Sizing of cable for 150kVA Transformer

[jaehlee99]

I am working with some project. For example,

1) 600V / 3PH panel. Feeding 150 KVA Xformr. 600V - 120/208. Primary current = 144.3 amps.....x 1.25 = 180 amps. Basing it on 75 degree column, so use 3/0 copper.
Secondary current 416.4 x 1.25 = 520 amps...so ( 2 ) parallel runs of 300 MCM copper.

2) 600V/ 3PH panel, Feeding 15 KVA Xformr. 600V - 120/208. Primary current = 14.3 amps.....x 1.25 = 18 amps. Basing it on 75 degree column, so use 12 copper.
Secondary current 41.5 x 1.25 = 52 amps...so #6 copper.

We have potential problem. For number 1) It was decided to use 250 amp breaker to feed Xfrmr, as inrush current might trip breaker. Would it be acceptable to use 250A breaker. If so, how can make out the sizing of secondary cable again? Would you explain the steps how to calculate this case?

 

[augie47]

1st you need to assure proper transformer protection per 450.3
Then you need to assure you conductors are protected per Art 240.

As to (1):

If you elect to increase your primary breaker to 250 amps, beyond the 125%, then your transformer secondary must be protected at 125% of the rated current. The next larger size OCP device is allowed for protection, however, secondary conductors must be equal or greater than the OCP device, so if you used your parallel 300's (separate raceway) you would be limited to a 500 amp OCP. Art 450 rules would allow you to use a 600 amp secondary protection device but your wire would need to be rated at 600 amps minimum

Keep in mind that since your transformers are 4 wire, 240.21(C) will not allow the primary overcurrent device to protect the secondary conductors so secondary overcurrent protection will be required in both scenarios.

 

[jaehlee99]

I appreciate your reply.

Many thanks,

 

[Smart $]

...

If you elect to increase your primary breaker to 250 amps, beyond the 125%, then your transformer secondary must be protected at 125% of the rated current. The next larger size OCP device is allowed for protection, however, secondary conductors must be equal or greater than the OCP device, so if you used your parallel 300's (separate raceway) you would be limited to a 500 amp OCP. Art 450 rules would allow you to use a 600 amp secondary protection device but your wire would need to be rated at 600 amps minimum

Keep in mind that since your transformers are 4 wire, 240.21(C) will not allow the primary overcurrent device to protect the secondary conductors so secondary overcurrent protection will be required in both scenarios.

Correct, but I'd like to touch on some of the finer points for the OP'ers sake....

We can use either Primary Only Protection or Primary and Secondary Protection as evident in Table 450.3(B). Using one in particular is never required. However, note we can only "round up" to the next size standard OCPD rating for the primary only protection scheme. For primary and secondary protection scheme, we cannot "round up"... but under the conditions of Note 3 we are permitted a higher rating. Regardless, under either scheme, the primary conductors must be sized to be protected by the OCPD per Article 240.

On the secondary side, up to three OCPD requirements affect rating determination (and as a matter of consequence, conductor rating):

1. transformer secondary protection if opting for primary and secondary protection scheme
2. transformer secondary conductor protection [240.21(C)]
3. supplied equipment protection such as a disconnect or panel

It is self evident the first OCPD protects the secondary conductors and serves as the transformer secondary protection where primary and secondary protection scheme is used. It also serves as the system disconnecting means. The OCPD rating cannot exceed the rating of the equipment. As such, we can see this OCPD has to comply with all three requirements.

You noted the 4-wire secondary [conductors] cannot be protected by the primary OCPD. I'd just like to add that it does not mean we must use the primary and secondary protection scheme. In most cases we will opt for that scheme, but in a few [rare] cases, it may be advantageous to use the primary only protection scheme for the transformer.

 

[jaehlee99]

Thank for your reply.

 

[GoldDigger]

Correct, but I'd like to touch on some of the finer points for the OP'ers sake....

We can use either Primary Only Protection or Primary and Secondary Protection as evident in Table 450.3(B). Using one in particular is never required. However, note we can only "round up" to the next size standard OCPD rating for the primary only protection scheme. For primary and secondary protection scheme, we cannot "round up"... but under the conditions of Note 3 we are permitted a higher rating. Regardless, under either scheme, the primary conductors must be sized to be protected by the OCPD per Article 240.

On the secondary side, up to three OCPD requirements affect rating determination (and as a matter of consequence, conductor rating):

1. transformer secondary protection if opting for primary and secondary protection scheme
2. transformer secondary conductor protection [240.21(C)]
3. supplied equipment protection such as a disconnect or panel

It is self evident the first OCPD protects the secondary conductors and serves as the transformer secondary protection where primary and secondary protection scheme is used. It also serves as the system disconnecting means. The OCPD rating cannot exceed the rating of the equipment. As such, we can see this OCPD has to comply with all three requirements.

You noted the 4-wire secondary [conductors] cannot be protected by the primary OCPD. I'd just like to add that it does not mean we must use the primary and secondary protection scheme. In most cases we will opt for that scheme, but in a few [rare] cases, it may be advantageous to use the primary only protection scheme for the transformer.

As I see it, the fact that primary protection cannot protect the transformer itself from damage in the case of an asymmetric secondary fault rules out the use of primary only protection. Unless. that is, you are willing to sacrifice the transformer.
I do not understand your assertion that primary only protection is allowed.

 

[Smart $]

As I see it, the fact that primary protection cannot protect the transformer itself from damage in the case of an asymmetric secondary fault rules out the use of primary only protection. Unless. that is, you are willing to sacrifice the transformer.
I do not understand your assertion that primary only protection is allowed.

The rules for transformer protection are completely separate from the rules for secondary conductor protection. Yes, there are a couple rules [240.4(F), 240.21(C)(1)] where the primary protection cannot serve as secondary conductor protection... but that is not the same as transformer secondary protection. While secondary conductor protection can serve as transformer secondary protection, it is not required.

 

[GoldDigger]

OK. I knew that the rules for secondary and secondary conductors are different, but I thought that there was a requirement to protect the transformer. And primary only protection cannot always do that. Meanwhile, the rule against tapping a tap would prevent using secondary conductor protection anywhere but at the transformer.
What other solution do you have for protecting the secondary conductors? I suppose you could use large secondary conductors and protect them with a breaker that is too large to provide protection to the transformer secondary. But what would be the point of that?

 

[Smart $]

... But what would be the point of that?

IDK. I would likely opt for primary and secondary protection scheme. Others corrected me when I tried to say secondary protection was required. I believe Jim Dungar explained a situation where it made sense, but I don't recall what it was.

 

[don_resqcapt19]

... Meanwhile, the rule against tapping a tap would prevent using secondary conductor protection anywhere but at the transformer.
What other solution do you have for protecting the secondary conductors? ...

How so? The secondary conductor rules have provisions for a 10' conductor, a 25' conductor and an 1,000,000' conductor (outside, unlimited length rule).

 

[Smart $]

How so? The secondary conductor rules have provisions for a 10' conductor, a 25' conductor and an 1,000,000' conductor (outside, unlimited length rule).

He means secondary conductor protection must be the same device as transformer secondary protection wherever primary and secondary protection scheme is used. There can be no secondary conductors after the first OCPD on the secondary side.

 

[Smart $]

... But what would be the point of that?

Just speculating here, but perhaps multiple secondary ocpds is where it'd be an advantage to use the primary only protection scheme. If you use primary and secondary protection scheme, the sum of secondary ocpd ratings cannot exceed the limit of imposed on one ocpd.

Say you have two machines powered by the same transformer, and you only use one at a time. If you use primary and secondary protection scheme, the transformer would have to be rated for powering both machines concurrently, even though you only use one at a time.

 

[GoldDigger]

How so? The secondary conductor rules have provisions for a 10' conductor, a 25' conductor and an 1,000,000' conductor (outside, unlimited length rule).

I have seen the argument made here, and generally accepted at the time, that the transformer secondary wires, not being protected at the source (whatever that may be in this case), are themselves tap wires, so connecting secondary conductors directly to them would be "tapping a tap."
In those transformer configurations where primary only protection effectively protects the secondary too (e.g. delta-delta), the secondary itself would not consist of tap conductors and so you could apply tap rules to the external secondary conductors.

 

[david luchini]

I have seen the argument made here, and generally accepted at the time, that the transformer secondary wires, not being protected at the source (whatever that may be in this case), are themselves tap wires, so connecting secondary conductors directly to them would be "tapping a tap."
In those transformer configurations where primary only protection effectively protects the secondary too (e.g. delta-delta), the secondary itself would not consist of tap conductors and so you could apply tap rules to the external secondary conductors.

I would say the transformer secondary wires aren't a "tap," they are a "source."

The feeder which is comprised of the transformer secondary conductors are allowed to be connected to that source, without overcurrent protection at that source, by 240.21(C).

 

[don_resqcapt19]

I have seen the argument made here, and generally accepted at the time, that the transformer secondary wires, not being protected at the source (whatever that may be in this case), are themselves tap wires, so connecting secondary conductors directly to them would be "tapping a tap."
In those transformer configurations where primary only protection effectively protects the secondary too (e.g. delta-delta), the secondary itself would not consist of tap conductors and so you could apply tap rules to the external secondary conductors.

If the wires you are calling the "transformer secondary wires" are not a part of the transformer itself, then they are transformer conductors covered by 240.21(C). If they are part of the transformer, then the rules in 240.21(C) apply to the field installed conductors that are connected to the "transformer secondary wires".

 

[iceworm]

I appreciate your reply.

Many thanks,

jae -
Your profile says you are in Canada. And 600V (575V) is pretty common in Canada - not so common in the US.

All the references given were US NEC. Do those references fit he Canadian electric Code very well?

ice