Secondary protection for an autotransformer?

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greenspark1

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Hello.
This is a new one for me. 600-480V three phase 45kVA autotransformer. It certainly requires primary overcurrent protection per 450.4. I believe it needs secondary overcurrent protection for the conductors, per article 240, but it's not clear reading through the Code. Also, it seems the 450.4 requirement for 125% protection of the primary side will limit the power on the secondary of this step down transformer.

Maybe it's simpler than I think and I can treat it like a standard delta-wye with overcurrent protection on primary and secondary side. Thoughts?
 
It seems to me that 240.4(F) disallows relying on the primary protection to protect the secondary conductors. I don't see an exception that fits. Further, 450.3(B) sets the maximum protection rating of the secondary over-current device. As it is a step down transformer the secondary can supply more current than the primary OCPD rating and perhaps more than the secondary conductor ampacities. Thus, they need OCPDs. Does the transformer have any purpose other than to step the 600V down to 480V?
 
An autotransformer with a single input connection, a single output connection, and a third connection common to input and output does not need a secondary OCPD provided the primary OCPD is properly sized.

Just as a standard transformer with a two-wire primary connection and two-wire secondary connection does not need a secondary OCPD. This because you can not overload the secondary without overloading the primary.

On the other hand a transformer with a 3-wire secondary (120/240) does need secondary OCPD because it is possible to place double the rated load on one of the 120V windings while fully loading the primary which could burn up the secondary without the primary OCPD seeing an overload (just seeing a full load).
 
starbolin said:
It seems to me that 240.4(F) disallows relying on the primary protection to protect the secondary conductors. I don't see an exception that fits. Further, 450.3(B) sets the maximum protection rating of the secondary over-current device. As it is a step down transformer the secondary can supply more current than the primary OCPD rating and perhaps more than the secondary conductor ampacities. Thus, they need OCPDs. Does the transformer have any purpose other than to step the 600V down to 480V?
Click to expand...

Please reread 240.4(F) carefully, it does allow the primary OCPD to protect the secondary under the circumstances I described above.
 
fmtjfw said:
Please reread 240.4(F) carefully, it does allow the primary OCPD to protect the secondary under the circumstances I described above.
Click to expand...
The primary OCPD is allowed to protect the secondary. Protecting the wires fed from the secondary is a seperate issue, yes?
 
GoldDigger said:
The primary OCPD is allowed to protect the secondary. Protecting the wires fed from the secondary is a seperate issue, yes?
Click to expand...

Maybe or maybe not:

240.4 Protection of Conductors.
(F) Transformer Secondary Conductors.
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
 
fmtjfw said:
Maybe or maybe not:

240.4 Protection of Conductors.
(F) Transformer Secondary Conductors.
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Click to expand...

I agree this would be the appropriate view of a delta-delta transformer.

(side note: The thread states "autotransformer"... I know of no autotransformer that would provide these voltages... perhaps a simple step-down transformer ? _)
 
GoldDigger said:
The primary OCPD is allowed to protect the secondary. Protecting the wires fed from the secondary is a seperate issue, yes?
Click to expand...
The secondary conductors would be considered protected if they are of an ampacity greater than the primary OCPD rating multiplied by the primary-to-secondary voltage ratio. Say primary is protected with a 125A OCPD... 125A ? 600/480 = 156.25A is the minimum ampacity for being considered protected by the primary OCPD. Otherwise, must comply with 240.21(C)(2, 3, 4, or 5).
 
starbolin said:
OP said 'three phase'. This is not the circumstances you described.
Click to expand...
EDIT: Not necessarily. Is it delta-delta or wye-wye. I don't think a 3? autotransformer can be wired delta-wye or wye-delta... have to think on that if need be. In meantime, need clarification from OP'er.
 
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fmtjfw said:
Maybe or maybe not:

240.4 Protection of Conductors.
(F) Transformer Secondary Conductors.
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Click to expand...

How is the OP not covered by the first sentence in (F) above? And how would the product of the secondary conductor ampacity and the secondary-to-primary transformer voltage ratio not exceed the value in 450.3(B)?
 
starbolin said:
How is the OP not covered by the first sentence in (F) above? And how would the product of the secondary conductor ampacity and the secondary-to-primary transformer voltage ratio not exceed the value in 450.3(B)?
Click to expand...

An auto transformer doesn't have a "primary" and a "secondary."

It has only an input and an output. I don't believe that 240.4(F) would apply at all.
 
Smart $ said:
EDIT: Not necessarily. Is it delta-delta or wye-wye. I don't think a 3? autotransformer can be wired delta-wye or wye-delta... have to think on that if need be. In meantime, need clarification from OP'er.
Click to expand...
IMO an autotransformer cannot have a delta delta configuration if the original delta is grounded unless you are willing to accept a partially open delta output (no autotransformer action on the third (both ends hot) winding.
If you have a full three phase autotransformer configuration, the three output leads are all at different voltages from the three input leads, and the line to line voltages are all rotated through a small angle.
A wye-wye autotransformer is, by comparison, very conventional. :) And it can be used to feed a delta load of course.
 
Let's hit the "reset" button for a second.
OP stated: "600-480V three phase 45kVA autotransformer."
Is there such an animal ? The only autotransformers with which I am aware would not provide that voltage range. however, if it is an autotransformer, , his reference would be 450.4 not 450.3, IMO.

Assuming it's a polyphase 600-480 transformer, Art 450 only deals with transformer protection, not conductor protection
240.21(C) would cover our conductor protection and as fmtjfw points out on a delta-delta configuration 240.21(C)(1) allows the secondary conductoors can be protected by the primary overcurrent device if the ratio is taken into account.
 
Good points and discussion.
Client says his transformer is a three phase 600-480V autotransformer, no more details.

I found one real quick with the help of google:
http://www.temcoindustrialpower.com/products/Transformers_-_General/AT3172.html
http://attachments.temcoindustrialpower.com/Data_sheet/Acme_Sec1_Autotransformers.pdf
These look like these are wye-wye since they have a XO.

I need to find out what type of autotransformer they have to see if it meets the exception for delta-delta 3 wire per 240.4(F). Talk about down in the weeds.

I can't say I understand why the exception exists for delta-delta 3 wire transformers. If delta-wye needs protection of the conductors on the secondary side, why wouldn't a delta-delta. I think we can agree the transformer protection requirements in 450 are pretty straightforward, but the conductor protection is what's tripping us up. Thanks for the good discussion!
 
Autotransformers have a primary and a secondary in that the primary is where the power comes into the transformer and the secondary is where the power goes out -- true of all transformers (almost but we'll not discuss zig-zags and their friends).

A buck-boost transformer with an actual primary coil and an actual secondary coil becomes an autotransformer when you connect a primary lead to a secondary lead -- which is by-the-way how they are used. If the primary and secondary leads are internally connected in a "true" autotransformer nothing electrical has changed.

A delta-delta 3-phase autotransformer is interesting, you can get ratios of from 1:1 to 1:2 but no greater. Also it only works (under the NEC) for ungrounded deltas.
 

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greenspark1 said:
Good points and discussion.
Client says his transformer is a three phase 600-480V autotransformer, no more details.

I found one real quick with the help of google:
http://www.temcoindustrialpower.com/products/Transformers_-_General/AT3172.html
http://attachments.temcoindustrialpower.com/Data_sheet/Acme_Sec1_Autotransformers.pdf
These look like these are wye-wye since they have a XO.

I need to find out what type of autotransformer they have to see if it meets the exception for delta-delta 3 wire per 240.4(F). Talk about down in the weeds.

I can't say I understand why the exception exists for delta-delta 3 wire transformers. If delta-wye needs protection of the conductors on the secondary side, why wouldn't a delta-delta. I think we can agree the transformer protection requirements in 450 are pretty straightforward, but the conductor protection is what's tripping us up. Thanks for the good discussion!
Click to expand...

delta-delta does not need secondary protection because it is impossible to impose a load on the secondary that is not reflected as a primary load based on the primary-secondary ratio. The time that secondary protection is required is when you can overload the secondary without having the overload appearing the the primary.

(1) you have a transformer with a two wire primary and two wire secondary, 240:120. Now if the secondary ampacity is 20A, then you would expect the primary ampacity to be 10A (in the real world a little more for copper and iron losses). If you fuse the primary for 10A, then you have effectively fused the secondary for 20A because of the transformer ratio.

(2) you have a transformer with a three wire secondary and a two wire primary: 480:120/240. Now if the secondary ampacity is 20A,then you would expect the primary ampacity to be 10A (in the real world a little more for copper and iron losses). You fuse the primary for 10A.

Now if you have a current of 20A flowing across the 240V leads, you have 10A flowing in the primary and the fuse does not blow. If you have 40A flowing across the 240V leads, you have 20A flowing in the primary and you protect the secondary because you blow the fuse.

But if you have 40A flowing across one of the 120V lead pairs, you have 10A flowing in the primary and the fuse does not blow. But the secondary is overloaded and needs protection (as is required by the NEC).
 
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