According to Table 450.3(B), for a three-phase transformer with primary and secondary ratings of nine amperes or more and with no coordinated thermal overload protection, the maximum primary OCPD rating is what percentage of the transformer rating?

According to Table 450.3(B), for a three-phase transformer with primary and secondary ratings of nine amperes or more and with no coordinated thermal overload protection, the maximum primary OCPD rating is what percentage of the transformer rating?

• A. 125%
• B. 250%
• C. 200%
• D. 300%

Answer: (B)

The test is about how the NEC specifies primary protection for a transformer when there is no coordinated thermal overload protection inside the transformer. Table 450.3(B) gives the maximum primary overcurrent protective device (OCPD) rating as a percentage of the transformer's primary rating. For a three-phase transformer with primary and secondary ratings of nine amperes or more and with no coordinated thermal overload protection, the primary OCPD is allowed to be as high as 250% of the transformer's primary rating. This higher limit accounts for the inrush and short-duration overload that can occur when the transformer energizes, preventing nuisance tripping while still providing protection. If thermal protection were provided, the permissible primary OCPD would be smaller, but under the given condition with no coordinated thermal overload protection, 250% is the correct maximum.

The test is about how the NEC specifies primary protection for a transformer when there is no coordinated thermal overload protection inside the transformer. Table 450.3(B) gives the maximum primary overcurrent protective device (OCPD) rating as a percentage of the transformer's primary rating. For a three-phase transformer with primary and secondary ratings of nine amperes or more and with no coordinated thermal overload protection, the primary OCPD is allowed to be as high as 250% of the transformer's primary rating. This higher limit accounts for the inrush and short-duration overload that can occur when the transformer energizes, preventing nuisance tripping while still providing protection. If thermal protection were provided, the permissible primary OCPD would be smaller, but under the given condition with no coordinated thermal overload protection, 250% is the correct maximum.