12.47kV Transformer Primary Feeder Sizing (code question)

[jstpie5]

Hello all,

I have a 1500kVA 12.47kV to 480V 3 phase transformer being installed and I was tasked with verifying the primary feeder sizing. I found another post that very closely described the situation I am facing but I have some additional questions. The transformer is being fed by a fused switch with a 100E fuse. The conductors will run in conduit underground for +10 feet and then hop in a cable tray where they will run to the primary of the transformer.

I noticed from 13.2kV Feeder Sizing that the primary conductor should be sized for the load of the transformer and future load growth. I'm good with this, but when it comes to sizing the conductors I am unsure of what I have found in the codebook (yet again).

The load of the transformer would be 1500kVA/(12470*1.732) = 69.45A. Since the highest power rating of the transformer nameplate is 1932kVA I'm thinking of using it to size the primary conductors. This comes out to be 89.45A.

I would like to use a 3/C armored cable - so, per 310.60(C)(79) - One Circuit - Detail 1 - I should be able to use MV-105 #6.

Is this correct (assuming the cable thermal damage curve coordinates nicely with the fuse curve)?

Thanks,

-jstpie5

 

[GroovyGuy]

Hi
Does anyone make a #6awg 15kV cable? You might want to check your local cable suppliers, but the smallest size might be as large as #1. Therefore cable size is a moot point.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[jstpie5]

I figured someone would say that

I almost added to my post that I would go with a #2 because that is the smallest stocked conductor (by Okonite).

For this particular application it is a moot point - I agree.

-BUT-

For future applications I would like to have a solid grasp on the code so that I can correctly apply it.

Does my sizing look correct if a #6 awg 15kV cable was available?

-jstpie5

 

[GroovyGuy]

Howdy,
Do transformer feeder conductors not have to be rated 125% of the NP full-load current of the transformer they are supplying (ie 112A @ 1932kVA)?
A couple of other issues to check include;
1) Check that your fuse can pass the expected inrush current of the transformer without melting (usually 10 - 12 X for 0.1s)
2) Check that your fuse will protect the transformer for s/c withstand (usually 2s)
3) Check that the fuse will protect the feeder cable from thermal damage due to primary fault current
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[jstpie5]

Hey - I'm supposed to be asking the questions here... Only joking of course.

I have not found anywhere in the Code that indicates a need to rate the primary conductors for 125% of the transformer load.

I would expect that to be the case but I haven't found it. I would love for someone point me in the right direction.

-jstpie5

 

[waross]

In the Canadian Code it is under;
Section:- Installation of Equipment.
Sub Section:- Transformers
Rule 26-258:- Conductor size for transformers.
Primary and secondary conductors @ 125% of rated current with exceptions for demand loads less than transformer ratings when protected in accordance with other sections of the Code.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

In my opinion, the cable has to withstand the short-circuit presumed current.
For instance according to:
IEC 60076-5 Power transformers –Part 5:Ability to withstand short circuit it is recommended:
If the short-circuit apparent power of the system is not specified, the values given in table 2 shall be used.
Table 2 – Short-circuit apparent power of the system
From 7.2 to 24 kV equipment highest voltage 500 MVA is recommended.
Isc=500/sqrt(3)/12.47=23.15 kA
For 23.15 kA and 1 sec the minimum 316 MCM copper conductor it is required
See:
NEC 2014 VIII Supervised Industrial Installations Art. 240.92 Location in circuit
Short-Circuit Formula for Copper Conductors:
(I^2/A^2)t = 0.0297 log10 [(T2 + 234)/(T1 + 234)]
if T1=85oC –loaded- and T2=250oC [EPR or XLPE insulation] then A=316 MCM.

 

[jstpie5]

7anoter4 - Wow, a 316 MCM seems like overkill imho just based off of the installations I have seen in the field. But that doesn't mean that the installations I looked at were correct though.

Also - It looks like the calculation you provided for short circuit of a 500MVA transformer is actually the FLA of the 500MVA transformer. I think short circuit current would be more like

Isc=500/sqrt(3)/12.47 *100/Z%

Assuming Z=5% Isc would be more like 463kA... I'd need a 6313 MCM. Doesn't smell right to me - but I am not very familiar with the calculations in IEC 60076-5.

What I did find in the code was that in 240.100(C) requires that the protective device (a 100E fuse in my case), short circuit current, and the conductor need to be "...coordinated to prevent damage or dangerous temperatures in conductors or conductor insulation under short-circuit conditions."

Let's assume the transformer feeding my switch is a 40MVA with Z=7%. Isc=40,000,000/(12470*sqrt(3))*100/7 = 26.5kA. (past experience we had a 34.5/13.8 kV 40MVA transformer feeding our plant motor loops)

Make sense to me so far...

Also, according to the trip curve for a 100E, a fault of 26.5kA would trip one cycle with a peak let-through of ~17 kA. For 17kA, t in the formula would be more like 0.01 seconds. Thus making the required conductor size 31.5 kcmil which is somewhere between a #6 and #4 (of course this is before any de-rating)

Hey now that sounds much more realistic.

What say you all?

-jstpie5

 

[7anoter4]

At first, I don't think the fault clearing time of any CB could be less than 6 cycles[0.1 sec].
Second, for a supply cable, I think the fault clearing time of a next circuit breaker [reserve protection] it is good enough. I think as a minimum delay could be 0.1 sec then 0.2 sec it could be fair enough. So 316*sqrt(0.2)=141.3 MCM.[3/0 awg].

 

[jstpie5]

7anoter4 - what CB are you referring to?

The primary conductors are protected by a 100E fuse. Clearing time at Isc (say 26.5kA) for a 100E (per trip this curve) should be ~0.01 if I'm looking at it correctly.

Please set me straight if I am interpreting this incorrectly.

 

[7anoter4]

If it is about fuses it seems to me it is correct.