112.5kVA tranformer Primary Protection

[anbm]

112.5kVA transformer 480Vdelta primary - 208Y/120V secondary.

We used to put 175A/3P breaker in the panel that feeds this transfomer.

Square-D recently issues a document and states that we have to use 250A/3P breaker to protect this xfmr instead of 175A/3P, they said because 175A/3P won't help transformer inrush current. This doesn't make much sense to me:

1. Never heard a xfmr can have inrush current, most of time we won't have motor load fed out of this type of xfmr, typical load shall be receptacle and lighting or small equipment.

2. If we increase primary CB from 175A to 250A as recommended, we may need to increase primary wire size to comply with NEC.

3. We may run into coordination issue between 250A breaker and its upstream breaker instead of 175A CB.

4. Not sure if other manufacturers have similar approach?

Anyone has any experiences with this matter? Thanks!

 

[augie47]

112.5kVA transformer 480Vdelta primary - 208Y/120V secondary.

We used to put 175A/3P breaker in the panel that feeds this transfomer.

Square-D recently issues a document and states that we have to use 250A/3P breaker to protect this xfmr instead of 175A/3P, they said because 175A/3P won't help transformer inrush current. This doesn't make much sense to me:

1. Never heard a xfmr can have inrush current, most of time we won't have motor load fed out of this type of xfmr, typical load shall be receptacle and lighting or small equipment.!

They definitely do (have inrush). Think of it as a motor that dosen't turn.

2. If we increase primary CB from 175A to 250A as recommended, we may need to increase primary wire size to comply with NEC.!

The conductors will have to correspond to the breaker

3. We may run into coordination issue between 250A breaker and its upstream breaker instead of 175A CB.!

An enginering study could determine

4. Not sure if other manufacturers have similar approach?

Anyone has any experiences with this matter? Thanks!

First time I have known of such input from a manufacurer, but it may be commonplace. I have seen 112.5s operating on a 175 amp breaker. In fact, without proper secondary overcurrent protection, I believe that would be the Code required primary OCP.
It might be helpful if your breakr has an adjustable trip curve.

 

[jim dungar]

Since 2009 a Square D 112.5kVA EE**T3H has had an inrush of 7.8X.

 

[Open Neutral]

1. Never heard a xfmr can have inrush current, most of time we won't have motor load fed out of this type of xfmr, typical load shall be receptacle and lighting or small equipment.

The first 3-5 cycles it looks almost like a short; then the flux gets back in line with the 50Hz and it drops to the core/magnetizing loss level; a few % of full load.

 

[Open Neutral]

The first 3-5 cycles it looks almost like a short; then the flux gets back in line with the 50Hz and it drops to the core/magnetizing loss level; a few % of full load.

Doh! sixty hertz of course....

 

[kwired]

450.3 is for overcurrent protection of the transformer.

Conductor size and conductor overcurrent protection are not addressed in 450 so you must go back to 240 and 310 for these.

240.21(3) allows transformer primary conductors to be protected above conductor ampacity if length of primary and secondary conductors together are not over 25 feet with a few other requirements included.

I don't see that the 175 amp breaker is allowed unless 240.21(3) applies or if using minimum 175 amp conductors.

Increasing the breaker to 300 amps would be allowed by 450.3 but increasing conductor size may have to come with it.

As for how much inrush there actually is - just like motors or any other inductive load, capacitiors, or short circuits and ground faults the amount of instantaneous current that will flow will depend on the impedance of the source as well as the impedance between the source and the load.

 

[iwire]

240.21(3) allows transformer primary conductors to be protected above conductor ampacity if length of primary and secondary conductors together are not over 25 feet with a few other requirements included.

Primary conductors? You lost me, 240.21?(3)

 

[kwired]

Primary conductors? You lost me, 240.21?(3)

Sorry 240.21(B)(3).

 

[david luchini]

240.21(3) allows transformer primary conductors to be protected above conductor ampacity if length of primary and secondary conductors together are not over 25 feet with a few other requirements included.

240.4(B) allows the transformer primary conductors to be protected above conductor ampacity, as long as the OCPD is not more than 800A.

#4/0 could be used on the primary with a 250A c/b, and 300MCM could be used on the primary with a 300A c/b.

 

[templdl]

I have always thought of it this way:
You must provide secondary protection if you have a 1ph3w or 3ph 4w secondary. As such the transformer is protected from overload by applying a properly sized OCPD.
What does the primary breaker bring to the party? It is used to protect the upstream distribution system should the transformer itself fail. Essentially it is short circuit protection because the overload protection for the transformer is already provided by a properly sized secondary breaker.
Then size the primary breaker so that it is no more than maximum allowed but such that it doesn't nuisance trip because inrush upon energizing the transformer.

 

[augie47]

I have always thought of it this way:
You must provide secondary protection if you have a 1ph3w or 3ph 4w secondary. As such the transformer is protected from overload by applying a properly sized OCPD.
What does the primary breaker bring to the party? It is used to protect the upstream distribution system should the transformer itself fail. Essentially it is short circuit protection because the overload protection for the transformer is already provided by a properly sized secondary breaker.
Then size the primary breaker so that it is no more than maximum allowed but such that it doesn't nuisance trip because inrush upon energizing the transformer.

I stand to be corrected, but I don't think there is any transformer related requirement on the size of the secondary protection as long as the primary is protected per 450.3(B). The "requirement" for secondary protection on a 1p3w or 3p4w secondary is in 240.4 and is conductor protection, not necessarily transformer protection, so, in such situations the primary protection does indeed protect the transformer.
If your secondary protection is such as to prevent transformer overload, then I agree, but, with a properly sized primary OCP device the secondary is only limited by the conductor sizing.

 

[david luchini]

I agree with Gus. If the primary of the 112.5kVA transformer was protected at 175A (using the 'primary only' protection per T450.3(B)) then nothing would prevent the secondary OCPD from being, say, 800A (256% of the rated secondary current.) The ampacity of the secondary conductors would have to be at least 800A.

 

[kwired]

240.4(B) allows the transformer primary conductors to be protected above conductor ampacity, as long as the OCPD is not more than 800A.

#4/0 could be used on the primary with a 250A c/b, and 300MCM could be used on the primary with a 300A c/b.

Above conductor ampacity meaning the next higher standard device up to an 800 amp device. This is general rules and not specific to transformers.

I have always thought of it this way:
You must provide secondary protection if you have a 1ph3w or 3ph 4w secondary. As such the transformer is protected from overload by applying a properly sized OCPD.
What does the primary breaker bring to the party? It is used to protect the upstream distribution system should the transformer itself fail. Essentially it is short circuit protection because the overload protection for the transformer is already provided by a properly sized secondary breaker.
Then size the primary breaker so that it is no more than maximum allowed but such that it doesn't nuisance trip because inrush upon energizing the transformer.

That logic makes sense but unlike motors that may have a circuit breaker with a trip setting well above the ampacity of the conductor there is no section I can find with similar allowances for transformers. Conductors must be protected by general overcurrent protection rules.

A transformer will have an instantaneous current upon energizing just like a motor, but will drop to normal operating levels as soon as the magnetic fields are up. A motor still has to accelerate the load and will draw a heavy current until the load gets close to full speed. Now a transformer feeding motors will have some of this problem but it is not the transformer causing the problem.