Transformer Primary, or is it Secondary?

[beanland]

I have a semantics problem that seems to be endemic in the PV industry. Which side of the transformer is the "primary" and which side is the "secondary"? I see no clarification in the NEC. For a normal distribution transformer, the primary is the side connected to the utility source, the secondary is the load side. But does the same rule apply when used as a PV step-down? For example, 208Y/120V service and 480Y/277V PV. To me, the primary is the side closest to the utility grid because that is where excitation of the core comes from. Just because the power moves toward the grid, that does not affect the primary as being closer to the utility grid. But, this impacts 450.3 location of overcurrent protection and that is not clear. Does anyone have a citation from NEC that spells this out?

 

[electrofelon]

I pretty much agree with your reasoning. There is a section in 690 or 705 which states that when transformers are used, the calculations shall be done on both sides, I think it is for the 450.3 requirements but I'm kinda striking a blank and don't have my code book close by.

 

[GoldDigger]

It is not in the NEC, but I consider the relevant distinction to be which side supplies that magnetizing current each time the transformer is energized. In the case of a grid-intetactivr inverter that will always be the POCO side.

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[jaggedben]

I pretty much agree with your reasoning. There is a section in 690 or 705 which states that when transformers are used, the calculations shall be done on both sides, I think it is for the 450.3 requirements but I'm kinda striking a blank and don't have my code book close by.

750.65 (B).

...

My advice on the general subject: Don't use the terms primary and secondary. Say 'utility side' and 'inverter side'. That is unless you really need, in the particular conversation, to be getting into the transformer specifications and making sure, per Golddiggers post, that is properly specified.

 

[pv_n00b]

I use HV and LV for transformers, works well except when I get a 1:1 matching transformer. Then I wing it. There are some transformers that are specifically designed for one direction of power flow. So always ask when looking at a new transformer if there are any issues with bi-directional power flow. When doing fault analysis I always keep in mind that the vast majority of the fault current in the system is going to come from the utility. It does not matter if the utility is on the HV or LV side, it will drive any fault current.