Transformer to Main Breaker matching 4

[Splash1]

I have an unusual (for me unusual) situation. Have been asked by electrical contractor to obtain fault current ratings for a transformer serving a shopping center that we are doing a tenant build-out for and have obtained this. The contractor is telling me that the main breaker for the service supplying the space is not the right RMS factor and must be changed….? This center has been operating since the 60’s with no fault occurrences. It is 3 phase, 150kVA, 4160 x 13200Y to 120/208Y V_3-Phase. Available Secondary fault currents (Amps) Are: Three Phase: 17591 and Phase-Neutral: 15881 the RMS of the Main Breaker is square D, 200 Amp three phase Breaker rated at 10,000 SYM RMS. The Center has 5 other operating business all with virtually the same equipment and no problems. I have never had this situation in the many years of tenant build outs and construction. Cannot find anything on this. Can you help me understand?
Thanks in advance.

 

[davidbeach]

Well, last I checked 10,000 is rather a bit less than 17,591. Sounds like an NEC violation to me. Past performance is not an indicator of future results. Just because you've been lucky to date doesn't mean there want be a fault tomorrow the causes the breaker to blow up instead of cleanly interrupt.

 

[mgtrp]

Are you certain about the fault current? 17000A secondary fault must mean a fairly low impedance transformer, it may be worth double-checking. You could also include system and cable impedances. Otherwise, it sounds like your contractor is probably correct.

 

[waross]

You could also include system and cable impedances.

Good advice. That has saved me a couple of times when the Available Fault Current was not too far over the breaker rating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Splash1]

Thanks for the responses. The transformer is supplied by the utility company and the center has 6 separate business all served by the same transformer. The RMS is still not clear to me in that there is, I'm sure, a complicated calculation that the utility would have done when installing the transformer to make it compatible with the equipment served. The only other info from them on wire was Service Conductor type: UG-250 kcmil Al 37 Strand - #2/0 Al 19 Strand Neutral. If the load that was being "re-installed" were more than the previous tenant then I could justify the changing of the panel and main and the additional $$$$expense. However, the load is less than the previous tenant and the panel is more than sufficient for the tenant's needs. Just never had the RMS matching ever come up in an existing space for tenant improvements. Can you suggest the RMS for the main breaker?

 

[waross]

I think that rather than RMS you mean Available Fault Current. The breaker must have an interrupting rating equal to or greater than the Available Fault Current.
Rated full load current divided by the percent impedance voltage, sometimes just called percent impedance. On the transformer nameplate it may be shown as %Z.
It is the contractor's responsibility to check the available fault current of the supply transformer and use properly rated equipment.
The full load current rating of the transformer will be 150,000 KVA / (208 Volts x √3) = 416 Amps
416 Amps / 17591 Amps = 0.0237 Your transformer should show a %Z of about 2.4%
Note: This is the symmetrical or steady state fault current. The actual fault current may asymmetrical and may be considerably higher until the offset or DC component decays. When equipment is tested and rated for available fault currents, it is actually tested at the much higher peak offset current to account for the asymmetrical fault current for representative cases.
In the worst case, the asymmetrical current may approach 2.8 times the RMS current. How close it approaches depends on the X/R ration of the transformer and the point on wave where the transformer is energized.
As noted, this factor is allowed for in the testing and rating process.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wayne440]

I wonder how many changes the utility has made since "the 60's"? My next call would be to the utility to ask very, very nicely if they can do anything to reduce that figure to less than 10,000A.

 

[Splash1]

Again, THANKS!
Waross you have given me a much better understanding. Yes, I agree that it is the contractor's responsibility to be sure of the serviceability and conformance but, mid-stream is a little late for an EXPENSIVE change order that could/should have been done at the onset. Wayne440's suggestion will be tried, but, I'm not too ambitious about their help. Haven't had much luck with their customer service.

 

[davidbeach]

Sorry, the utility fault value is what it is and you are on the hook to supply suitable equipment. They're not going to change the transformer for your convenience. I'm pretty sure that their number is based on the lowest impedance they use for that size and may not represent the fault current from the actual transformer installed, but that transformer could fail at any time and utility will replace it out of their stock with whichever one they grab first. That one could be at the very low end of the impedance range. The utility will know what range (could be fairly broad) of impedance the transformer will fall into, but they won't have specific impedance data for every service transformer.

 

[wayne440]

It is very true that "your mileage may vary" when dealing with the utility guys. But asking nicely doesn't usually cost much, if anything. Good luck.

 

[Splash1]

Thanks.
Yeah, I'm sure that the utility won't accommodate me. But, everytime they change a transformer "from Stock" I guess the consumer has to change their service panel/main breaker to be compliant. if they don't match the RMS. Still sounds hokey to me. LOL
Asking nicely and repeatedly hasn't gained much ground thus far.

 

[unclebob]

Check if your breaker comes with limiter fuses. If so, you're ok and the breaker+fuse interrupting rating shoud be way higher than 10 kA.
Dunno if Square D have this ''Tripac'' type available though.

 

[wbd]

Typically a utility will provide the secondary fault current based on an infinite bus and the lowest impedance for the transformer that they will use. This will provide the absolute maximum fault current that can be delivered to the customer.

You may ask the utility for the AVAILABLE fault current and X/R ratio versus the MAXIMUM fault current. You may find that your existing equipment may be suitable for the conditions that exist now. This may give you some breathing room until you can plan an upgrade.