Sparky
Get Started
Practice question · NEC 430.52
What Size Fuse for a 22A Motor at 300%?
Motor fuse sizing is a two-step question: multiply, then round up. The exam knows most people stop after step one — so step two is where the points are.
Exam-style question (Sparky QOTD)
For a 22A FLC motor, a non–time-delay fuse at 300% gives 66A — not a standard size. The maximum permitted fuse is ___.
A. 70A
B. 66A
C. 60A
D. 80A
Answer: A) 70A. 66A isn't a standard rating, so 430.52(C)(1) Exception 1 lets you use the next higher standard rating per 240.6.
Where to Find It
Open your Code book to Table 430.52 and read it together with 430.52(C)(1) Exception 1. Table 430.52 gives a non–time-delay fuse at 300% of full-load current: 22 × 3 = 66A. But 66A isn't a standard rating — standard ratings live in 240.6 — so the Exception lets you go to the next higher standard rating: 70A.
Why This Works
Step one is straight table math: Table 430.52 × full-load current. For a non–time-delay fuse at 300%: 22 × 3 = 66A. But there's no such thing as a 66A fuse, and the Code knows it — that's what Exception 1 to 430.52(C)(1) is for.
The Exception permits the rating to be increased to the next higher standard size per 240.6, which is 70A. Note the direction: the allowance goes up, never down. 60A (option C) would be rounding the wrong way, and the question asks for the maximum permitted size, so you round up to 70A — not down to 60A.
Tab Table 430.52 in your book with a note: this is the same round-up rule as breakers. And if you want the percentage steps drilled into muscle memory, grab our formula sheet — the 22 × 3 step should be automatic.
The Trap
Option B (66A) is the trap — the answer you get if you do the math and stop. The exam always plants the "math only" answer for people who skip the Exception. Read the whole question: "not a standard size" is the stem telling you the round-up step is being tested. If 66A were a standard rating, the answer would be 66A. It isn't, so the maximum permitted is 70A.
Exam Tip
Watch for the phrase "maximum permitted" — that wording is your cue that the round-up step is the whole question. The exam does the same dance for breakers off the same table. Whenever the calculated value doesn't land on a 240.6 standard size, go up one step, and never round down.
Keep Drilling
This is one of 33 NEC topics. Sparky's 1,087 codebook-first questions drill all of them — every question a lookup rep with your book open, 15 minutes a day.