Utility Transformer Exceeds FLA Clarification

[RRoc]

My Pump Station Load is listed below and the Main Utility Transformer Supplying this load is 150KVA. From adding up all the current draw. The Pump Station Load is greater than the Secondary Transformer FLA. The Utility Engineer informed although the FLA has been exceeded the Operational Limits of the transformer has not. Can someone explain why this is ok? Also what NEC standard section to reference as well?

Pump Station Load
50HP, Current Draw = 65
50HP, Current Draw = 65
50HP, Current Draw = 65
15HP, Current Draw = 21
15HP, Current Draw = 21
25HP, Current Draw = 34
2HP, Current Draw = 3.4
Total = 274.4

Utility Transformer
Pri Volt: 16.34 KV, Secondary Volt: 480VAC
Rating: 150KVA, Secondary FLA: 180 FLA

 

[iwire]

It is very straight forward, the NEC load calculations do not apply to the power company.

The power company knows what they are doing and will sometimes run their transformers beyond FLA.

 

[Ingenieur]

Utility sets its own ratings basically
xfmrs like motors have a service factor, ie, 1.15 can run at 115% fla indefinitely

on a more technical note
do all your motors run at the same time?
or for example the 50's: 2 will alternate (both will run in hi flow conditions) the third is a back-up

 

[jim dungar]

NEC article 450 is about Amps not kVA. There is no NEC requirement or information, that assists you in determining either the primary or secondary FLA of a transformer.

Things like forced air cooling and acceptable loading profiles (e.g. motor starting) allow the manufacturer to be flexible with terms like Nameplate kVA, Continuous kVA, Emergency kVA. But 450.3 seems to be primarily concerned with transformer conductors protection, so just stay with FLA and transformers might be easier.

 

[RRoc]

I have asked this question about the motors all running at the same time and the answer is yes. I am assuming not all pumps are running at 100%.
I don't believe there are any issues because the pump station is up and running. I just don't understand how is this possible.

Maybe a Utility Engineer/Lineman can help out.

 

[Jraef]

And to reiterate, the NEC does not apply to Utilities. Don't do things by their example.

 

[iwire]

And to reiterate, the NEC does not apply to Utilities. Don't do things by their example.

If you do the inspector will have a good laugh as they fail you.

 

[iwire]

I just don't understand how is this possible.

What specifically are you not understanding?

A transformer is capable of supplying more current than the FLA, it is just a mater of how much over the FLA, how long you do it and what you do to keep it cool.

With an oil-filled power company distribution transformer there is a lot of thermal mass so you can run it above the FLA for many hours without actually overheating the unit.

 

[peter d]

I use this example a lot (kind of like someone who wired a lot of condos in the 1980's) but there is a street near me where there's 11 houses on a single 50 kva transformer. The houses are fairly small, the street has a gas main, and central A/C is uncommon around here, but that's a lot even for this area. Still, haven't seen any sign of a problem with it.

 

[peter d]

What specifically are you not understanding?

A transformer is capable of supplying more current than the FLA, it is just a mater of how much over the FLA, how long you do it and what you do to keep it cool.

With an oil-filled power company distribution transformer there is a lot of thermal mass so you can run it above the FLA for many hours without actually overheating the unit.

And even the overloading scenario is not very likely because the poco knows the true load will be far less that what the NEC calls for.

 

[Jraef]

This whole subject reminds me of an encounter I once had with PG&E on a pump station service. We were starting up a project with large pumps and even with soft starters, the voltage drop was so bad that everything shut down, including the HID lighting. After hooking up a PQ meter, I confirmed that the voltage was collapsing. Walked around to the outside to look at the pad mount utility xfmr, and it was WAY too small to be correct for a pump station with 400HP of total potential load. Called PG&E, they said it was a 750kVA transformer, just as specified. The pad mount was maybe 50" cube, nowhere NEAR what I would expect of a 750kVA. Nonetheless, they insisted it was right. Went back in and turned everything down as low as possible on the soft starters, this time the transformer fuse blew. PG&E technician shows up 2 hours later, opens the box and it says, right on the inner panel, 75kVA! I point that out to him, but he keeps insisting that their records show that it was 750kVA. He replaces the MV fuses, we try again, fuse clears immediately right in front of him. Finally he calls their Engineering dept., they start over with the ridiculousness of how "You can't go by what you see, we have different rules for transformers." I told them I understood that, but this was NOT that kind of situation. 4 hours after we began this issue, 4 PG&E engineers showed up to look at it and eventually came to the conclusion that yes, someone had "dropped a zero" on the order.

 

[iwire]

And even the overloading scenario is not very likely because the poco knows the true load will be far less that what the NEC calls for.

For sure often the case but add up just his motors.

 

[peter d]

For sure often the case but add up just his motors.

I did, it's very interesting to see how the poco goes about sizing. :happyyes:

 

[topgone]

My Pump Station Load is listed below and the Main Utility Transformer Supplying this load is 150KVA. From adding up all the current draw. The Pump Station Load is greater than the Secondary Transformer FLA. The Utility Engineer informed although the FLA has been exceeded the Operational Limits of the transformer has not. Can someone explain why this is ok? Also what NEC standard section to reference as well?

Pump Station Load
50HP, Current Draw = 65
50HP, Current Draw = 65
50HP, Current Draw = 65
15HP, Current Draw = 21
15HP, Current Draw = 21
25HP, Current Draw = 34
2HP, Current Draw = 3.4
Total = 274.4

Utility Transformer
Pri Volt: 16.34 KV, Secondary Volt: 480VAC
Rating: 150KVA, Secondary FLA: 180 FLA

I noticed your nameplate amps refer to the FLA of a 3-phase transformer, 150kVA, 480V secondary!
Your load amps when computed as single-phase loads will give you 131.712 kVA (very near the 150kVA rating); but should be 228.13kVA if computed as 3-phase!