# 3-Phase Transformer Question?

Please indulge an ME for asking this, but here goes. We have a building with a 600A, 120/208V service; what size transformers will the power company install.?This is an overhead service, so I assume they will use 3 single-phase line transformers?

In simple terms, the connected load is:
600 amps x .208 kV x √3 = 216 kVA

In reality, the NEC does not allow you to load your 600 amp switch beyond 80% of its rating, (480 amps) therefore the worst case demand you can achieve is 80% of 216 kVA or 173 kVA.

The electric utility is concerned about what your demand is going to be. This is the maximum rate of consumption over a 15 or 30 minute period. This value is FAR lower than your switch size. It is VERY common for your actual loads to be far less than even 173 kVA. This is due to the electrician following the NEC, when tends to be VERY conservative and overstate the loads of various equipment. I would not be surprised if your building's actual demand is closer 100 kVA.

Assuming your building's load is 100 kVA, the utility would be considering either 3-25 kVA transformers or 3-37.5 kVA transformers. Liquid filled distribution transformers are rated for an insulation life of 20.55 years while loaded continuously at nameplate kVA with a top oil temperature of 110° C. In reality your buildings load will cycle throughout the day, and the ambient temperature will flucuate through the day and the seasons.

Assuming your single phase load is evenly balanced, the load seen by each of the 25 kVA transfomers would be 33.3 kVA or 133% of nameplate. Depending on your building's "peak load duration" and geographic location, this might be completely suitable and result in a 30+ year transformer life.

Installing three 37.5 kVA transformers will only load them to 89% of nameplate. (33.3 ÷ 37.5 = 89%) Since this is your "peak load", the transformers will be lightly loaded much of the year. Becaue they are never pushed beyond their nameplate, the transformers will likely last "forever".

As a real world example, most fast-food restaurants have at least a 600 amp switch. Yet most do not need a transformation larger than three 37.5 kVA transformers.
3 -75KVA
3 125KVA transformers (the exact figure is 124.8KVA) - provided the 208VAC also delivers 600A.
transformer design cannot be computed with just your main breaker only, computation is based on your connected loads with demand factors, but for the sake of giving you the maximum transformer design, heres the computation based on your main breaker, but Im sure the utility company won't follow this kind of computation.
Main protection= 600 amps, 3 phase
incoming voltage=208 wye, with neutral at 110 volts
Formula for determining KVA;
KVA=sq. root 3*E*I
=1.732*208*600/1000
=216.153 kva
KVA per transformer= 216.153/3= 72.05 kva
since the standard transformer nearest this value is 75 kva, then it will be 3-75 kva transformer.
but if they will compute with demand factor, then mostly they will use 80%,
so, 80% of 216.153= 172.92 kva,
172.92/3=57.64 kva per transformer
still the nearest standard transformer is 75 kva and 50 kva which will be smaller than the computed load, so, the most probable bet that they'll install is 3- 75 kva transformer.
Phase power = 600 x 120 = 72000 VA

So, use 3 single phase transformers banked in delta primary and y-secondary each having 72 KVA rating. Or directly a 3 phase delta / wye transformer having rating of 216 KVA.

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