NEC 695.5(A)covers sizing of a transformer when supplying a pump from a single source. Basically it's 125% of the pump load plus 100% of other load.
If two sources are used with a transfer switch, 695.5(C)(1) covers the sizing and is the same as 695.5(A), BUT, 695.5(C)(2) requires that 450.3 and Table 450.3(A) be also satisfied for protection by the over-current devices sized to carry locked rotor current. In cases where there is very little "other" load (non fire pump), it is necessary to increase the transformer size so the upsized OC devices protect it per 450.3.
Why is this required on dual fed pump systems and not on single feed systems?
If two sources are used with a transfer switch, 695.5(C)(1) covers the sizing and is the same as 695.5(A), BUT, 695.5(C)(2) requires that 450.3 and Table 450.3(A) be also satisfied for protection by the over-current devices sized to carry locked rotor current. In cases where there is very little "other" load (non fire pump), it is necessary to increase the transformer size so the upsized OC devices protect it per 450.3.
Why is this required on dual fed pump systems and not on single feed systems?