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Transformers + Substations Handbook: 2014
The efficiency standards in NEMA TP-1 [1] are based on certain
assumptions that may result in the selection of less-than-optimally
efficient transformers. One key assumption is that low voltage (600 V
class), dry-type (typical commercial or industrial) transformers are
loaded at 35% of their nameplate rating. For medium voltage and liq-
uid-filled transformers, the assumed loading is 50% of the nameplate
rating. Another underlying part of the economic rationale for the stand-
ard is an assumed electricity cost of six cents (US) per kWh (which is
equivalent to 62 cents per kWh in South Africa).
These assumptions could be inaccurate for industrial and commer-
cial users, who can often more accurately predict their load require-
ments and who may be paying more or less than six cents per kWh,
particularly at peak times. In fact, recommended loading for economic
sizing of a transformer is typically around 75% of nameplate; a 35%
load, if constant, means the transformer is oversized and wasting core
loss as well as being higher priced.
The operating cost of a transformer,
over its long life span, can greatly
exceed its initial price, so the
selection must include examining
the unit’s capacity and efficiency.
Copper windings
Table 1
compares a ‘standard efficiency’ 75 kVA transformer to an al-
uminium-wound TP-1 model, a copper-wound TP-1 model and a ‘pre-
mium efficiency’ copper-wound unit, at various loading levels. As
shown, choosing a more efficient, copper-wound transformer that
exceeds the minimum efficiencies of TP-1 (and Energy Star) can pay
back its price premium in as little as one year.