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Joined: Mar 2003
Posts: 849
Y
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Table 310-77 Hight Voltage cable 2001-5000v
90c MV 90 and 105C Mv 105 Wire size as an Example #1 = 90c(MV90)=170amps
#1 = 105c(MV105)=180 amps

Terminations 110.40
States terms to be rated at 90c unless otherwise idenitfied.
Question is Wire rated at 90c for terms 105 for derateing.
Simlar to under 600 volts 75% terms 90% for derating.

The way it reads I;d say is if you use MV105 the amps are rated at 180.
Question Do all other factors come into play as for Parallel conductors , I know derating goes buy the details (1-2-3 ect).
Needs others opinions .
This is in a underground ductbank

2017 / 2014 NEC & Related Books and Study Guides
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Yooper- I'd agree with you that your example of 105C wire could be used at it's value of 180a for derating purposes but the maximum ampacity would be 170a. Maybe it would be a good idea to consult an Engineer on this one since they are smarter than us inspectors [Linked Image]


George Little
Joined: Feb 2002
Posts: 182
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Bob Offline
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"Terminations 110.40
States terms to be rated at 90c unless otherwise idenitfied.
Question is Wire rated at 90c for terms 105 for derating."

No. There are cables that have 105 temp ratings. If your terminal is not rated at 90
or 105 C degrees then you have to reduce the cable rating. Problem there are no tables in the NEC to get that figure. Contact the cable MFG for assistance.

"The way it reads I'd say is if you use MV105 the amps are rated at 180.
Question Do all other factors come into play as for Parallel conductors".

You are correct. MV105 is rated at 180 amps
provided you conform to the information given
by Table 310.77. 3 conductors per duct,
Temp 20C, 100% LF, Earth Rho factor 90 and
conductor max temp either 90 or 105C.
Note that you must use Fig 310.60 for the duct bank arrangments.
Under Table 310.77, the first table is for
1 circuit, 3 cables, Detail 1 Fig 310.60.
The second table under 310.77 is for 3 circuits, detail 2 and so on thru the rest of the tables.
You would not have parallel cables in the ducts so no derating would be needed.

Are you installing 5 kv cable?


[This message has been edited by Bob (edited 11-12-2004).]

Joined: Mar 2003
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Gets more interesting. There are 13.8 ,4160,and 480 cables(all single conductors in this ductbank. The 480 I know 75% for terms , the 4160 and 13.8 I;d say 90% unless terms rated for 105% which i'm sure there not. They also had a 100 pair phone cable in this bank passing thru the manholes. I feel per 800.11 A this cannot be in there. (cables pass thru several manholes)This is all customer owned.
Also since theses are all feeders a Ground wire sized per overcurrent device in all conduits required. just would like to have others opinons . Thank you in advance for your imput

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Bob Offline
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I assume that the 13.8, 4kv and 480 cables are in individual ducts. I hope so.
Your post was concerned about the terminal temperature. I do not think this is a problem with the 13,8 and 4 kva cables since the load is not likely to be near the max capacity of the cable. It might be a problem with the 480 volt cables. One thing you need to check is the panel terminal for the 480 volt service. If its old it might have 60C terminals. It new it has 75C termsinals.

"They also had a 100 pair phone cable in this bank passing thru the manholes. I feel per 800.11 A this cannot be in there. (cables pass thru several manholes)This is all customer owned."

The communication installation is not per the code unless there is a barrier intalled per 800.11. You might be able to split a sch 80 conduit and get an ok from the inspector. I hope these are racked on the oposite side of the man hole.

"Also since theses are all feeders a Ground wire sized per overcurrent device in all conduits required."
I think you mean an equipment grounding conductor. The 480 volt service should have an EGC per table 250.122.
What do the 13.2 and 4 kv serve?

[This message has been edited by Bob (edited 11-23-2004).]

Joined: Mar 2003
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Bob
13.8 & 4160 serve switchgear in other buildings from the power plant. I feel since they are feeders high voltage or not they require equipment grounding .The 600 volt no question EGC per 122 .As far as terms on HV the cable is rated at 90% 105% but most if not all terms are rated at 90% the 105 can be used for derating. I have brought several of theses questions up to the code panels but found out theres not a great deal of good info out there on H.V. questions. I am aware of the 60% on terms on older panels . Thanks for the interchange of ideas. The wires by the way each in there own 4 inch.


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