Line-side "tap" and max. current back-feed thru Utility XF

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electron_ranger

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Lilburn, GA
Line-side "tap" and max. current back-feed thru Utility XF

500kva utility, pad-mounted transformer
400A 480/277v MSB to the building

I need to calculate what the maximum size PV array is that I can connect. It'd be a line side tap for sure, and I'd have to upsize the conductors between the tap and the XF I know.

Question is, how many electrons can I push backwards through that 500kva transformer? If I said that NEC section 450, Transformers, doesn't apply to utility owned transformers, would I be right? Is that a trick question?

Thanks for any suggestions or code references on this.
-Montana B.
 
POCO and NEC will not let you push more current back than the nominal size if the service. If POCO says that their transformer will handle your expected load on that size service it does not necessarily mean that it can handle a full current backfeed. That is their problem, although they may try to charge you for the transformer upgrade.
Since the load current and PV current are in opposite directions it is very unlikely that a residential PV system will strain the capacity of the service.

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electron_ranger said:
...
Question is, how many electrons can I push backwards through that 500kva transformer? If I said that NEC section 450, Transformers, doesn't apply to utility owned transformers, would I be right? Is that a trick question?
Click to expand...

You are right and the practical import is probably that only the utility's opinion matters.
 
jaggedben said:
You are right and the practical import is probably that only the utility's opinion matters.
Click to expand...

I've notified the utility of the customer's desire to max out the PV array. They are checking to see what the pad mounted XF can handle. I would think any typical XF can handle the same amount of current in reverse that it can handle in normal, voltage step-down operation.

Thank you.
 
electron_ranger said:
I've notified the utility of the customer's desire to max out the PV array. They are checking to see what the pad mounted XF can handle. I would think any typical XF can handle the same amount of current in reverse that it can handle in normal, voltage step-down operation.

Thank you.
Click to expand...
They will probably calculate the effect on voltage stability for any other customers on the same transformer if it is shared. Upstream of that not much effect unless a neighborhood installs and maxes at the same time.
Saw a description from a user, possibly in Puerto Rico, whose 240 rose to around 300 when the nearby Coast Guard station was delivering from an array bigger than their normal load.

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GoldDigger said:
They will probably calculate the effect on voltage stability for any other customers on the same transformer if it is shared. Upstream of that not much effect unless a neighborhood installs and maxes at the same time.
Saw a description from a user, possibly in Puerto Rico, whose 240 rose to around 300 when the nearby Coast Guard station was delivering from an array bigger than their normal load.

Sent from my XT1585 using Tapatalk
Click to expand...
At SPI a few years ago I had lunch with a grid admin from Puerto Rico. He told me that that PV system is causing him much grief.
 
jaggedben said:
And that's fine up to the service point, where the NEC applies. Once you're on the utility's side of the service point the NEC doesn't apply and the utility gets to say what goes.
Click to expand...
...including possible PV feedback from the NEC side of things.
 
Smart $ said:
...including possible PV feedback from the NEC side of things.
Click to expand...

Yeah and outside of the codes they can make up some of them regulate their own policies like an EMC where the "members" never get to vote on squat. The utilities in Georgia are hard on solar. Tough market. But the people in GA are starting to get it.
 
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