3 Utility owned 167 KVA single phase xformers - Whats the % of aceptable overload

[John Seaton]

If I do the calculations on these utility owned transformers we are far exceeding their rating, I've been told 150% and up to 200% overload is the norm.

 

[kwired]

If I do the calculations on these utility owned transformers we are far exceeding their rating, I've been told 150% and up to 200% overload is the norm.

I don't know exactly how they figure it, but I believe they factor in how long the peak demand is expected as well as how long there will be low demand to allow for cooling. Keep the demand period short enough and allow sufficient cool down time and you can load them more then if you don't have those variances in load.

 

[Ingenieur]

Take a look at this
https://www.usbr.gov/power/data/fist/fist1_5/vol1-5.pdf

 

[electrofelon]

If I do the calculations on these utility owned transformers we are far exceeding their rating, I've been told 150% and up to 200% overload is the norm.

Are you talking calculated load or actual load? Usually a big difference.

 

[Jraef]

Also keep in mind, it's THEIR equipment, so how THEY want to abuse it is THEIR business, technically. In other words if they chose to run them at 200% of nameplate continuously and it cooks them, it's technically their risk. In reality it's also the USER who is at risk of down time, but the user can attempt to get compensation if the utility screws up.

On the other hand if the utility TOLD YOU that is a 200kVA service and you attempt to pull 400kVA from it, they might come back on YOU if it fries their transformers. But if THEY tell you it's a 400kVA service that THEY are squeezing out of 200kVA worth of transformers, it's a risk they have decided to take and they will have to suck up the damages if they happen.

 

[junkhound]

If I do the calculations on these utility owned transformers we are far exceeding their rating, I've been told 150% and up to 200% overload is the norm.

PSE/potelco told us the same thing (e.g 150%) when we questioned their use of a 25 kVA to feeds 5 2 BR habitat units with elec kitchens (but gas WH and furnace). AFAIK, that 25 kVA has been there about 5 years now.

 

[kwired]

Also keep in mind, it's THEIR equipment, so how THEY want to abuse it is THEIR business, technically. In other words if they chose to run them at 200% of nameplate continuously and it cooks them, it's technically their risk. In reality it's also the USER who is at risk of down time, but the user can attempt to get compensation if the utility screws up.

On the other hand if the utility TOLD YOU that is a 200kVA service and you attempt to pull 400kVA from it, they might come back on YOU if it fries their transformers. But if THEY tell you it's a 400kVA service that THEY are squeezing out of 200kVA worth of transformers, it's a risk they have decided to take and they will have to suck up the damages if they happen.

They may also hit you with demand charges if you are pulling too much and that can help encourage customer to manage the load better. If customer determines they need to use more power anyway then they may increase transformer and adjust demand rate structure. For some services there is a minimum demand charge that you pay whether you use the energy or not.

 

[publicgood]

They may also hit you with demand charges if you are pulling too much and that can help encourage customer to manage the load better. If customer determines they need to use more power anyway then they may increase transformer and adjust demand rate structure. For some services there is a minimum demand charge that you pay whether you use the energy or not.

The load profile has influences additional charges also.

 

[publicgood]

If I do the calculations on these utility owned transformers we are far exceeding their rating, I've been told 150% and up to 200% overload is the norm.

Beyond 150%, if this is real load and not assumed demand, the utility typically adds fan packages or other means of additional capacity. This would be the case more so for pad mount three-phase transformers rather than single-phase.

 

[kwired]

PSE/potelco told us the same thing (e.g 150%) when we questioned their use of a 25 kVA to feeds 5 2 BR habitat units with elec kitchens (but gas WH and furnace). AFAIK, that 25 kVA has been there about 5 years now.

If you were supplying same load from a single service/feeder using NEC 5 ranges would have an allowable demand factor of 20 kw. Reality is they probably almost never see that much cooking load at one time, unless all five families are on the exact same schedule and they all cook using multiple heating elements on the range.

 

[gadfly56]

If you were supplying same load from a single service/feeder using NEC 5 ranges would have an allowable demand factor of 20 kw. Reality is they probably almost never see that much cooking load at one time, unless all five families are on the exact same schedule and they all cook using multiple heating elements on the range.

Thanksgiving?

 

[retirede]

I have heard a story that I consider urban legend. 5 homes all connected to a single undersized transformer. Homeowners had dimming lights and premature appliance failures. POCO would not do anything about it, so one hot summer day with A/C running, they all got together and energized everything they could and smoked the transformer.

 

[kwired]

Thanksgiving?

One time situation where it could heat the transformer some, but will have about a year to cool down.

Unless they all turn on ovens and surface elements at exactly the same time, still will likely be less demand then one may think. Elements will cycle on/off as needed to maintain temperature once setpoint is initially achieved.

That and the Thanksgiving is at that time of year when the transformer isn't likely to see too high of an ambient temp which helps keep it running cooler then if we all did similar cooking on July 4, when the AC load is also a contributing issue.

 

[John Seaton]

These transformerrs are feeding a 60,000 sq ft manufacturing facility

These transformerrs are feeding a 60,000 sq ft manufacturing facility

Thanks for the great comments so far, we want to add more equipment I'm concerned we are maxed out, I'm putting together a request for the utility to either add another set of pole mounted transformers or to replace them with 250 KVA units. Also considering a pad mount transformer close to the building. We have alot of resistive loads controlled by SSRs also alot of 3 phase motors.

The utilities primary side voltage is 12,470 we average 450 to 460 on the secondary side. 167 KVA x 3 = 501 KVA total so that gives us available continuous current of 630 amps at 100% nameplate rating.

I've put a amp clamp on the service entrance cables due to the 20 + machines with SSRs the amps are constantly changing. So its never a fully continuous load.

 

[gadfly56]

Thanks for the great comments so far, we want to add more equipment I'm concerned we are maxed out, I'm putting together a request for the utility to either add another set of pole mounted transformers or to replace them with 250 KVA units. Also considering a pad mount transformer close to the building. We have alot of resistive loads controlled by SSRs also alot of 3 phase motors.

The utilities primary side voltage is 12,470 we average 450 to 460 on the secondary side. 167 KVA x 3 = 501 KVA total so that gives us available continuous current of 630 amps at 100% nameplate rating.

I've put a amp clamp on the service entrance cables due to the 20 + machines with SSRs the amps are constantly changing. So its never a fully continuous load.

If you do that AND you have a fire pump on the premises, you would want to make sure the service cables go underground to the pad. In the event of fire, the responding department may deenergize your service before they enter the site if you have overhead cables, thus shutting down the fire pump.

 

[John Seaton]

Good point

Good point

If you do that AND you have a fire pump on the premises, you would want to make sure the service cables go underground to the pad. In the event of fire, the responding department may deenergize your service before they enter the site if you have overhead cables, thus shutting down the fire pump.

Good point, we have no pump and we have alot of molten metal sitting in melting pots so we hold anual classes with the FD so they dont get hurt or destroy our equipment if theyre called out.

 

[kwired]

Chances are POCO can give you demand history, that will tell you what your actual loading conditions are like.

If your actual demand isn't high enough they will not want to upgrade transformer(s), if you want it bad enough and pay them what they ask for they may do so anyway.

If peak demands are at overload levels they will want to also consider how long they typically remain at that level when determining if upgrade is necessary.

You have a high level of heating equipment load - that may be pretty demanding after a cold start, but average demand maybe is considerably less once up to normal running temperature.

 

[mbrooke]

1. Utility transformers are oil filled, meaning they have thermal inertia inside of them. If the load cycles with enough time to cool down they can easily be loaded 125%-200% (sometimes 250%-300% in resi) without any significant life loss.

2. NEC and calculated load if often 2x-3x more then what the POCO actually encounters.


All in all its more than possible for a 250kva service to be fed with say a 100 kva bank. FWIW around me there are endless (perhaps thousands) of 400amp services for businesses and small strip malls fed via a 45 or 75kva pole bank. Auto body or welding shop might get 150. All going strong for decades- not even an issue with dimming lights.

 

[John Seaton]

Thank you thats exactly the type of brain power I was hoping to find on this forum.

Thank you thats exactly the type of brain power I was hoping to find on this forum.

1. Utility transformers are oil filled, meaning they have thermal inertia inside of them. If the load cycles with enough time to cool down they can easily be loaded 125%-200% (sometimes 250%-300% in resi) without any significant life loss.

2. NEC and calculated load if often 2x-3x more then what the POCO actually encounters.


All in all its more than possible for a 250kva service to be fed with say a 100 kva bank. FWIW around me there are endless (perhaps thousands) of 400amp services for businesses and small strip malls fed via a 45 or 75kva pole bank. Auto body or welding shop might get 150. All going strong for decades- not even an issue with dimming lights.

There is no significant cooling time during first shift say 6 am to 3 pm then the load diminishes a little then quite a bit after 10 pm. I'm concerned about peak load during the day. Thermal inertia is my phrase of the day. Thanks

 

[mbrooke]

There is no significant cooling time during first shift say 6 am to 3 pm then the load diminishes a little then quite a bit after 10 pm. I'm concerned about peak load during the day. Thermal inertia is my phrase of the day. Thanks

A power data log can determine the actual loading. In a case like this I wouldn't load the pigs past 120% during the peak period.