I have a customer that we have done a bunch of work for and on one of the building we replaced a huge piece of service gear and the inspector had no issues whatsoever, but this same customer has another building with a really old piece of junk gear that needs to be replace (they will but probably a year or so out) that is rated at 42k and what the issue is the power company replaced their transformer about six months ago from a 300kva to 500kva. Now we have to get a permit signed off, but we cannot because the fault current of this transformer exceeds the rating of the gear. The power company is dodging all the questions that I'm asking them and this is their last response "Per Std 0650.3010, the available fault current at the secondary bus of a 3-phase 208Y/120V pad-mounted transformer is 60,300A. However, this fault current drops to a lower level at the customer's gear. You can provide the transformer's impedance of 2.3 % and R/X=0.2 to the customer and let him calculate the available fault current at the gear." Can someone give me some advice? This gear is rated at 42k 120/208V 800amp fused (no idea what the fuses are rated at because they are 30 years old) and feed with parallel runs of 750MCM AL about 20'. Everything I'm coming up with still puts this fault current at way above what the gear is rated at. The power company is PSE.
Fault Calc's on existing gear when utility company change transformer
- Thread starter nicknorth
- Start date
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kwired
Electron manager
- Location
- NE Nebraska
I didn't try to run any of your numbers, but available fault current drops fairly quickly just by adding length to the conductors, you may find you only need 10 or 20 feet more conductor length to get the available current to an acceptable level,then you just need to find a way to make those installed conductors longer. Not saying that will always be easy or cheap to do but may be easier or cheaper than other alternatives.
Changing size and number of parallel sets or switching to aluminum conductors could also help you out.
Changing size and number of parallel sets or switching to aluminum conductors could also help you out.
Joe Villani
Senior Member
- Location
- Westchester County NY
It looks like (based on some assumptions) that your cables will need additional length.
I'm not an engineer but maybe the power company will let you install some cable limiters on the secondary terminals of the new transformer?
http://www.cooperindustries.com/con...l_library/BUS_Ele_Tech_Lib_Cable_Limiters.pdf
Again, other here will know more, just something to look into.
Joe Villani
I'm not an engineer but maybe the power company will let you install some cable limiters on the secondary terminals of the new transformer?
http://www.cooperindustries.com/con...l_library/BUS_Ele_Tech_Lib_Cable_Limiters.pdf
Again, other here will know more, just something to look into.
Joe Villani
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
Couple of things I'm not clear on:
This is a likely a dedicated service. 300kva at 208V is 832A. Is your customer is the only one connected? Seems odd the power company would just jump in change a transformer and not tell anyone that was their plan.
And you may have to upgrade some of the subpanels. The effect of the increased fault current continues down the distribution. You have to follow the increased calcs down stream until the new fault durrent falls below the equipment ratings.
Or, maybe the customer is starting their service upgrade a year ahead of time
ice
Why do you need a permit for power company work? Did your customer ask to have the xfm changed? And the inspector is saying, "Oh you asked for a service upgrade. Now your service equipment has to stand the new available SCC?
This is a likely a dedicated service. 300kva at 208V is 832A. Is your customer is the only one connected? Seems odd the power company would just jump in change a transformer and not tell anyone that was their plan.
That doesn't sound like they are dodging anything. They are giving you the available fault current at the transformer secondary terminals with an infinite primary. You have the necessary information to calculate the available fault current at the service equipment
Welcome to the world of being the "Engineer of Record". Get the rest of the data, cable lengths, raceways. Fire up ETAP or SKM or equivalent. Run your calcs. Or, you could use the Busmann paper for a hand calc. You either have to add impedance or upgrade the service equipmnt. You could add a reactor between the xfm and the main. Enough impedance to effectively increas the xfm %IZ from 2.3% to 3.4%. That would drop the SCC from 60kA to 40KA.
And you may have to upgrade some of the subpanels. The effect of the increased fault current continues down the distribution. You have to follow the increased calcs down stream until the new fault durrent falls below the equipment ratings.
Or, maybe the customer is starting their service upgrade a year ahead of time
ice
Exactly. The substation transformer supplying the primary is not an infinite current source.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Most POCOs do not calculate 'exact' available fault current values for customers that purchase secondary power (e.g. 240V or 480V). It is far easier to provide worse case values (which are some times called design levels). This is partly due to not knowing how their grid connections are made. The other reason is the transformer impedance often depends on which transformer was actually pulled from the 'yard', where they may have been purchased in bulk from different vendors.
It is much easier to provide more accurate fault information to primary rate customers as the transformer has often been special ordered, or allocated.
This is such a normal process almost all 'design engineering' firms have standard procedures to cope with it.
The additional information helps with understanding but provides no resolution to the OP's predicament. IMO the onus to certify SCCR of the existing service equipment has not been exceeded should be on the POCO. The OP'er may need to get his public utilities commission involved.
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Every POCO 'metering manual' I have ever read says it is the responsibility of the customer to provide adequately rated equipment.
And I agree that would be the case when the service is established. But not when the service parameters are changed at a later date at the sole discretion of the POCO where the consumer's equipment was adequately rated prior to the change.
kwired
Electron manager
- Location
- NE Nebraska
If you figure actual SCC with consideration of primary side conditions and your installation is close to max SCC permitted, what happens a few years down the road when the substation transformer gets upgraded for some reason, or a new sub is constructed - maybe at a closer distance to the customer. I still think designing by a somewhat worst case scenario is best. If OP can consider primary side effects and they do lower the SCC enough to get this gear by for another year or so until it is scheduled to be changed, I may consider it to be acceptable if I were his inspector.
I did a service upgrade last summer that also involved POCO relocating the transformer. No significant load changes happened this year, but the plan is to add some significant load in 3-5 years. POCO used existing transformer because load is still about the same but intends to increase the size when the load is added. I had to design that install for what I expect to be worst case for the new transformer not knowing exactly what it will be when that time comes, because we don't want to have to redo everything that was done this year when the new transformer does get installed.
I did a service upgrade last summer that also involved POCO relocating the transformer. No significant load changes happened this year, but the plan is to add some significant load in 3-5 years. POCO used existing transformer because load is still about the same but intends to increase the size when the load is added. I had to design that install for what I expect to be worst case for the new transformer not knowing exactly what it will be when that time comes, because we don't want to have to redo everything that was done this year when the new transformer does get installed.
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
This is from Xcel Energy's Blue Book:
"Note: Because an overloaded transformer is typically replaced by the next larger size transformer, and an under-loaded transformer may be replaced by the next smaller size transformer, the customer is encouraged to use this range of transformers to perform their studies and select equipment such as current limiting fuses, breakers and switchgear bus bar bracing."
A reasonable expectation on the part of the POCO. Service history would be involved... and the onus would be back on the customer if they have added substantial load since the service was established, enough for the POCO to deem the original size transformer to be overloaded.
The POCO should have coordinated with the customer on this matter before outright replacing the transformer, especially if the previous transformer was still in operation. There is a possibility they did, and the customer just overlooked the SCCR of the existing equipment.
It may be best to commission a PE who is qualified in doing thorough SCC studies to sort this out. That person will go through the entire distribution system to identify where the deficiencies may be. That person would consider if there any components that can be series rated or make recommendations where the system can be upgraded to meet the SCC rating requirements which may include using series rated devices.
Again, unless such a study is done it is of my opinion that you are just beating around the bush and shooting at shadows. At least you can refer to a document that points out what problem areas need to be addressed.
The sooner that you do this the faster you can provide a properly coordinated system.
Again, unless such a study is done it is of my opinion that you are just beating around the bush and shooting at shadows. At least you can refer to a document that points out what problem areas need to be addressed.
The sooner that you do this the faster you can provide a properly coordinated system.
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I should add that given the previous transformer was rated 300kVA, which is 833A on a 208/120V system, and the gear has 800A fuses, seems to indicate the transformer was not overloaded.
That said, I'm wondering if the customer actually initiated the POCO upgrade by revealing its intention to change the service too far in advance of actually doing so...???
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
AIC = Amps interrupting Capacity; this is a term applied only to protective devices intended to interrupt fault current. For the most part this AIC applies to items covered by NEC 110.9
SCCR = Short Circuit Current Rating; is now the industry preferred term for equipment/items which are intended to carry fault current until a protective device operates. For the most part, this term is interchangeable with the power equipment term 'withstand rating', which was often misapplied. For the most part SCCR applies to items covered by NEC 110.10.
SCCR takes into account all fault handling capability, including the AIC of protective devices. For example: a switchboard may be built with bus bracing for 65kA of short circuit current and it may be able to accept protective equipment with AICs that go from 10kA up to 300kA (i.e. fusible switches with and without rejection clips). The equipment SCCR would be equal to the lowest value of bracing or protective device.
No the customer did not initiated an upgrade this was done by POCO (which is PSE out here) because their transformer blew up one night and made a huge mess. Actually this transformer was leaking for months and they would barely band-aide it then it had a complete meltdown one night so they had to bring out haz-mat to cleanup and this is where and why I think they swamped it out with the next size larger because it was such a rush job. "The fault current for 500 kVA, 120/208V xfmr is 60,300a. In our design standard for a 300 kVA xfmr, the fault current is 39,700a. For 750 kVA xfmr, the fault current is 39,100a. The customer says the old gear is only rated at 42,000a and the transformer is within 5' of this gear. Is the fault current for the 3 ph 120/208V 500 kVA xfmr correct at 60,300a" They could of went to a 750kVA and been okay with the fault current, but I'm guessing no one from POCO looked into this or cared about it.
Yes, but these conductors are super short and are already AL.
We had to get a permit because someone removed a meter at some point and the POCO took it away from them and now when they requested a meter be installed in the existing meter socket they request the inspection that has now lead to this mess. I have been dealing with the POCO for two months now and they don't answer anything and just keep passing it to the next person that says the same thing...nothing... The bussing on this gear is rated at 42k amps so there's not much that can be done beside replacing the gear. My issues is the POCO did this own their own without even telling anyone that they did upgrade this transformer.
Yes it's 2.3%
How do you get into contact with the Public Utilities commission? This same POCO has also done some other what I would called shady practices on some other current jobs. They also supply gas service in this area and a customer at a residential had a gas line 3" below his grass called and had locate done and hit it with a shovel. They came out and fix the leak and said they had to lower the line and then they never heard anything back from them and one day a couple of guys show up and dig up the yard and bore in a new line about 20' leave a big mess and take off. A month later they get a bill for $3,800. The bill has a phone number to call but goes straight to voicemail and they have left several message but no call back. To me this seems like medical billing where at the point you do get someone on the line they might waive the entire bill, but they actually write off the amount so they can show this on their books. This just seems shady.
Assuming PSE is Puget Sound Electric and in the U.S.:
http://www.utc.wa.gov/consumers/Pages/ConsumerComplaints.aspx
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
NickNorth...
Here's a solution based on Joe Villani's suggestion:
Given:
o Isc(e) = SC-Current Available (300kVA Xfmr) = 42,000A.
o Isc(f) = SC-Current Available (500kVA Xfmr) = 60.000A.
Find:
o Zi = Impedance to limit SC duty, inserted between new Xfmr and existing layout!
Equation(s):
o Eq 1) : Ze = Vs/Isc(e) = Vs / [?3 x Isc(e) ] = 2.86 mOhm.
o Eq 2) : Zf = Vs/Isc(f) = Vs / [?3 x Isc(f) ] = 1.91 mOhm..
Solution:
o Zi = Ze-Zf = 0.91 mOhm.
Regards, Phil Corso
Here's a solution based on Joe Villani's suggestion:
Given:
o Isc(e) = SC-Current Available (300kVA Xfmr) = 42,000A.
o Isc(f) = SC-Current Available (500kVA Xfmr) = 60.000A.
Find:
o Zi = Impedance to limit SC duty, inserted between new Xfmr and existing layout!
Equation(s):
o Eq 1) : Ze = Vs/Isc(e) = Vs / [?3 x Isc(e) ] = 2.86 mOhm.
o Eq 2) : Zf = Vs/Isc(f) = Vs / [?3 x Isc(f) ] = 1.91 mOhm..
Solution:
o Zi = Ze-Zf = 0.91 mOhm.
Regards, Phil Corso
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