A set of paralleled feeder conductors has a total allowable ampacity of 1,140 amperes after every correction and adjustment. The load is noncontinuous, the conductors supply no receptacles, and the standard device ratings bracketing that value are 1,000 and 1,200 amperes. What is the largest overcurrent device permitted to protect these conductors?
- A1,200 amperes, using the next higher standard rating
Rounding up is only permitted for devices of 800 amperes or less. Applying it here would leave the conductors protected at a value above their ampacity.
- B1,140 amperes
This figure is the conductor ampacity, not a standard device rating. Overcurrent devices are selected from the standard ratings the Code lists.
- C800 amperes
Eight hundred amperes is the threshold where the rule changes, not the answer. Choosing it throws away several hundred amperes of usable capacity.
- 1,000 amperes
Why D is correct
Two rules cover conductor protection and the boundary between them is 800 amperes. At or below that value you may round up to the next standard rating. Above it, the conductor must carry at least as much as the device is rated, so you round down. Recognizing which side of the boundary a problem sits on is the entire skill being tested.
What this question is testing
This item tests whether you know the 800 ampere boundary in the conductor protection rules and whether you round the correct direction on each side of it.
On the job
This shows up on service and main switchgear sizing, where every 100 amperes of frame size is real money and where a design that rounds the wrong way either fails inspection or wastes copper. The practical move is to size the conductors so their ampacity lands at or above a standard rating rather than just below one, which usually means one more parallel set or one size larger conductor. Catching it at design is far cheaper than discovering it at energization.
Memory technique
Under eight hundred you may round up; over eight hundred the wire must carry the whole device.
Exam tip
Ask whether the device is above or below 800 amperes before you decide whether to round up or down. That single question decides these problems.
Where to look it up
Protection of conductors is at the front of Article 240, with the next-higher-rating allowance and the above-800-ampere rule in consecutive subdivisions.