# Re: [RE-wrenches] Transformer Nomenclature/Protection/Grounding

```hi I have copied your text. My comments are in red

- Is there any consensus on what circuit is considered the "primary" and
which is the "secondary" in this type of scenario?  Yes, the energy supply
side is the primary, so the solaredge side is the Primary
- NEC 690.9(D) speaks of "considering first one side of the transformer,
then the other side of the transformer, as the primary". Is this relevant? Yes;
you must protect from the grid side (the grid has all the real power) then
the high-voltage side inverter side is protected by the inverter
capacity.   i.e.
Is it possible for both sides of the transformer to be labeled as a
the energy flows from primary to secondary with a slight voltage drop, the
turns ratio reflects this slight voltage change.  Also, the low voltage
side is wound next to the core and the higher voltage side is wound away
from the core.```
```
- I understand that 450.4(A) requires overcurrent protection on the "input
conductors" (aka "primary conductors"?), correct? Yes, the grid side must
be protected in the event of a fault. And the solar side must be protected
or have conductors sufficient to carry the current as limited by input
(grid) side OCPD.  The solar side by OCPD or a power limited supply, the
inverters, but the input from the grid must be protected.  Must we provide
overcurrent protection on the "other" side in the above scenario? Yes, the
solar side has to have protection from the grid.

- We were originally planning to land each inverter output on a 50A
breaker inside a 200A main lug panel - must we also include a main breaker? No,
not from the inverters, but if you are you are using this as the OCPD on
the solar side of the transformer yes.  I think the calculation is 125%
transformer OCPD power or the power of the transformer is the maximum
current.  For example, 112 kVA / 480 x 125% =292 amps is possible, the
conductors need to be 292 amp or you protect with OCP. If you protect then
the conductors need to be 100 kW x125% /480/1.73= 150 amp minimum.

- How do we size the grounding conductors on either side of the
transformer? For standard distances, the equipment ground for the
transformer is from 250.122 based on the grid side OCPD.  For 100 kW x 1.25
/208/1.73 = 350 amps, is #3 cu or equiv.
For the solar side it is a separately derived system, The equipment ground
from 250.122 is #6, However, if you use a corner grounded delta, (if SE
will allow)  you do not run a separate equipment ground.  Being a
separately derived system you need an GEC, which is sized from 250.66and it
will go to a series of ground rods, with a #6

- Is the grounding conductor between MDP and transformer considered an
EGC (250.122)? Yes,  for the transformer on the grid side

On Tue, Apr 10, 2018 at 7:38 PM, Corey Shalanski <co...@joule-energy.com>
wrote:

> I'm seeking some guidance on how to treat transformers when included in a
> PV system.
>
> Specific system details:
> - (3x) SolarEdge 33.3 inverters combined in a 200A panel
> - 480/208 transformer to be installed between 200A panel and main
> distribution panel (service is 208 wye)
>
> My questions:
>
> - Is there any consensus on what circuit is considered the "primary" and
> which is the "secondary" in this type of scenario?
>    - NEC 690.9(D) speaks of "considering first one side of the
> transformer, then the other side of the transformer, as the primary". Is
> this relevant? i.e. Is it possible for both sides of the transformer to be
> labeled as a primary?
>
> - I understand that 450.4(A) requires overcurrent protection on the "input
> conductors" (aka "primary conductors"?), correct? Must we provide
> overcurrent protection on the "other" side in the above scenario?
>    - We were originally planning to land each inverter output on a 50A
> breaker inside a 200A main lug panel - must we also include a main breaker?
>
> - How do we size the grounding conductors on either side of the
> transformer?
>    - Is the grounding conductor between MDP and transformer considered an
> EGC (250.122)?
>    - Is the inverter-side of the transformer considered a separately
> derived system, in which case the grounding conductor would be considered a
> GEC (250.102(C)(1))?
>
>
> --
> Corey Shalanski
> Joule Energy
> New Orleans, LA
>
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