Sizing a Transformer For PV only Use.

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JasonC

Member
Location
N. Andover MA
My question regards to the 120% rule as it applies to transformers. The site I have has 3ph 4w 208/120 service 1600A cabinet 1000A main. It has a spare 400A and 250A breakers. I am planning on using 4 20kw Tl inverters (480V), that is where I need some assistance. The 400A equals 143kVA, I assume I need to derate (Constant Current) that 400A since it will be in a PV Array. That would mean the 400A would really equal (333.3A) 119kVA. Now that 119kVA on the secondary side (480) would equal 144A (not including transformer losses). Since I have covered the OCPD on the primary side and it is derated for constant current, I now look at the secondary side. If i take the 144A and figure for the OCPD on either side 150A (125kVA) and 125A(103.8 kVA), I still have to derate those OCPD's for continuous use. If I use the 150A, I can only input 125A into that OCPD which equals 103.8 kVA. I have lost roughly 15 kVA in code losses. If the 150A was the corrected amperage the uncorrected amperage would be 180A. I guess my real question is would a 125 kVa transformer be the correct fit and is the 150A on the 480 side correct.
 

Smart $

Esteemed Member
Location
Ohio
Let's take a step back and get some missing pertinent details...

Are these 1? or 3? inverters?

If 1?, you have 4, which cannot be balanced across the three phases. Are you looking at one transformer or four?

What is the inverter [continuous] output current rating?
 

JasonC

Member
Location
N. Andover MA
All the inverters are 3? inverters (SMA 20000TL), the max output current from the spec sheet is 24A. They will be feed out of Load Center from a single transformer. Sorry if the post seems to run on, it was late and trying to work the question out as I was typing.

I appreciate the help. I know this will have to get the drawing stamped, but if I can understand how to size it for its most efficient and cost feasible use it helps in designing.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
My question regards to the 120% rule as it applies to transformers. The site I have has 3ph 4w 208/120 service 1600A cabinet 1000A main. It has a spare 400A and 250A breakers. I am planning on using 4 20kw Tl inverters (480V), that is where I need some assistance. The 400A equals 143kVA, I assume I need to derate (Constant Current) that 400A since it will be in a PV Array. That would mean the 400A would really equal (333.3A) 119kVA. Now that 119kVA on the secondary side (480) would equal 144A (not including transformer losses). Since I have covered the OCPD on the primary side and it is derated for constant current, I now look at the secondary side. If i take the 144A and figure for the OCPD on either side 150A (125kVA) and 125A(103.8 kVA), I still have to derate those OCPD's for continuous use. If I use the 150A, I can only input 125A into that OCPD which equals 103.8 kVA. I have lost roughly 15 kVA in code losses. If the 150A was the corrected amperage the uncorrected amperage would be 180A. I guess my real question is would a 125 kVa transformer be the correct fit and is the 150A on the 480 side correct.

I am a bit confused by the barrage of information here, but at first blush it appears that you are sizing the transformer for the 400A breaker. If that is what you are doing then it seems to me that you are working the problem backwards. I think you should start with the inverters and figure out what is going on from that side, and size your transformer and backfed breaker to fit.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
My question regards to the 120% rule as it applies to transformers.

Applying the 120% rule on both sides of a transformer is something would be really open to interpretation within the code. What code cycle are you on?

To me, it would be reasonable to calculate the 120% rule on the secondary (PV) side of the transformer using whatever breaker is protecting the conductors from the combiner panel to the transformer.

...The 400A equals 143kVA, I assume I need to derate (Constant Current) that 400A since it will be in a PV Array....

Well, no. As ggunn said, you should be doing it the other way around. I calculate that you need your output on the primary (utility) side of the transformer to be rated 277A. (Too bad that just misses your spare 250A breaker.) Using the 400A breaker is fine then, but it may require oversizing the conductors between it and the transformer with respect to the PV load.
 

JasonC

Member
Location
N. Andover MA
I am working in the 2014 code,

The secondary side would be fused since the load center will most likely go up on the roof with the inverters (MLO Load Center)

And since I have room on the roof for potential growth in the system I would want to size the transformer for the 400A at the 208V. I would like to be able to "plug in" additional PV generation when when the funds are available.
 

Smart $

Esteemed Member
Location
Ohio
...it seems to me that you are working the problem backwards. ...

... As ggunn said, you should be doing it the other way around. ...
Thus my first reply (if you read between the lines... accurately ;)).


Applying the 120% rule on both sides of a transformer is something would be really open to interpretation within the code. What code cycle are you on?
Well he lists his location as MA, so I'll assume 2014.

The MDP appears to be multi-breaker service equipment and the panel bus on the service side of the service disconnecting mean, so connecting the PV system would be a line-side connection. The PV system side would not be subject to the 120% rule?

Individual inverter OCPD minimum rating: 24A ? 125% = 30A.
Combined inverter output circuit current rating: 4 ? 24A = 96A
Bus/conductor contribution: 96A ? 125% = 120A

Actual load on transformer: 96A ? 480V ? sqrt(3) = 79,813VA
Primary current: 79,813VA ? 208 ? sqrt(3) = 221.7A (continuous)

If we went with a Sq D dry type made-for-step-up transformer, the minimum best fit transformer would be a 112.5kVA 208D480Y (EE112T212H).

Primary rated current: 112.5kVA ? 208 ? sqrt(3) = 312.5A
Primary-only protection maximum: 312.5A ? 125% = 391A (round up to 400A permitted)

Note primary-only protection is not required, and schema is interchangeable with primary plus secondary protection. Shown for emphasis.
 
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