Transformer - 3 phase 575 to 480 using two single phase transformers

[Rcarter]

In this old factory I am working at now, they have 3 phase 575Vac 1000A CB supplying power to two single phase transformers as shown below.



The wire from CB to transformer is 2- 1/c 500Kcmil CU for each phase. The same wire size is used on the secondary to a bus duct.
Is this ok with the wire rated at 760A?

Thanks

 

[Jraef]

Are the conductors in raceway?

 

[Smart $]

In this old factory I am working at now, they have 3 phase 575Vac 1000A CB supplying power... Is this ok with the wire rated at 760A?

No. 240.4(C) requires the conductor ampacity of circuits with ocpd rated over 800A to be not less than the ocpd rating. If adjustable trip circuit breaker, and adjust to 800A or less, must meet the requirements of 240.6(C).

 

[Jraef]

The 500kCMILs in open air are rated 620A each (assuming 75C), hence my question as to how this is wired. If in raceway, it's a problem. In open air, maybe not.

Also, if the primary is utility equipment, they don't have to follow the NEC, it's their stuff.

 

[Tony S]

OK I’m trying to get my head around this set up.

Is the 600V corner grounded delta? I drew it out as 600V star and it made no sense what so ever, corner grounded delta seems logical.

 

[Jraef]

OK I’m trying to get my head around this set up.

Is the 600V corner grounded delta? I drew it out as 600V star and it made no sense what so ever, corner grounded delta seems logical.

No, it's just open delta, they are not showing a corner ground in that drawing. As to whether or not that meets our code without having a corner ground, that will depend on other facts not in evidence here.

 

[Tony S]

Cheers JR, I hadn’t considered open delta.

My brain has just gone out to lunch

 

[david luchini]

OK I’m trying to get my head around this set up.

Is the 600V corner grounded delta? I drew it out as 600V star and it made no sense what so ever, corner grounded delta seems logical.

This is an Autotransformer setup. It is "bucking" the 600V to 480V.

They are using 2 50kva transformers to get 432kVA, 3phase (or 2 75kVA transformers to get 650kVA, 3 phase,etc.)

 

[Smart $]

... In open air, maybe not.
...

If under NEC purview, terminal temperature limitations are per Table 310.15(B)(16)... so 380A per conductor is the maximum for 75°C-rated terminations.

 

[Ingenieur]

If under NEC purview, terminal temperature limitations are per Table 310.15(B)(16)... so 380A per conductor is the maximum for 75°C-rated terminations.

if in free air why not Table 310.15(B).17
75C Cu 500 kcmil 620 A
???

 

[Smart $]

if in free air why not Table 310.15(B).17
75C Cu 500 kcmil 620 A
???

Here's the general statement of 110.14(C)(1)...

(1) Equipment Provisions. The determination of termination
provisions of equipment shall be based on 110.14(C)(1)(a) or
(C)(1)(b). Unless the equipment is listed and marked otherwise,
conductor ampacities used in determining equipment termination
provisions shall be based on Table 310.15(B)(16) as
appropriately modified by 310.15(B)(7).

 

[Ingenieur]

Here's the general statement of 110.14(C)(1)...

Makes no sense
renders all the other tables (free air, messenger, etc) null and void

 

[Smart $]

Makes no sense
renders all the other tables (free air, messenger, etc) null and void

The "tables" establish the allowable ampacity, and from that you determine the adjusted and corrected ampacity.

Circuit ampacity is the lesser of the above or the ampacity correlating to the minimum conductor size determined under 110.14(C).

http://www2.schneider-electric.com/...7/en_US/Wire Terminations 0110DB9901R2-02.pdf
Dated but still valid in principle.

 

[coop3339]

Makes no sense
renders all the other tables (free air, messenger, etc) null and void

You could splice to larger conductors before entering the equipment. Similar to a service drop to SE conductors.

 

[Rcarter]

The conductors are in a raceway.

 

[Smart $]

The conductors are in a raceway.

Well, that's definitely a problem. Choices are: increase conductor ampacity or reduce ocpd rating.

What is the bus duct rating?

 

[Rcarter]

2000 amp

 

[Smart $]

2000 amp

Okay. Just wanted to make certain it will not be a limiting factor.