Transformer to Main Breaker matching
Transformer to Main Breaker matching
(OP)
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.
Thanks in advance.






RE: Transformer to Main Breaker matching
RE: Transformer to Main Breaker matching
RE: Transformer to Main Breaker matching
Bill
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"Why not the best?"
Jimmy Carter
RE: Transformer to Main Breaker matching
RE: Transformer to Main Breaker matching
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
RE: Transformer to Main Breaker matching
RE: Transformer to Main Breaker matching
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.
RE: Transformer to Main Breaker matching
RE: Transformer to Main Breaker matching
RE: Transformer to Main Breaker matching
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.
RE: Transformer to Main Breaker matching
Dunno if Square D have this ''Tripac'' type available though.
RE: Transformer to Main Breaker matching
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.