Inrush 480-240/120V 50kVA Xfmr 3

[RonShap]

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.

 

[busbar]

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.

 

[RonShap]

The code states 250% max for the primary when there will be a secondary OCPD. There is no minimum that I can find, could you give me the article indicating the minimum?
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.

 

[cuky2000]

Take advantage of the advance search feature in this forum. Search for "Inrush current". See sample of result below:

thread238-10885

thread238-35609


Good luck

 

[busbar]

Occasionally there is no good substitute for well-composed/unambiguous contract verbiage.

 

[DanDel]

There is no bottom limit for the primary C/B as far as overcurrent protection. Your concern about inrush current is valid and may limit the choice of C/B, however.

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?

 

[RonShap]

It is a Cutler-Hammer (actually an ITE) Type EHD. The left side of the instantaneous characteristic is 10x the trip. I realize the rule of thumb for inrush is generally 12x the FLA for .1 seconds per ANSI C57.109. I am trying to plot it on SKM software to display to the client, but for some reason the inrush indentifier (a cross on the drawings) is at approx 720A, not as expected (104FLA x 12 should end up at 1248A). I wanted to confirm that my approach is fair, and I will contact SKM to find out why the inrush symbol is so far from expected.

 

[DanDel]

IEEE 242 recommends using 10x for transformers under 2000kVA. Typically, the smaller the transformer, the less the inrush per unit. I've seen 8x used for transformers larger than yours. Perhaps SKM uses a default multiplier based on size.
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.

 

[jbartos]

Suggestion: It is not unusual to have a sensitive protector fine tuned, if need to be. Fine-tuning can start with a circuit breaker on its larger rating side according to NFPA 70 NEC. Then, the inrush current can be measured by oscilloscope and the protective device rating reduced accordingly over the circuit breaker time-current curves.

 

[RonShap]

The circuit breaker is an ITE. SKM does not have an ITE libray anymore, so I have to use the Cutler Hammer curves as a substitute.
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.

 

[jghrist]

SKM is using the FLA of a 50 kVA 480 volt 3-phase transformer which is 60A.

 

[RonShap]

jghrist,
Sometimes the obvious is so not-obvious for me. Thanks for the idea, I think your right.

 

[jbartos]

Suggestion: Check with Siemens regarding ITE breakers. Visit

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

 

[DanDel]

RonShap, according to ETAP software, the inrush point for the 50kVA single-phase 480/208 transformer (plotted at 10X or above) and the Instantaneous portion of the time-current curve for the ITE EHD 100A C/B are in definite conflict.

 

[RonShap]

Thanks