Pole Mount Transformer calculation??

[powerplay]

On a private farm there is an 75KVA Pole mount Power Authority transformer that is being replaced with an private 50KVA Pole mount Transformer with 120/240volt secondary lines to power a home with a 100amp panel, and a 200amp Lighting panel for a Commercial Greenhouse.

... The question before deciding where to add another meter and panel for the Common Well Pump shed as well as another "House" panel for a warehouse is how much can be drawn off the private 50KVA Pole mount transformer? The customer is skeptical because they have an existing 75KVA transformer that was permitted through the Power Authority to feed a 200amp 120/240volt Commercial Panel and the 100amp Home Panel, and now that they are returning the 75KVA pole mount Transformer to the Power Authority and putting in their own private pole Transformer, they were told from a salesman it would handle far more load than the existing 300amps. I was told when the Power Authority sizes the transformer, they allow the pole mount Transformer to be loaded over 150%, but when the same Transformer is on private property the derating calculation changes and they reduce the amount it can be rated for...... Seems to me 50000VA/240volts=208amps ... What is the allowable load for a private 50KVA Pole mount transformer with 120/240volt Secondary output??

Thank again in advance for your help!

 

[kwired]

Just because you have a 200 amp panel and a 100 amp panel doesn't necessarily mean you have 300 amps of load. What kind of load do you get after going through calculations in art 220? You may be surprised at how low it may be.

 

[mivey]

The pole-mount distribution transformers can handle temporary overloads just fine.

200 amps would be fine for a continuous load. 300 amps would be fine for a peak load. Even 300% loading is ok for very brief periods depending on the pre-loading. The key is to determine the level and duration of the overload as well as the leve and duration of the pre-load. There are guidelines for these calculations and they require more load information than you have provided.

If you have extended periods per day of more than 200 amps when it is hot outside, I would be concerned with using a 50 kVA transformer.

Don't forget to account for loading unbalance when looking at the loads. A safety factor would be in order.

 

[Hv&Lv]

I would agree with mivey here for transformers in general. But, now that the transformer is now owned by the customer, different rules come into play IMHO (NEC vs NESC or POCO practices)
As such, does the NEC allow for load diversity when sizing transformers? How would you size this if it were a dry pack for a building?

 

[Smart $]

I would agree with mivey here for transformers in general. But, now that the transformer is now owned by the customer, different rules come into play IMHO (NEC vs NESC or POCO practices)
As such, does the NEC allow for load diversity when sizing transformers? How would you size this if it were a dry pack for a building?

TTBOMK, the NEC has no requirement or guidelines for sizing such a transformer. It seems the NEC is only concerned with overcurrent protection. So such transformer sizing by the NEC would be indirect at best.

 

[iwire]

In my opinion the NEC requires a transformer to be sized for the calculated load of the service and that is what I see done for customer owned transformers.

110.3 Examination, Identification, Installation, and Use
of Equipment.
(A) Examination. In judging equipment, considerations
such as the following shall be evaluated:

(7) Classification by type, size, voltage, current capacity,
and specific use

(B) Installation and Use. Listed or labeled equipment
shall be installed and used in accordance with any instructions
included in the listing or labeling.

If a transformer is labeled 75kVA in my opinion the NEC does not allow me to connect more load than that to the transformer.

 

[Smart $]

In my opinion the NEC requires a transformer to be sized for the calculated load of the service and that is what I see done for customer owned transformers.

The NEC cannot and does not require any size service transformer.

If a transformer is labeled 75kVA in my opinion the NEC does not allow me to connect more load than that to the transformer.

I too am of that opinion. I'm just saying the NEC does not say that explicitly.

 

[kwired]

If the transformer is customer owned, we need to look harder at where is the "service point". The transformer may still be on POCO side of service point, but POCO maybe wants to recover some cost by making customer pay for the transformer, even though they still install and maintain it.

 

[Smart $]

If the transformer is customer owned, we need to look harder at where is the "service point". The transformer may still be on POCO side of service point, but POCO maybe wants to recover some cost by making customer pay for the transformer, even though they still install and maintain it.

Perhaps relevant for some installations, but essentially no matter who pays for or owns a transformer on the utility side of the service point, the NEC will not apply.

 

[kwired]

Perhaps relevant for some installations, but essentially no matter who pays for or owns a transformer on the utility side of the service point, the NEC will not apply.

Whether or not NEC applies is what my point was.

If this transformer is on the load side of the service point then the service equipment is connected to the primary voltage and the transformer is a separately derived system.

Many POCO charge their customers for equipment these days but it is still controlled and maintained by POCO.

 

[Hv&Lv]

Whether or not NEC applies is what my point was.

If this transformer is on the load side of the service point then the service equipment is connected to the primary voltage and the transformer is a separately derived system.

Many POCO charge their customers for equipment these days but it is still controlled and maintained by POCO.

That sounds like a crappy thing for any POCO to do. The basic facilities charge should more than cover the initial cost of the XF over the years of service. Maybe I could understand some type of charge for an unusually small usage account, but a 50 or 75 kVa XF ain't one of them.

 

[kwired]

That sounds like a crappy thing for any POCO to do. The basic facilities charge should more than cover the initial cost of the XF over the years of service. Maybe I could understand some type of charge for an unusually small usage account, but a 50 or 75 kVa XF ain't one of them.

I can't say I disagree with you. But most POCO around here are charging fees for things they never used to. Almost every service now has some construction costs from POCO in one way or another, or contractor is installing underground raceways and POCO pulls conductors through or something. Within cities and villages they may not charge for as many things, but for rural services, they want some compensation for almost every foot of line they run, and transformation charges, metering equipment, etc. These charges may not be full price of the equipment, but they are things that they never charged for a few years ago. They still put a disconnect on the pole on the farms and it is considered owners equipment, and they can do so for less than I can charge to put up same equipment, so they end up installing it nearly 100% of the time.

 

[mivey]

That sounds like a crappy thing for any POCO to do. The basic facilities charge should more than cover the initial cost of the XF over the years of service. Maybe I could understand some type of charge for an unusually small usage account, but a 50 or 75 kVa XF ain't one of them.

Perhaps the energy use is still relatively small. A lot of rates spread annual facilites costs over 12 months and in the energy blocks. Loads with low load factors or that tend to be seasonal won't recover the costs in the standard rates. There are many ways to skin that cat and better rates is one means. Additional facilities/equipment charges can be part of that strategy.