Sizing Cables for a Xfmr 1

[Redskinsdb21]

Hello,

I have a 13.8 Kv-480/277 V, 1000 KVA Xfmr.

Using S = V*I*sqrt(3) I got:

I available on primary side = 41.84 Amps

I available on secondary side (Using 277 Volts) = 2,084 Amps

My questions:

1) Do I simply take 41.84 Amps (I on primary side) and multiply by 1.25% = 52.3 Amps and choose a cable with at least this Amp rating to provide a safety factor? Then do same for secondary side?

2) Is it best to use 480 V or 277 Volts on secondary side when calculating Xfmrs rated amps capability on that side? Above I calculated 2,084 Amps but 480 would only provide 1,202 Amps. Which is best to use? Would'nt you want to provide the ability to use either voltage level?

 

[waross]

I get 1202 Amps for both 277 and 480 Volts.
Hint: Use 3 instead of root three for 277 Volt calculations.
Or; use root three times 480/277.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

The rated voltage is 480 V. Then Irat=1000/sqrt(3)/0.480=1202.813 A
Or you may use 0.48/sqrt(3)=0.277128 kV and then Irat=1000/3/0.277128=1202.814 A

 

[b_gibb]

The cable is sized for the protection, not the transformer full load rating. For example, if your fuse is 30A, you need to size the cables for this value.

In other words, calculate the protection first, then the cable size.

ben
bengibb.ca
powerdesignerpro.com

 

[waross]

The cable is sized for the protection, not the transformer full load rating

26-258 Conductor size for transformers
(1) The conductors supplying transformers shall have an ampacity rating
(a) not less than 125% of the rated primary current of the transformer for a single transformer;

In other words, calculate the protection first, then the cable size.

26-252 Overcurrent protection for power and distribution transformer circuits rated
over 750 V
(1) Except as permitted in Subrules (2), (3), and (4), each ungrounded conductor of the transformer feeder or
branch circuit supplying the transformer shall be provided with overcurrent protection
(a) rated at not more than 150% of the rated primary current of the transformer in the case of fuses; and
(b) rated or set at not more than 300% of the rated primary current of the transformer in the case of
breakers.
(2) Where 150% of the rated primary current of the transformer does not correspond to a standard rating of
a fuse, the next higher standard rating shall be permitted.
(3) An individual overcurrent device shall not be required where the feeder or branch circuit overcurrent device
provides the protection specified in this Rule.
(4) A transformer having an overcurrent device on the secondary side rated or set at not more than the values
in Table 50 or a transformer equipped with coordinated thermal overload protection by the manufacturer
shall not be required to have an individual overcurrent device on the primary side, provided that the
primary feeder overcurrent device is rated or set at not more than the values in Table 50.

The cable size is dependant on the transformer primary current. Long cable may be oversized to reduce voltage drop.
The overcurrent protection is dependant on the transformer primary current, the type of overload protection or Table 50.
Under certain conditions, table 50 may allow a primary breaker to be rated at up to 600% of the transformer primary current.
My code edition is several years old.
Have there been code changes, or are you working under a local revision?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[b_gibb]

I believe we are saying the same thing. The cable size depends on the transformer's primary current — my point is that it is indirectly dependent. It is directly dependant on the protection size. The cable size changes if the protection size is selected at 125%, 150%, 175%, 200%, etc...

ben

https://www.bengibb.ca

https://www.powerdesignerpro.com

 

[waross]

The cable size changes if the protection size is selected at 125%, 150%, 175%, 200%, etc...

Have you considered a code upgrade course?
Remedial reading?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Redskinsdb21]

waross (Electrical)28 Nov 23 18:43
I get 1202 Amps for both 277 and 480 Volts.
Hint: Use 3 instead of root three for 277 Volt calculations.
Or; use root three times 480/277.


I see how you did this calculation but not sure why you use root 3 for 277 V calculations. If everything being fed from Transformer was 277 Volts I would think you would need larger cable than if everything ran on 480 V. So to calculate amps pulled from 277 V loads:

S= V*I*(sqrt(3))

I = S/(V*sqrt(3)) regardless of voltage being used?

What am I missing here?

 

[waross]

If you put a 100 Amp load across each phase of 480 Volts, the line current will be 173 Amps. The total KVA will be 3 x 480V x 100A = 144 KVA.
If you put a 100 Amp load across each phase of 277 Volts, the line current will be 100 Amps. The total KVA will be 3 x 277V x 100A = 83 KVA.
Or,
If you put a 48 KVA load across each phase of 277 Volts, the line current will be 173 Amps. The total KVA will be 3 x 277V x 173A = 144 KVA.
Because the line current is 1.73 of the phase current, you must use a factor of 1.73 when calculating phase voltages or phase currents from total KVA.
Because the line current equals the phase current for line to neutral connected loads you must use a factor of 3 when calculating phase voltages or phase to neutral currents from total KVA.

Think of it as line to neutral KVA = total KVA / 3
Phase to phase KVA = total KVA x 1/3 x 1.73
(3 / 1.73 = 1.73 so the factor is still 1.73)


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Redskinsdb21 (Mining)(OP)28 Nov 23 15:53
" I have a 13.8 Kv-480/277 V, 1000 KVA Xfmr. Using S = V*I*sqrt(3) I got: I available on primary side = 41.84 Amps. I available on secondary side (Using 277 Volts) = 2,084 Amps. My questions: #1. 1) Do I simply take 41.84 Amps (I on primary side) and multiply by 1.25% = 52.3 Amps and choose a cable with at least this Amp rating to provide a safety factor? Then do same for secondary side? #2. 2) Is it best to use 480 V or 277 Volts on secondary side when calculating Xfmrs rated amps capability on that side? Above I calculated 2,084 Amps but 480 would only provide 1,202 Amps. Which is best to use? Would'nt you want to provide the ability to use either voltage level? "
I have the following opinion for your consideration.
1. Attention: NEC does NOT covered installations underground in mines and..... See NEC 90-2 (b) (2) . The factor 1.25 may differ? for mining installations.
2. A Xfmr connected to 3-phase 400 V loads , the winding and the line current 1202 A for 1MVA loading.
2.1 However, the Xfmr connected to 3 banks of 1-phase 277 V L to N loads, the winding and the line 2084 A for 1MVA loading.
3. For info: In SI unit, kVA NOT KVA, kV NOT Kv
Che Kuan Yau (Singapore)

 

[waross]

2. In Canada a transformer connected to 3-phase 400 Volt loads, the winding current will be 833 Amps. The line current will be 1443 Amps for 1 MVA loading.
For a 1 MVA load connected to a 480 Volt delta system the winding current will be 694 Amps.
For a 1 MVA load connected to a 480 Volt delta system the line or phase current will be 1202 Amps.
For a 1 MVA load connected to a 277/480 Volt wye system the winding current and the line or phase current will be 1202 Amps.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

For info; while not si compliant, KVA is still in common use in North America.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Redskinsdb21]

b_gibb (Electrical)29 Nov 23 15:48
The cable is sized for the protection, not the transformer full load rating. For example, if your fuse is 30A, you need to size the cables for this value.

In other words, calculate the protection first, then the cable size.

Should'nt the cable be sized larger than the protection? Example, protection is 30 Amps. Size cable for 1.25% larger?

30*1.25 = 37.5 Amps...so pick next cable size above 37.5 Amps since most cables are a whole number?

 

[waross]

Hi Redskinsdb21.
A word of caution;
Most of us here are professionals and as such we are bound by codes and regulations.
Conductor sizing and protection is determined by code, not by b_gibb's beliefs.
I strongly suggest that you ignore all posts by b_gibb.
While his methods may work in special cases they will not pass any inspection nor will they pass a professional review.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Redskinsdb21]

waross (Electrical)
Noted

Finally sit down and was able to review above equations to fully understand. Your feedback is appreciated.

Quetiosn:
What locale code are you referring to above? My location we would be required to use NEC Article 450 for determining Xfmr protections and while the located code you have listed looks similar I do not think its exact.

Finally, I think anytime you see a Xfmr listed as above

13.8KV-480/277 this is telling us the 480/277 V side (secondary in this case) is a WYE (Star) connected side in this case?

 

[waross]

"What locale code are you referring to above?"
The Canadian Electrical Code.

"13.8KV-480/277 this is telling us the 480/277 V side (secondary in this case) is a WYE (Star) connected side in this case?"
Yes.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

I’ve been reading along this post and there appears to be a big difference in Canadian and NEC codes regarding transformer primary conductor sizes- NEC 240.4 in my 2020 code handbook requires conductors to be protected against overcurrent, and transformer primary conductors are not specifically exempted. So it appears to me that unless the system falls under the tap rules in 240.21, the transformer primary feeder conductors must be upsized from the minimum specified in 215.2 (feeders) or 210.19 (branch circuits) depending on the setting of the transformer primary protection device to comply with 240.4.

 

[waross]

Hi Casey. Overcurrent is short circuit protection. Transformer inrush may be up to 25 times rated full load current.
Conductor protection is not based on overcurrent protection.
Conductor size is based on rated full load current.
If there is a possibility that an ONAN transformer may be upgraded by the addition of fans, the AHJ may require the ONAFfull load current to be used.
It comes down to the conductor size or ampacity must be 125% of the transformer full load rating.
Does this agree with the NEC?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

In some cases, overload protection (not over-current protection) may be on the transformer secondary, but the ampacity of the primary conductors will be a minimum of 125% of the full load current.
Note:
The original question was to do with conductor size.

There may be some exceptions where the calculated demand load may be used in place of the rated full load current,
But;
There are pit-falls.
Will the AHJ sign off on the installation.
The customer swears that the load will never be increased and a year or so later increases the load.
The conductors are overloaded and the breaker trips.
The customer blames you.
The customer forgets his earlier assurances that the load will never be increased.
Been there, seen that.
The reduced rating of the breaker trips on energization.
Been there, seen that also. (New equipment supplied by others. Not my design.)
There are very few special circumstances that will justify sizing the conductors at less than the transformer rated current.
This may not be used as a general method for sizing conductors.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Redskinsdb21]

waross (Electrical)29 Dec 23 17:14
Conductor size is based on rated full load current.
If there is a possibility that an ONAN transformer may be upgraded by the addition of fans, the AHJ may require the ONAFfull load current to be used.
It comes down to the conductor size or ampacity must be 125% of the transformer full load rating.
Does this agree with the NEC?

I tried to copy Tables 450.3 (A) and (B) to this post but it wouldn't paste. May be a copyright infringement or something similar. Anyways, from your comments above:

Conductor size is based on rated full load current.

Nec Table 450.3 (A) requires a Xfmr (over 1000 V)primary side overcurrent/short circuit protection device to have a rating of 300% or even 600% of Xfmr primary side rated current (depending if you are using a fuse or circuit breaker)in most cases. Would this not require the cable to be sized to handle the 300 % or 600 % short circuit current the cables would experience in a short circuit situation? Or can the cables be sized at 125% of Xfmr full load current rating because when the cable experiences a short circuit situation, amps cables experience will be very large and cables should experience these short circuit faults only a short amount of time due to the fact the OCPD should trip quick enough under fault/short circuit conditions thus protecting cables before they are damaged?