MV Wire on LV Circuits

[ThomasSchlitt]

Are there any regulations, rules of thumb, etc. against using 5kv or even 15kv single insulated copper conductors such as transformer jumper lead for 480V circuits in protected installations? I haven't been able to locate anything in the code that inhibits its use.

NEC Table 310.60(C)(69) indicates an ampacity of 695A at 90deg C for a 5KV, 500MCM cable. On a 2000A circuit, one could install 3 conductors per phase instead of 6 if terminated effectively at 90 deg C and installed properly.

 

[iwire]

Seems like voltage drop would be a factor.

That said I don't think the NEC prevents using MV cable for LV applications.

 

[ThomasSchlitt]

Thanks for replying. It could be on a longer run. But for short runs, it wouldn't be an issue. With the price of Copper over $4.30/lb, cutting your number of runs in half would save a lot of money.

 

[roger]

What is meant by "protected installations"?

Roger

 

[iwire]

Thanks for replying. It could be on a longer run. But for short runs, it wouldn't be an issue. With the price of Copper over $4.30/lb, cutting your number of runs in half would save a lot of money.

Not if the runs are short as you say. :grin:

 

[ThomasSchlitt]

What is meant by "protected installations"?

Roger

Guarded against public / unauthorized personnel. Usually a locked fenced in area.

 

[roger]

Guarded against public / unauthorized personnel. Usually a locked fenced in area.

So these will be in free air in lieu of being in a raceway?

Roger

 

[ThomasSchlitt]

So these will be in free air in lieu of being in a raceway?

Roger

Yes. Actually we're considering installing per the 2011 code in a cable tray per section 392.80(B)(2)(b) which refers back to 310.60(C)(69).

 

[don_resqcapt19]

You are not gaining any ampacity by using that table. The ampacity of 500kcmil copper under those same conditions is 700 amps in Table 310.17. Well almost the same conditions. T310.69 is based on a 40?C ambient and T310.17 is based on a 30?C ambient.

 

[roger]

You are not gaining any ampacity by using that table. The ampacity of 500kcmil copper under those same conditions is 700 amps in Table 310.17. Well almost the same conditions. T310.69 is based on a 40?C ambient and T310.17 is based on a 30?C ambient.

This is where I was going with my questions too.

Roger

 

[ThomasSchlitt]

Thank you all for your assistance. I was looking into a practice that had been done over the years and seeking justification. They will be installing this in the near future for a 2000A service.

Another problem.
The wire being used is designed such that it should not be in contact with other conductors and should maintain a spacing of at least the conductor diameter between wires. For parallel feeders in ventilated cable trays, the code indicates that conductors should be grouped together and securely fastened in a triangular or square fashion if the wire is not 1/0 to 4/0. 392.20(C) Not only does that lessen the ampacity, but it violates the wire's recommended installation method. How does the code address this?

We are planning on a single layer method of installation with the conductors arranged AAA, BBB, CCC. The code vaguely touches on this but doesn't seem to prohibit it. What are your thoughts on this?

 

[kingpb]

A 500KCMIL MV cable is going to cost more than a 500KCMIL LV cable. Copper is copper. The difference between the cable is the insulation.

Also, you are not going to find 90deg C terminations in LV equipment. The reason MV cable is rated for 105 and 90 deg C is because MV equipment terminations are rated for 90 deg C.

MV cable can be used in LV installations. But once you do a cost analysis, it will be better to stick with LV cable. As an alternative to copper look at running aluminum.

 

[ThomasSchlitt]

90 Deg Terminations on the transformer won't be an issue, and the distribution panelboard will probably have a power distribution block rated at 90 Deg C. From there we'll run properly sized wire for 75 Deg terminations to the breaker.

As far as the MV cable, with its shielding and multiple layers of insulation, it tends to be more robust than 600V wire. (And it's what they've already purchased.)

I'll check into the Aluminum on future projects. Thanks for the tip.

 

[jdsmith]

90 Deg Terminations on the transformer won't be an issue, and the distribution panelboard will probably have a power distribution block rated at 90 Deg C. From there we'll run properly sized wire for 75 Deg terminations to the breaker.

As far as the MV cable, with its shielding and multiple layers of insulation, it tends to be more robust than 600V wire. (And it's what they've already purchased.)

I'll check into the Aluminum on future projects. Thanks for the tip.

WHen you say MV wire is more robust than 600V, what type of 600V wire are you comparing it to? I agree with you on THHN, but XHHW is another story - it is much tougher than THHN.