The Primary voltage(s) stated in standard transformer specifications (or
customer-specified in an inquiry) are the nominal voltages. These voltages may vary during times when line supplies are at heavy loads (nearly always around dinnertime), or light loads (when most people are asleep). Drops or rises in this voltage can be as much as 10% of the nominal and vary considerably from country to country. This change in voltage will be reflected on the secondary voltage (on and off-load) by the standard transformation relationship:
A toroidal transformer is rated at 12v and the primary is rated at 117v 60 Hz. The line supply regulation is stated as ± 6% by the utility company.
The secondary voltage when the line is at its lowest voltage will be: