some more transformer protection

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robeward

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raleigh
been sifting through the archives. got a fairly good understanding i think.

couple questions.

can the primary protection be any distance from the xfmr as long as it is before the point of service for the xfmr? haven't really found any rules on this.

240.21(C)(4), outside secondary conductors. Does this include conductors buried or in concrete encased duck and still outside of any building or structure? i guess the main deal is the code is just trying to keep unprotected feeders outside of structures should something go wrong.

table 450.3(A). what if you have a step-up xfmr that goes from low voltage to over 1000V?

thanks.
 

Carultch

Senior Member
Location
Massachusetts
been sifting through the archives. got a fairly good understanding i think.

couple questions.

can the primary protection be any distance from the xfmr as long as it is before the point of service for the xfmr? haven't really found any rules on this.

240.21(C)(4), outside secondary conductors. Does this include conductors buried or in concrete encased duck and still outside of any building or structure? i guess the main deal is the code is just trying to keep unprotected feeders outside of structures should something go wrong.

table 450.3(A). what if you have a step-up xfmr that goes from low voltage to over 1000V?

thanks.


This also brings up another interesting question. What is the ultimate definition of which side of the transformer is primary, and which side is secondary, as far as the NEC is concerned?

Transformer voltages? Direction of power flow? Side of energization (which isn't necessarily directly determined from the direction of power flow)? Side of the utility?
 

ron

Senior Member
I am not familiar with a formal definition of primary or secondary, but the name implies that the primary is the source or upstream side of the transformer.

The protection upstream of a transformer can be any distance away, although in 2011, there is a requirement for a disconnecting means for the primary (450.14).
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I am not familiar with a formal definition of primary or secondary, but the name implies that the primary is the source or upstream side of the transformer.

The protection upstream of a transformer can be any distance away, although in 2011, there is a requirement for a disconnecting means for the primary (450.14).

To avoid ambiguity in the case of a grid tied inverter feeding a transformer, you might want to say that the primary winding is the one that supplies the magnetizing current when the transformer is initially energized. That way the POCO facing side will be the primary regardless of the direction of power flow at any particular point in time.
Otherwise "source" could be taken to be the GTI side rather than the POCO side.
 

robeward

Member
Location
raleigh
i agree it all comes down to 'definition'.

i always thought of the primary as the source side.

if i have a 480V to 4160V xfmr then should i still apply table 450.3(A) and just say the high voltage side is the primary and low voltage side is the secondary?
 

big john

Senior Member
Location
Portland, ME
i agree it all comes down to 'definition'.

i always thought of the primary as the source side.

if i have a 480V to 4160V xfmr then should i still apply table 450.3(A) and just say the high voltage side is the primary and low voltage side is the secondary?
I would say no. Because those tables appear to me to be sized such that the "primary" fusing takes into account inrush current and efficiency losses from exciting the transformer, whereas the "secondary" side is just sized to prevent the load from creating an overcurrent.

If you sized them based on voltage alone, then you would probably have inadequate protection on the secondary and you might blow fuses on the primary.

I don't have access to the IEEE dictionary, but I would bet a donut that "primary" is determined by which side excites the transformer and "secondary" is determined by which side loads the transformer.

In generation systems these sides can reverse based on direction of power flow, so we refer to them as "high side" and "low side" because it avoids any miscommunications.
 

robeward

Member
Location
raleigh
I would say no. Because those tables appear to me to be sized such that the "primary" fusing takes into account inrush current and efficiency losses from exciting the transformer, whereas the "secondary" side is just sized to prevent the load from creating an overcurrent.

If you sized them based on voltage alone, then you would probably have inadequate protection on the secondary and you might blow fuses on the primary.

I don't have access to the IEEE dictionary, but I would bet a donut that "primary" is determined by which side excites the transformer and "secondary" is determined by which side loads the transformer.

In generation systems these sides can reverse based on direction of power flow, so we refer to them as "high side" and "low side" because it avoids any miscommunications.

i agree with you Big John but still a little confused as to how to apply the code
 
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