Autotranformers are protected by primary OCPD only via 450.4(A)

[Russ_Wildcat]

I am regularly installing autotransformers on 3 phase 480/277 VAC to buck 500 VAC down to 480 VAC. This autotransformer is in series with the actual device which is 480/277 VAC @ 192 FLA. I cite NEC 450.4(A) for OCPD protection for transformer, which only requires individual protection by which it receives from the branch breaker OCPD(250 A CB). So often I have AHJ's state I must use NEC Article 450.3 and provide primary and secondary protection for the autotransformer. Is there a convincing way to present this to AHJ's that I do not need secondary protection for the autotransformer? I feel that when the AHJ sees that is 3 phase, they then think it can't then be a 2 -wire situation. But when analyzing 3 phase systems, it is customary to analyze only one phase, they disregard this immediately because it is 3 phase so you can't have 2-wire if there are 3 wires....?

 

[kwired]

You can have two transformers connected in open delta manner, but you still have three input and three output conductors. One conductor is essentially "common" though, but if it is ungrounded and an input to the transformer, I read it as needing overcurrent protection in 450.4(A).

A grounded input conductor doesn't need overcurrent protection.

 

[Russ_Wildcat]

Primary Protection only via NEC 450.4(A)

Primary Protection only via NEC 450.4(A)

Title: But do you agree that there only needs to be protection on the primary side only, not also secondary protection per NEC 450.4(A). I do agree that you can go to NEC 450.3(B) for sizing OCPD for the autotransformer, but from this you should be able to select Primary protection only to adequately protect the Autotransformer.:happyyes:

 

[MTW]

Buck Boost 500 to 477V 3Φ 3W Protection

Buck Boost 500 to 477V 3Φ 3W Protection

You need to run the numbers, to be sure, but you should be able to use primary protection only, on a delta/delta configuration. A 3Φ buck transformer bank is normally wired as an 3W open delta. Checked an Acme catalog, and the configuration for a 500 to 477V 3 or 4W input, and a 3W Delta output configuration would be connected as open delta. If your bank was set up as a Wye/Wye, then you would need secondary protection,




This Acme connection diagram shows OCP on the primary side only.

240.4
(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.

An adjoining handbook clip.

The fundamental requirement of 240.4 specifies that conductors
are to be protected against overcurrent in accordance
with their ampacity, and 240.21 requires that the protection
be provided at the point the conductor receives its supply.
Section 240.4(F) permits the secondary circuit conductors
from a transformer to be protected by overcurrent devices in
the primary circuit conductors of the transformer only in the
following two special cases:
1. A transformer with a 2-wire primary and a 2-wire secondary,
provided the transformer primary is protected
in accordance with 450.3
2. A 3-phase, delta-delta-connected transformer having a
3-wire, single-voltage secondary, provided its primary
is protected in accordance with 450.3
Except for those two special cases, transformer secondary
conductors must be protected by the use of overcurrent
devices, because the primary overcurrent devices do not provide
such protection.

Section 240.4(F) permits the secondary circuit conductors
from a transformer to be protected by overcurrent devices
in the primary circuit conductors of the transformer
only in two special cases.

The first case is a transformer with
a 2-wire primary and a 2-wire secondary, provided the transformer
primary is protected in accordance with 450.3.

The second case is a 3-phase, delta-delta-connected transformer
having a 3-wire, single-voltage secondary, provided its primary
is protected in accordance with 450.3. Where the primary
feeder to the transformer incorporates overcurrent
protective devices rated (or set) at a level not to exceed those
prescribed herein, it is not necessary to duplicate them at the
transformer.

MTW Ω

 

[kwired]

You need to run the numbers, to be sure, but you should be able to use primary protection only, on a delta/delta configuration. A 3Φ buck transformer bank is normally wired as an 3W open delta. Checked an Acme catalog, and the configuration for a 500 to 477V 3 or 4W input, and a 3W Delta output configuration would be connected as open delta. If your bank was set up as a Wye/Wye, then you would need secondary protection,

View attachment 23394



View attachment 23395
This Acme connection diagram shows OCP on the primary side only.

240.4
(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.

An adjoining handbook clip.



MTW Ω

OP is talking about autotransformers. I think he only needs to comply with 450.4 - which usually isn't hard to do if the unit is sized per the load as the overcurrent protection for the supply circuit is already in the range where it needs to be to protect the autotransformer anyway when using said autotransformer for a specific load.

 

[Russ_Wildcat]

Auto Transformers don't require secondary protection; 450.4(A)

Auto Transformers don't require secondary protection; 450.4(A)

Thanks for feedback! Can I look at it another way. Since an auto transformer is not isolated, can that exclude me from 450.3(B) ? Then the focus would be on 450.4(A) only. Also I want to stress that even though my example circuit is a 'Wye' Bucking Auto Transformer, electrically it is truly a 2-wire circuit; Thus, one phase is analyzed, assuming a balanced load.

 

[kwired]

Thanks for feedback! Can I look at it another way. Since an auto transformer is not isolated, can that exclude me from 450.3(B) ? Then the focus would be on 450.4(A) only. Also I want to stress that even though my example circuit is a 'Wye' Bucking Auto Transformer, electrically it is truly a 2-wire circuit; Thus, one phase is analyzed, assuming a balanced load.

View attachment 23402

Current in is proportional to current out, single overcurrent device (per phase) should be sufficient.

 

[synchro]

Thanks for feedback! Can I look at it another way. Since an auto transformer is not isolated, can that exclude me from 450.3(B) ? Then the focus would be on 450.4(A) only. Also I want to stress that even though my example circuit is a 'Wye' Bucking Auto Transformer, electrically it is truly a 2-wire circuit; Thus, one phase is analyzed, assuming a balanced load.

View attachment 23402

Is the wye autotransformer made up of three separate single phase autotransformers or is it a single unit with a 3 or 5 leg magnetic core? The single phase units are truly independent but 3-phase units still have a small coupling into a phase that has an open input. I think this might be why they exclude a wye-wye transformer from protection on only the input side.
In reality though, the current through the shunt winding to neutral of a 500/480 bucking autotransformer is only 4.2% of the input current, and so the amount of current in the neutral when there's an open phase will be very limited.

 

[Russ_Wildcat]

450.4(A) For Auto transformers

450.4(A) For Auto transformers

Yes it is a 3-legged core, not 3 individual Bucking autos....Yes there is the common core, but what you see on the primary side reflects directly to the secondary, else why does 450.4(A) say "individual overcurrent device installed in series with each ungrounded input conductor". I leave you with that.