Inrush 480-240/120V 50kVA Xfmr
Inrush 480-240/120V 50kVA Xfmr
(OP)
Considering FLA for the primary of the 480-240/120V 50kVA single phase transformer is 104A; and the secondary load is fixed and relatively low; how tight (low) can I set the primary thermal magnetic circuit breaker without false trips of the primary breaker due to the inrush current? 80%, 90%, 100% or ????? of FLA.






RE: Inrush 480-240/120V 50kVA Xfmr
Based on NEC article 450, 125-250% is specified. What is the purpose for anything lower? In any case, three-wire secondaries must have their own overcurrent protection—primary alone won’t do it per NEC.
RE: Inrush 480-240/120V 50kVA Xfmr
The secondary circuit breaker will be 200A.
The contractor is attempting to save his money by re-using an existing panelboard which will not accept a breaker bigger than 100A. I am trying to establish a code violation to force a new panel be installed with a 125-150A primary breaker to avoice nuisance trips of the main.
RE: Inrush 480-240/120V 50kVA Xfmr
Thread238-10885
Thread238-35609
Good luck
RE: Inrush 480-240/120V 50kVA Xfmr
Occasionally there is no good substitute for well-composed/unambiguous contract verbiage.
RE: Inrush 480-240/120V 50kVA Xfmr
A typical 100A molded case C/B is so rated because it has a thermal overload trip of 100A. This means that eventually, the C/B will trip for current over 100A. Believe it or not, it can take well over 2 minutes for most 100A C/Bs to trip at a 300% overload(300A). This C/B also has a magnetic short-circuit trip function(usually non-adjustable for this size C/B) which will trip with no intended time delay at current values approximately 10x the current rating.
A 50kVA transformer may have an inrush current of between 8 and 10 times its FLA, for a maximum time of 0.1s. This may cause the nuisance trips you are concerned about. A quick look at the time-current curve of the C/B plotted against the inrush point of the transformer(a coordination graph) will indicate if there is a possibility of interference.
What is the manufacturer and model of the C/B?
RE: Inrush 480-240/120V 50kVA Xfmr
RE: Inrush 480-240/120V 50kVA Xfmr
I will try tomorrow with my ETAP program and check for you.
Is the C/B a C-H or ITE? I believe they both have a model EHD. Some C-H devices used to be Westinghouse, and some ITE devices are now Siemens, but I've never heard of both C-H and ITE together on one C/B.
RE: Inrush 480-240/120V 50kVA Xfmr
RE: Inrush 480-240/120V 50kVA Xfmr
The SKM damage curve is adjustale, and is currently set at 12x for .1 seconds and indicates approx 720A (obviously not 104A x 12) on the TCC drawing. I though maybe the 12x was something other than FLA, but I believe that ANSI indicates 12x the FLA, so it should be 1248A instead.
Since it is such a small transformer, the manufacturer will not indicate the design parameters, and it is not worth testing prior to installation.
RE: Inrush 480-240/120V 50kVA Xfmr
RE: Inrush 480-240/120V 50kVA Xfmr
Sometimes the obvious is so not-obvious for me. Thanks for the idea, I think your right.
RE: Inrush 480-240/120V 50kVA Xfmr
http://www.siemens.com
http://www.siemens.de
http://www.ad.siemens.de/cd/energie/index_76.htm
http://www.ad.siemens.de/cd/energie/html_76/leistung_3vf.htm
http://www.ad.siemens.de/cd/energie/html_76/products.htm
etc. for more info
RE: Inrush 480-240/120V 50kVA Xfmr
RE: Inrush 480-240/120V 50kVA Xfmr