Utility transformer sizing and thoughts on across-the-line motor starts

fastline

Member
Location
midwest usa
Met with the utility guys today for a 400A 240/120 service. I was trying to avoid them adding a utility easement in the middle of my property (which they have right to access) but my secondary distance was getting expensive at 300ft. I think we will have no choice but to run the primary closer to the structure.

I have more than enough loads to justify the 400A and will certainly use it but I am getting the option on transformer sizing, however I get to pay for it....:D They then charge $/KVA minimums against the transformer size.

I know utilities way under size transformers, which is fine, as long as I don't have issues. He said they will replace it if blown up. As it stands, I was probably leaning at 100KVA and he was at 50KVA, but we might negociate at 75KVA. Most of my loads are motor loads so PF considerations should be considered I think.

So what kind of head room do these pad mount transformers really have?



Also, wading into thick water but they put a limit of 10HP for motors on the single phase line but will give me all the amps I want. I realize they are trying to consider inrush on motor starts but I am curious how sticky this can get when I run bigger motors on VFDs? Pretty much my machines are 15-20HP but with about a 2-4sec spool up time. I run a bank of 10HP motors and caps to make a balanced 3PH bank. My inrush would be limited but not uncommon for my machines to go from min to max loads a few times/minute.

Across the line starts can be 10x the name plate current for starts but VFDs are usually limited to no more than 150% of rated continuous current for acceleration. I guess I am basing some of this on that fact to consider what I might reasonable be able to run without a call from them....
 
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meternerd

Senior Member
Location
Athol, ID
As you have probably already figured out, you pay for what you want, but over-sizing a transformer is pretty much a waste of money. A few things to consider....KVA is exactly that. Independent of power factor. Residential customers usually only pay for Kwh, but commercial can also be charged for KVarh and demand (Kw). In that case, getting your PF close to zero is to your advantage. Frequent starts and stops may increase your demand charge, but demand is actually calculated at 15 min interval averages, so small motor starts probably wouldn't make a big difference. Most utilities have numerous rates, so you need to find out what category they are going to put you in. Long secondary can be an issue if you draw enough current to create unacceptable voltage drops, but I'm not sure the why utility would really care. They may give you advice or have maximum distances, but it wouldn't impact their revenue. Running primary closer to your transformer would probably be a higher cost up front, but you will likely save yourself headaches down the road if you don't have to deal with voltage sags. Our utility charges for installed equipment up front, but monthly charges are based on service size, not transformer or wiring. Lots of options, none of which will probably be cheap. Good luck.
 

fastline

Member
Location
midwest usa
They are just charging per KWH, no KVARS. However, I was simply mentioning the PF due to the increase in current, which is what kills transformers.

He did not care about the secondary length but was trying to help, indicating I can reduce wire cost and voltage drop. However, IIRC, the Vdrop was around 3% at full capacity with my wiring design but that is 3% I am losing to mother nature too.


Basically they charge $/KVA of transformer size as a minimum charge and then once your KWH breaks over that, you are just paying for KWH. It is not an additional charge.
 

meternerd

Senior Member
Location
Athol, ID
Not sure about your utility, but ours charges a "facilities fee" which is a flat rate based on service size. If you have a transformer installed that only feeds your service, you pay fees based on what they install. You didn't mention demand charges. If you aren't billed for demand, the motor starting current would probably not impact your bill much, since startup is such a small percentage of continuous load. Voltage drop would be the big consideration, especially with long secondary. But if the metering point is at the end of the secondary service drop, any voltage losses would reduce the metered KWh, so I don't see how mother nature would benefit. Utilities size transformers based on expected or declared load of the customer. Most padmounts have no problems at 125% of full load, and that's continuous, which start/stop motor loads would not be. Like I said, over-sizing may sound safer or more reliable, but probably not cost effective. Especially if they'll replace it if it's overheating. All utilities do things differently, but they have the historical experience to back up their suggestions. You get what you pay for, so if you want to go bigger and can afford it, they'll probably be more than happy to oblige. If you have plans for later expansion, bigger up front would make sense. Just my humble opinion, though. Don't profess to be a power engineer.
 
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fastline

Member
Location
midwest usa
Thanks. That link answers some questions! In regards to HP limits here, that is something stated BUT they are not charging demand. This comes down to voltage drop that the PC can detect. I am concerned they might see an issue and cause problems for me.
 

Sahib

Senior Member
Location
India
An undersized transformer will cause excessive voltage drop during motor starts and across the line motor contactors may drop out. Check for that.

A dry type transformer is intolerant of over loads. Check whether the POCO would charge you for repair/replacement of the dry type transformer damaged due to over loads.
 

wirenut1980

Senior Member
Location
Plainfield, IN
An undersized transformer will cause excessive voltage drop during motor starts and across the line motor contactors may drop out. Check for that.

A dry type transformer is intolerant of over loads. Check whether the POCO would charge you for repair/replacement of the dry type transformer damaged due to over loads.
Utility transformers are typically oil filled, not dry type. The amount of overload they can handle without loss of life depends on how much they are overload and for how long of a time, the loading prior to the overload period, how much time between when the transformer is overloaded. More detailed information can be found in ANSI C57.91.

If the revenue metering is located on the secondary side of the transformer, then the utility is typically responsible for maintenance and replacement of the transformer.
 

fastline

Member
Location
midwest usa
Thanks for the information and I will be sure to look into the ANSI references.

I would like to ask if there are any references for actual voltage drop estimations for over current loads? It was mentioned that an undersized transformer will have drop when starting a load and that might drive my sizing. The meter is on the secondary side and they are responsible for the transformer. I know they use liquid filled only.


Also, no one has replied in regards to their motor sizing. Does anyone see any other concerns with my thought process regarding larger VFD driven loads? My thought process is this, a 10HP load FLA is about 50A and figure a worst case of 10x for inrush, that is 500A. I have one drive that is 90A 3PH 30min rating and accel is 120% of that so that is 186A inrush on 1PH power. The way I am looking at it is to control my inrush on loads to stay under the 500A threshold. I know the POCO can technically look up data off my meter from their office and probably see my inrush BUT they already know I have several loads and how could they really know if I start 15HP motor across the line or happen to have 3, 5HP motors start up simultaneously? I have brought that up and they said, "just don't have anything bigger than a 10HP connected".

IIRC, they have 6ga, 7.2KV for primary OH.
 
I would like to ask if there are any references for actual voltage drop estimations for over current loads? It was mentioned that an undersized transformer will have drop when starting a load and that might drive my sizing. The meter is on the secondary side and they are responsible for the transformer. I know they use liquid filled only.
I would be curious about this too. It seems that most of the discussion on overloading transformers is in regards to the heat generated, how much and for how long, and life reduction. What about the concerns of excessive voltage drop and how is that factored into the sizing decision. Say for example I have a 10KVA transformer and a motor that draws 10KVA at full load, but draws say 8 times that during start up. Obviously heat is not an issue for the short duration of the starting cycle, but would it be prudent to up size the transformer for lower voltage drop and easier starting? I believe that is essentially your question also.
 

Sahib

Senior Member
Location
India
II have a 10KVA transformer and a motor that draws 10KVA at full load, but draws say 8 times that during start up. Obviously heat is not an issue for the short duration of the starting cycle, but would it be prudent to up size the transformer for lower voltage drop and easier starting? I believe that is essentially your question also.
As I said in my earlier post, the voltage drop along the conductor during motor starting to be found out to see that across the line motor starters do not drop out. The starters may withstand a voltage drop of up to 30%.
 

Sahib

Senior Member
Location
India
fastline:
Consider the voltage drop for largest across the line motor starting and see that it is within 30%.
 

meternerd

Senior Member
Location
Athol, ID
Of course transformer undersizing would increase voltage drop, but would oversizing be any advantage? Would the POCO size the secondary wire for the service size or the transformer size? Likely the POCO would go by service size. One question would be....if you have many motors, is it likely they would all start at the same time? Impedance is the issue, regardless of where...long secondary, wire too small, transformer too small, poor connections, etc. And yes, limiting inrush would help reduce drop, but that's not something the POCO would likely consider. And no, it's not really possible for a utility to record motor starting current or inrush. Most high end solid state KWh meters can measure and record energy (KWh), PF, Peak Demand (KW) and can also display instantaneous demand, but short duration surges would not be recorded. Peak demand is normally averaged over a 5 or 15 minute interval. Some can record min and max voltages as well, but they aren't really very high resolution. Power quality or disturbance analyzers would work, but they aren't cheap. Just another of my humble opinions, though, so no arrows, please.:D
 

robbietan

Senior Member
Location
Antipolo City
Utilities on my side of the world charge $/kVA in addition to kWh for commercial and industrial customers. The utility helps you figure out your kVA demand then sets a minimum kVA to charge you with. No problem as long as you actually exceed the minimum kVA you have contracted the utility with. Utility gets to place a transformer with the "right size" for the customer

The utility also limits motors to 5 HP if they have across the line starting. To limit inrush currents that may blow up the fuses protecting the transformers. Motors equipped with reduced voltage starters (VFDs, wye-delta) are allowed, regardless of size but it is important to mention this while talking to the utility during the kVA computations
 

kwired

Electron manager
Location
NE Nebraska
Thanks for the information and I will be sure to look into the ANSI references.

I would like to ask if there are any references for actual voltage drop estimations for over current loads? It was mentioned that an undersized transformer will have drop when starting a load and that might drive my sizing. The meter is on the secondary side and they are responsible for the transformer. I know they use liquid filled only.


Also, no one has replied in regards to their motor sizing. Does anyone see any other concerns with my thought process regarding larger VFD driven loads? My thought process is this, a 10HP load FLA is about 50A and figure a worst case of 10x for inrush, that is 500A. I have one drive that is 90A 3PH 30min rating and accel is 120% of that so that is 186A inrush on 1PH power. The way I am looking at it is to control my inrush on loads to stay under the 500A threshold. I know the POCO can technically look up data off my meter from their office and probably see my inrush BUT they already know I have several loads and how could they really know if I start 15HP motor across the line or happen to have 3, 5HP motors start up simultaneously? I have brought that up and they said, "just don't have anything bigger than a 10HP connected".

IIRC, they have 6ga, 7.2KV for primary OH.
If your motor is driven by a VFD you are not going to hit the line with 20 hp demand (or more) instantaneously, the drive is going to ramp the motor up to speed. As it does so the motor will have reduced voltage and current (which equates to less kVA from the source) initially and gradually increase to the level demanded by the load.

If you had 20 kVA of resistance heating they would have no problem throwing it on across the line because there is no surge of current well over 20kVA like there would be for a 20kVA motor started across the line.

What is the big deal with the mentioned easement in the OP? If the line needing easement serves other customers - I can see some concerns, but if it only serves your facility, you got to get power there somehow and if it needs maintenance/repairs you either have to do it or POCO does depending on who actually owns it.
 

fastline

Member
Location
midwest usa
Thanks guys. I think when I talk to the POCO engineer this week, I will try to gather the models of Xformers they use so I can determine any voltage drop across the transformer over ratings.

As to the inrush, I tend to think the this POCO put a very vague limit of 10HP on the books with no provisions for VFDs and such. I think the mention of fuses is probably another valid point as they would have to fuse according to the Xformer and since we will be pulling over capacity, that could be an issue. Not saying they fuse for the 50KVA but I bet when I have 100KVA of constant draw for hours, we could have a problem.

I have made mention of having multiple smaller motors but some trying to start at the same time. I mentioned a 5hp table saw with a 10hp vac system that is switched to come in when the saw turns on. That is 15hp across-the-line. He did not say much.



As for the easement, I will probably have to swallow that. I am just a person that does not like governments having the right to enter my property whenever but I guess that is a utility easement only and running underground means unless I blow something up, I cannot see them needing to be out too much. The whole place will have hot fence around it anyway.
 

kwired

Electron manager
Location
NE Nebraska
Thanks guys. I think when I talk to the POCO engineer this week, I will try to gather the models of Xformers they use so I can determine any voltage drop across the transformer over ratings.

As to the inrush, I tend to think the this POCO put a very vague limit of 10HP on the books with no provisions for VFDs and such. I think the mention of fuses is probably another valid point as they would have to fuse according to the Xformer and since we will be pulling over capacity, that could be an issue. Not saying they fuse for the 50KVA but I bet when I have 100KVA of constant draw for hours, we could have a problem.

I have made mention of having multiple smaller motors but some trying to start at the same time. I mentioned a 5hp table saw with a 10hp vac system that is switched to come in when the saw turns on. That is 15hp across-the-line. He did not say much.



As for the easement, I will probably have to swallow that. I am just a person that does not like governments having the right to enter my property whenever but I guess that is a utility easement only and running underground means unless I blow something up, I cannot see them needing to be out too much. The whole place will have hot fence around it anyway.
Starting the 5 and 10 hp at same time might be an issue, but probably is not with most POCO. They are generally concerned about how a surge from an across the line started motor will impact other customers. Usually is not much of a problem unless you are over 50 to 100 hp, with single phase - often multiple customers are on a single transformer and this is probably where they are coming up with the 10 hp limit - since you are likely the only customer on the transformer they may waive that rule or at least raise how much of a surge they will permit before it has impact on the primary voltage and effects to other customers.

The easement has no government ties, unless you have a municipal POCO - but even then just allows POCO representatives access not government in general. You possibly have similar easement to water, sewer, telephone lines, or even if you had overhead service drop - there would be easement unless the meter was at the property boundary and you owned the drop. You may get slight variances from area to area but this is pretty common for all utilities to a facility.
 
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