & 4160 in duck bank over 3 circuits Detail 3 in code required for conductor rateings (310.61)Table 310.77 If 6-350 mcm wires in same conduit rating them dn an additional percentage 310.15(b)(2)(a)4-6 80%.
thus use table 310.77 Detail 3 then an additional derateing for 6 current carrying conductors in conduit.


350mcm 90c =315x2=630x80percent=504 amps
So parallel 350 in a single conduit with 4 other hv circuits in the ductbank .Rateing of conductors = 504 amps

Remember this is 4160 volts

2017 / 2014 NEC & Related Books and Study Guides
Re: High Voltage ductbank Derating #90902 12/17/04 01:51 PM
Joined: Feb 2002
Posts: 182
B
Bob Offline
Member
Table 310.15B is for tables 310.16 thru 310.19. Why would you want to install these
higher voltage cables in the same duct?

Re: High Voltage ductbank Derating #90903 12/17/04 04:10 PM
Joined: Mar 2003
Posts: 849
Y
Yoopersup Offline OP
Member
Bob

The cables in same duct are single conductors 4160 350 mcm x 6 plus ground. Legal and designed that way not by me but by the eng. I just feel if you should derate for amount of circuits in the duct per details in code and if you put 6 current carring condutors in one pipe derateing must also apply.They are parallel conductors going to the same overcurrent device.

Re: High Voltage ductbank Derating #90904 12/17/04 07:20 PM
Joined: Feb 2002
Posts: 182
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Bob Offline
Member
I agree with your concern. I could not find any derating info for the hi voltage cable. Suggest you contact Okonite at their site
Okonite.com. Just for information what size is the cable and overcurrent device?

Re: High Voltage ductbank Derating #90905 12/20/04 03:28 PM
Joined: Mar 2003
Posts: 849
Y
Yoopersup Offline OP
Member
Cable 2 sets 350 mcm
fusing per 450.3(A) transformer
Wire 240.100 & 240.101 a
By code can be 300% on primary (breaker)600%
Fuse if meets both code sections.

Re: High Voltage ductbank Derating #90906 12/21/04 04:37 PM
Joined: Feb 2002
Posts: 182
B
Bob Offline
Member
In your other post you said the transformer was rated at 1935 kva at 4.16 kv. Thats about 277 amps primary. One ckt of #350 cable is rated in duct at about 385 amps in table B310.77. Since you have 2 circuits of #350, even if you derate 50%, you should have adequate capacity for the transformer.

Re: High Voltage ductbank Derating #90907 12/22/04 08:08 AM
Joined: Mar 2003
Posts: 849
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Yoopersup Offline OP
Member
Read my ist post for conductor rateings . Detail 3 90% rateing 315 not 385 amps on the wire . Also the derateing was my interest as I know full load current of the Transformer and even with voltage drop I feel the wire is oversized for the projected use.(remember Over 3 circuits ductbank detail 3 not detail 1(310.77).

Re: High Voltage ductbank Derating #90908 12/22/04 06:58 PM
Joined: Feb 2004
Posts: 494
M
mustangelectric Offline
Member
Hi,
Read 310.4, a parallel ckt is considered a single conductor so 310.16 tables apply.

You did not mention what the insulation rating is?

Where you have more than three Conductors carrying current they must be derated whether in parallel or not to prevent overheating. Except the neutral..310.15(b)(4)..

-regards

greg


[This message has been edited by mustangelectric (edited 12-22-2004).]

Re: High Voltage ductbank Derating #90909 12/22/04 08:50 PM
Joined: Mar 2003
Posts: 849
Y
Yoopersup Offline OP
Member
310.16 tables (0-2000volts)
310.15 Ampacties for conductors rated 0-2000 volts.
This is 4160 -13800 volts . read ist post I put up for temp rateings and derateing.

310.4 last sentence (conductors in parrallel shall comply with the provisions310.15 (b)(2)(a)
But again 310.15 Ampacities for CONDUCTORS Rated 0-2000 volts!
I think derateing of HV conductors should apply to this section but only 310.77 really deals with them directly.
as far as Insulation 100% or 133% read notes bottom of table 310.64
328.80 Ampacitys of MV cable Shall be determined by 310.60 ( which I referenced in orginal post)


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