Transformer Primary/Secondary

[HecThor]

Good Day!

Just would like to seek guidance when applying Table 450.3(B) when using a transformer, if my PV system uses a 400V to step down to 230V then connect it to tapping point, which is the primary side? the 400V system where the PV system is or the 230V side to tapping point? Thanks in advance!

 

[ggunn]

Good Day!

Just would like to seek guidance when applying Table 450.3(B) when using a transformer, if my PV system uses a 400V to step down to 230V then connect it to tapping point, which is the primary side? the 400V system where the PV system is or the 230V side to tapping point? Thanks in advance!

It differs. Some call the side nearest the service the primary no matter what the voltage or direction of current flow. Some call the side that current is flowing into under normal operation the primary. Some just dodge the issue and call them the high voltage side and the low voltage side.

 

[electrofelon]

Good Day!

Just would like to seek guidance when applying Table 450.3(B) when using a transformer, if my PV system uses a 400V to step down to 230V then connect it to tapping point, which is the primary side? the 400V system where the PV system is or the 230V side to tapping point? Thanks in advance!

In case you were not aware, 690.9(F) addresses this.

 

[jaggedben]

In case you were not aware, 690.9(F) addresses this.

690.9(D) in the 2017 NEC.

Also 705.30(B), same language. (Probably should be removed from 690.)

 

[HecThor]

690.9(D) in the 2017 NEC.

Also 705.30(B), same language. (Probably should be removed from 690.)

If I read it correctly, the secondary side is where the Pv System is right?

 

[jaggedben]

If I read it correctly, the secondary side is where the Pv System is right?

Well, yes, but that code section doesn't say that exactly. From the point of view of complying with 450, you treat both as both. That is, both sides must meet requirements for both secondary and primary. You call each of them one and then switch them each to the other. :happyyes: I would refer to them as 'utility side' and 'inverter side', and make a calculation table with the entries, 'Utility Side as Primary', 'Utility Side as Secondary', 'Inverter Side as Primary', 'Inverter Side as Secondary'.

My understanding in these cases is that designating a particular side as 'primary' really only matter in terms of selecting an appropriate transformer, since the transformer may be engineered to expect energizing from only one side (and that side would typically be called the 'primary', and should be towards the grid). In terms of NEC code requirements, I'm not sure it ultimately needs to be said which 'is' which.

 

[don_resqcapt19]

The impact of the code rule requiring you to look at each side of the transformer as both primary and secondary is that the OCPD on both sides is limited to 125% of the full load current for that side.

 

[pv_n00b]

The impact of the code rule requiring you to look at each side of the transformer as both primary and secondary is that the OCPD on both sides is limited to 125% of the full load current for that side.

The exception to 690.9(D) allows no overcurrent protection for the transformer when looking from the inverter side as long as the transformer rating is equal to or greater than the inverter output rating. Typically the OCPD is sized based only on the current available from the utility side.