208V/3 to 480V/3 Trans. Installation

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steveve1

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Hi Guys! Well I'm sure this has been hashed out before, but I missed it. So here's the situation: On a service call to an existing industrial faculity with a 208 volt service. A standard 480v delta to 208Y/120 transformer is fed 208/3 to derive 480/3 for motor load (no neutral required). The transformer is 75kva, the primary over current protection (208v side) is 200a (LPN-RK), no secondary over current protection until the motor controller, 150' away, 100a circuit breaker then fusing. Secondary voltages (480v) seem fine when all is well. But in switching off the transformer for lock-out, then switching back on with no load, a primary fuse blew. Nothing to ground anywhere, all seems clear, installed new fuse, switch on - all good. Next day same thing. I'm thinking in-rush current is taking out the fuse (different phase each time). As the 208v side of things is connected wye, should X0 be grounded? It currently is not. I'm also thinking wrong transformer for the application, not to mention some over current concerns on the 480v secondary. Any thoughts? By the way, Happy Holiday Season to you and yours!
 
Re: 208V/3 to 480V/3 Trans. Installation

Usually when I hear about a 480 to 120/208 volt transformer installed backwards, to provide 480, x0 is grounded and that causes nuisance tripping because the connection will balance the 208 panel it is being fed from. When the 208 neutral at the transformer is lifted, the tripping stops. I have used the ungrounded transformer with success before, it should work.

Are you sure there is no ground on the transformer 208 neutral?

Jim T
 
Re: 208V/3 to 480V/3 Trans. Installation

I recall reading that the impedance of the 208 volt side is lower on this type of transformer than on one built for the purpose and that many times the primary OCPD will open on start up.
Don
 
Re: 208V/3 to 480V/3 Trans. Installation

Thanks for your quick replies. Yes, all XO bonding to ground appears to have been removed, I'll 2x check that. What if fusing was changed to FRN type? Whould that slow the in rush current trip?
 
Re: 208V/3 to 480V/3 Trans. Installation

Steve
The FLA for a 75 kva at 208 volts is 208 amps. A transformer may see 10 to 15 times the FLA when energized. NEC table 450.3B allows the primary fuse to be 125% of the FLA or 260 amps to handle inrush currents. You can go the next size up or 300 amps. I think you are under fused.
Another option is to use a 225 amp time delay fuse. I don't have the Bussman Fuse Info at this time.

[ December 08, 2004, 08:08 PM: Message edited by: bob ]
 
Re: 208V/3 to 480V/3 Trans. Installation

Thank You Bob for your input. As this is an existing installation, changing out to a 400a switch and fusing up would be a tough sell. What about slower blow fuses? Also what of the 480v secondary over current protection in this case?
 
Re: 208V/3 to 480V/3 Trans. Installation

Yes, do not bond or ground (earth) "X0". Do not connect anything to "X0".

The sec. (480V. side) should have O/C protection close enough to the xfmr to meet Art. 240 tap rules. 125A time lag fuse or bkr. should be plenty for the 90A rated sec. FLA.

Ideally, O/C protection on the pri. (208V. side), would be at least 250A, but like you say, will not fit in 200A fused sw.

Study the fuse curves yourself or check with knowledgable Bussmann engr. to verify whether LPN or FRN has more time lag at inrush levels.

[ December 08, 2004, 08:44 PM: Message edited by: kiloamp7 ]
 
Re: 208V/3 to 480V/3 Trans. Installation

Steveve1-

higher rated or time delay fuses may stop the blowing fuses, but will probably only mask the problem. Fuses don't "false" blow - they blow for a reason. It seems there is power flow that is blowing the fuses. There is nothing wrong with running a transformer "backwards" in this way, so inrush is not the problem, anymore than it would be on a 480 volt primary transformer. Per UL, time delay fuses must hold 500% rated current for 10 seconds. A non-time delay fuse will hold for something less, but well over the duration of the inrush, which might be a couple cycles.

Have you looked at the 480 volt side? is there an intentional or inadvertant ground on the 480 volt side? With no fuse at the 480 volt source, how do you know the 480V circuit is OK? I suggest adding fuses within 10 cable feet here. If the 480 volt fuse(s) blow, you have a better idea that the problem is on the 480 volt side. Since you have no OCP now on the 480 volt side (or 150 feet away) there may be a problem which is showing up as the 208 volt fuses blowing. And of course with OCP 150 feet away, there is a good chance that there could be a problem in the 480v circuit, and the breaker at the END of the 150 feet will do nothing, since the problem is likely upstream. (What if there is a fault at 149 feet? The circuit breaker 1 foot away does not see the current, therefore does not trip.)

Check out the 480V side, and add a fused disconnect per 240.21(C)(2). Waiting to hear how this turns out....
 
Re: 208V/3 to 480V/3 Trans. Installation

You might also look at the size of the motor load. They may be pushing the transformer to near its limit.

And do the motors automatically start when the transformer is turned on? If so, do they start under a load? Then you would have transfromer in-rush current followed by motor starting current. That's a good way to blow a fuse.
 
Re: 208V/3 to 480V/3 Trans. Installation

I always read these transformer application issues with interest.
As a former applications engineer for both distribution dry type transformers and molded case circuit breakers the transformer product line manager charged me with doing a study of the relationship of different sizes and types of transformers and overcurrent protection as included in the NEC. The experience and data that I have and information as provided by a seasoned and now retired transformer design engineer for a leading transformer manufacturer shows that The inrush for a transformer feed conventionally on the HV side will be proportionally the same as the same transformer feed on the LV side. The average 150degC DTDT is about 10x the FLA. This goes up significantly for 115degC and even more with 80degC and K-factor transformers. This information was compared to the trip curves of breakers that would be commonly applied.
The bottom line is that the pri.cable feeding the transformer and the pri. OCPD should be sized based upon the FLA of the transformer and not the load that is to be served.
The FLA of the LV side of a 75KVA transformer is
208a. It is my understanding from the NEC that the cable be sized 125% or 260a, that the cable must carry that load (1-300kcmil/ph.) And the breaker sized based upon 208v x 125%=260a you are allowed to go up to the next standard rating of 300a.
If the inrush is 208x10=2080a then a 300a breaker with an instantaneous of 3000a will be adequate. A 250a breaker would also work. A 200a breaker would be Russian roulette.
A 250a breaker may be marginal at best when applied with a 115degC and K4 transformer and a 300a breaker may even be marginal for an 80degC an K13 designs.
It is my interpretation of the NEC that sec. protection is not required with a 3ph3w and, as such, you are stuck with the 125% for the pri. protection but are allowed to go up to 250% if you do provide a sec. OCPD.
The data that I have accumulated is not meant to be an exact science but does provide some good ball park values that show possible application problems as their causes.
 
Re: 208V/3 to 480V/3 Trans. Installation

Based on my understanding of the installation, based on 75 Kva, 208V, 3-phase primary, the OCP in the primary is allowed to be 125 % of primary current which calculates to 208A x 1.25 = 260A. The Code allows the next standard size to be used which is 300A. However, unless the 480V secondary conductors are routed outdoors for 150 ft, they must have OCP in accordance with 240.21(C). High in rush current in transformers can be cuased by the orientation of the molecular structure when the transformer is de-energized. Re-energizing the transformer with the core molecular structure in alignment, allows higher inrush current. This can be countered by having a small load connected to the secondary when energizing the transformer.
 
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