Are these transformers overloaded?

[ShannonM]

Ok, so the following measurements were taken on two 408/208 transformers in a hospital. The tech in the field considered them to be overloaded, and therefore needing replacement (not by us, we're an inspection-only company). If the transformers were balanced, this would be clear-cut, but they're not, and so it isn't.

T1 (75 kVA):
H1:50.2 H2:47.5 H3:42.4 X1:97.8 X2:121 X3: 95.4 X0:48.9

T2 (112.5 kVA):
H1: 97.5 H2:62.4 H3: 65.7 X1:181.7 X2: 121.3 X3: 99.8 X0 81.7

Also, is there an easy (usable in the field) formula that I can give to the techs to determine whether a transformer is overloaded given nothing but the nameplate and a clamp-on ammeter? I've been looking for hours, and can't find one. All of the basic formulas assume balanced currents, and those are as rare as an honest politician in New Jersey.

Thanks for the help,

--Shannon

 

[iwire]

Ok, so the following measurements were taken on two 408/208 transformers in a hospital. The tech in the field considered them to be overloaded, and therefore needing replacement (not by us, we're an inspection-only company). If the transformers were balanced, this would be clear-cut, but they're not, and so it isn't.

T1 (75 kVA):
H1:50.2 H2:47.5 H3:42.4 X1:97.8 X2:121 X3: 95.4 X0:48.9

75 KVA trans 480 to 208

Rated input current. 90 amps, your highest reading 50.2.

Rated output current. 208 amps (not a typo, 208 amps), your higest reading 121.

T2 (112.5 kVA):
H1: 97.5 H2:62.4 H3: 65.7 X1:181.7 X2: 121.3 X3: 99.8 X0 81.7

112.5 KVA trans 480 to 208

Rated input current. 135 amps, your highest reading 97

Rated output current. 312 amps, your highest reading 181.


I see no problem at all.

How or why did the tech feel these where overloaded?

Also, is there an easy (usable in the field) formula that I can give to the techs to determine whether a transformer is overloaded given nothing but the nameplate and a clamp-on ammeter?

For three phase.

KVA * 1000 / volts / 1.73

All of the basic formulas assume balanced currents,

That does not change the formula, each phase is still well below the transformer capacity.

The loads could be better balanced but that has noting to do with the transformers.