mv cable ampacity

[mariomanansala]

I have a motor load with a nameplate rating of 850 amps at 6.9kv. i will use three sets of three conductor MV 105 deg C cables which will run 380' in a cable tray and 20' in conduits.
Do I have to use 750kcmil per Table 310.60(C)(79)(Three Circuits) subject to temperature correction? Or can I use 250kcmil per NEC Table 310.60(C)(71) and NEC Sections 310.60(B)(1) Exception and 392.80B?

 

[Julius Right]

First of all you have to check if the connected with apparatus may withstand 105 oC [usually only 70-75 degree].
Second, if you intend to employ cable tray according to 392.80(B)(2) you have the following possibility:
a) single conductors cabled together (triplexed, quadruplexed, etc.)- without free distances:
75% 310.60(C)(69) [or 70% if it is covered]. For 750 mcm 742 A for 250 mcm 371
b) single layer and 1 o/d clearance 100% 310.60(C)(69) for 750 mcm 990 A for 250 mcm 495 A.
c) in groups of 3*c[or 4*c] and 2.15*o/d clearance:
Table 310.60(C)(67) for 750 mcm 820 for 250 mcm 430 A.
Table 310.60(C)(77) [Embedded Conduits] 3 circuits Detail 2:
For 750 mcm 475A for 250 mcm 280 A.
According to 310.60(A)(1) Selection of Ampacity if the underground run is more than 3m[10 ft.] you have to select the minimum ampacity: For 750 mcm 475A for 250 mcm 280 A.

 

[bob]

Quote "I have a motor load with a nameplate rating of 850 amps at 6.9kv".

That's a lot of amps at that voltage. What size motor is this?

 

[Julius Right]

Quote "I have a motor load with a nameplate rating of 850 amps at 6.9kv".

That's a lot of amps at that voltage. What size motor is this?

I think it is about 10,000 hp. It could be a Boiler Feedwater Pump for 700 MW steam turbine?:?

 

[Smart $]

First of all you have to check if the connected with apparatus may withstand 105 oC [usually only 70-75 degree]...

See below...

110.40 Temperature Limitations at Terminations. Con-
ductors shall be permitted to be terminated based on the
90?C (194?F) temperature rating and ampacity as given in
Table 310.60(C)(67) through Table 310.60(C)(86), unless
otherwise identified.

 

[mbrooke]

I think it is about 10,000 hp. It could be a Boiler Feedwater Pump for 700 MW steam turbine?:?

I want one FWIW 6.9kv is a voltage I have seen used commonly in power generating facilities and large industrial customers.

 

[mariomanansala]

The motor is 12,500hp synchronous driving a big water pump.

@Julius, the cable is 3 conductors in an overall jacket, so use table 310.60(C)(79) instead of table...(77)?

 

[Smart $]

The motor is 12,500hp synchronous driving a big water pump.

@Julius, the cable is 3 conductors in an overall jacket, so use table 310.60(C)(79) instead of table...(77)?

Table 310.60(C)(79) is for underground duct banks. Your original post says cable will be 380' in cable tray and 20' in conduit.

Does the conduit exceed 10' at either end?

 

[mariomanansala]

The conduit exceeds 10' at one end.

 

[Smart $]

The conduit exceeds 10' at one end.

This will limit your ampacity to that of 310.60(B)(75) and the lesser of the following:

• adjusted and corrected from 105?C value
• the 90?C value (termination temperature limitation imposed by 110.40)

JMO

E2A: Assumed you were using copper conductor. Table 310.60(B)(76) if using aluminum.

 

[Julius Right]

I think since in the OP it was mentioned Table 310.60(C)(79) and Table 310.60(C)(71) it could be both: underground or in OPEN [no conduit] air. Underground running is for less ampacity, of course.
If the conduits are running in open air one has to keep a certain clearance and to protect against sun radiation [I did not see any NEC reference for 2001 to 35,000 V cable in conduit in air].
If the cable is three-conductor copper cable then you have to follow Table 310.60(C)(77), of course.

 

[mariomanansala]

This will limit your ampacity to that of 310.60(B)(75) and the lesser of the following:

• adjusted and corrected from 105?C value
• the 90?C value (termination temperature limitation imposed by 110.40)

JMO

E2A: Assumed you were using copper conductor. Table 310.60(B)(76) if using aluminum.

Thanks for your quick and thorough tips.
i think you mean table 310.60(B)(77) 3 circuits detail 2, right? Subject to the temp adjustments above, of course.
There are two other motors in this same installation. one is 4300hp @ 294 FLA, and the other is 9000hp @ 613 FLA. if i sue the same size cable but only one set for the first and two sets for the second, do i use the same table but 1 circuit detail 1 on the first? What about the second?

 

[Smart $]

Thanks for your quick and thorough tips.
i think you mean table 310.60(B)(77) 3 circuits detail 2, right? Subject to the temp adjustments above, of course.
There are two other motors in this same installation. one is 4300hp @ 294 FLA, and the other is 9000hp @ 613 FLA. if i sue the same size cable but only one set for the first and two sets for the second, do i use the same table but 1 circuit detail 1 on the first? What about the second?

Read the title. Table 310.60(B)(77) is for underground electrical ducts. You have yet to mention anything about any part of the installation being underground IIRC.

 

[kingpb]

I have a motor load with a nameplate rating of 850 amps at 6.9kv. i will use three sets of three conductor MV 105 deg C cables which will run 380' in a cable tray and 20' in conduits.
Do I have to use 750kcmil per Table 310.60(C)(79)(Three Circuits) subject to temperature correction? Or can I use 250kcmil per NEC Table 310.60(C)(71) and NEC Sections 310.60(B)(1) Exception and 392.80B?

Please don't take offense, but if you are having to ask these questions, you are not appropriately qualified to handle MV circuits and such large motors. I would assume that someone in your office may be capable of sitting down with you and mentoring you in this area.