Transformer underground secondary cable sizing (Canada)

[minus3db]

Hi folks,

I'm currently sizing the secondary feed for a 1000kva 13.8kV:600V transformer. The feed will be underground in 4" conduits.

If someone could check my secondary cable sizing calculations, I'd be grateful!

I am assuming copper conductors, rated for 75 deg C. My calculations are as follows:

(1) Secondary Current I_sec = (1000kVA/sqrt(3)/600V) = 963A

(2) From Rule 26-256, the conductors must be sized not less than 125% of the secondary current rating of the transformer. Therefore 963A * 1.25 = 1204A

(3) If we assume 4 cables per phase, minimum ampacity of each = 1204 / 4 = 301A

(4) Assuming Diagram D11, Detail 4, and using Table D11A, I select 500kcmil which is good for 357A at 90 deg C. Adjusting for 75 deg C: 357A * 0.886 = 316A.

Since 316A is greater than the minimum ampacity of 301A found in step (3), I think I am OK.

I have neglected voltage drop as the distance between transformer and main incoming switch is only a few meters.

Have I missed anything? This is my first time sizing an underground service - I'm used to Tables 1-4 and the associated derating factors.

Final question - from an electrician's standpoint, I imagine that it might be more difficult to pull 4 500kcmils in a pipe rather than 5 or 6 smaller ones... should I re-calculate for, say, 6 conductors per phase?

Much thanks in advance,
-minus3db

 

[waross]

The largest cable that I have seen was 3600 kcm. Yes, 3.6 million circular mills.
I have seen fairly long runs of 3 conductor 500 kcm pulled in tray.
Don't underestimate your electricians.
But let's look at a smaller cable size.

1204 Amps / 6 = 201 Amps.
201 @ 75 deg /.886 = 227 Amps
227 Amps @90 deg = 300 kcm (241 Amps)
So you could use 6 x 300 kcm.
6x300/4x500 = 90%.
I doubt that the saving in copper would equal the extra labour and material to use 6 conduits.
As far as the electricians are concerned, any industrial electrician who has trouble with 500 kcm will have trouble with 300 kcm.
Take a close look at rule 14-100-d
If you are feeding a 1200 Amp breaker or fused switch you may be able to drop the cable ampacity to 250 Amps.
That would be 283 Amps after temperature correction.
That would be 4 x 350 kcm. (299 Amps D11A)
For the sake of 4 Amps per cable the inspector may allow a reduction to 300 kcm.
You may be able to justify this with a projected load schedule.
For future reference; When using TECK cable it is well to check several configurations. The saving is often well worth using more, smaller cables.

For long runs, consider using the 90 degree rating with 1.2 meter pigtails at each end sized at 75 degrees.

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

 

[minus3db]

Hi Bill,

Thank you for the detailed reply! Everything you have said makes sense.

For the record, I never underestimate electricians. I've been fortunate enough to work with and learn from some good ones. I try to put myself in the installer's position whenever possible... I figured that, if doing an installation in cold weather, 500kcm might be less flexible and more difficult to work with than a smaller gauge... but your point regarding cost savings is definitely valid.

Thanks again,
minus3db

 

[7anoter4]

If you have already done a duct bank of 4 ducts of 4" pvc [schedule 40] you may run 7*300 MCM [6 lives and 1 neutral per each duct].
The ampacity [per one conductor] it is 180 A-calculated [2*4*180=1440 A]. [Tc=90,Ta=20,RHO=120]
If the duct is pvc schedule 80 the allowable area it is only 4.5 inch^2[40%] and the area of 7 cables 300 MCM it is 4.96
However, I am not sure that it is easier to pull 6-7 cables of 300 mcm in a 4" conduit instead of 4*500 mcm.

 

[7anoter4]

Sorry!!!.The ampacity of 6*300 mcm per 4" duct it is only 151 A [as per CEC requirements]

 

[7anoter4]

I am sorry again!151 A it is for 4/0 awg only.I apologize!!