STANDBY CAPACITY OF PLAIN CABLE FEEDERS AND TRANSFORMER FEEDERS

For switchboards other than those for the generator or intake feeders it is established practice to
add some margin in power capacity of their feeders so that some future growth can be accommodated.
The margin is often chosen to be 25% above the TPPL.

If the feeders are plain cables or overhead lines then it is a simple matter to choose their
cross-sectional areas to match the current at the 125% duty.

For transformer feeders there are two choices that are normally available. Most power transformers
can be fitted with external cooling fans, provided the attachments for these fans are included
in the original purchase order. It is common practice to order transformers initially without fans
and operate them as ONAN until the demand increases to justify the fan cooling. Thereafter the
transformer is operated as ONAF, see sub-section 6.5. Adding fans can increase the capacity of
the transformer by 25% to 35%, depending upon the particular design and ambient conditions. The
alternative choice is simply to rate the ONAN transformer for the 125% duty, and initially operate
it at a lower level. The decision is often a matter of economics and an uncertainty about the
future growth.

When standby or future capacity is required for transformers it is necessary to rate the secondary
cables or busbars correctly at the design stage of the project. Likewise the secondary circuit
breakers and switchgear busbars need to be appropriately rated for the future demand. The decision
to over-rate the primary cables or lines may be made at the beginning of the project or later when
demand increases. Again this is a matter of economics and forecasting demand.