When a group of not more than 6 overcurrent devices is used as the transformer’s secondary protection (1000 volts, nominal or less), the sum of all device ratings shall not exceed what value?

When a group of not more than 6 overcurrent devices is used as the transformer’s secondary protection (1000 volts, nominal or less), the sum of all device ratings shall not exceed what value?

• A. The value of a single overcurrent device
• B. The sum of all device ratings
• C. Twice the largest device
• D. The average of device ratings

Answer: (A)

When protecting a transformer’s secondary with a small group of fuses or breakers (not more than six), the sizing rule is that the total of all their ampere ratings must not exceed the rating of a single protective device. This keeps the protection on the secondary in proper proportion to the transformer and helps maintain coordination, so that the group as a whole behaves like one device in terms of overcurrent protection. For example, if the transformer secondary is rated for 100 A, you could use multiple devices whose combined ratings do not exceed 100 A (such as six 15 A devices totaling 90 A, or a single 100 A device). You cannot have the sum exceed the rating of one device, which is why the correct total is the rating of a single overcurrent device. The other ideas (sum of all ratings, twice the largest, or the average) don’t align with this protection approach.

When protecting a transformer’s secondary with a small group of fuses or breakers (not more than six), the sizing rule is that the total of all their ampere ratings must not exceed the rating of a single protective device. This keeps the protection on the secondary in proper proportion to the transformer and helps maintain coordination, so that the group as a whole behaves like one device in terms of overcurrent protection.

For example, if the transformer secondary is rated for 100 A, you could use multiple devices whose combined ratings do not exceed 100 A (such as six 15 A devices totaling 90 A, or a single 100 A device). You cannot have the sum exceed the rating of one device, which is why the correct total is the rating of a single overcurrent device. The other ideas (sum of all ratings, twice the largest, or the average) don’t align with this protection approach.