13.4 Motor Short-Circuit / Ground-Fault Protection

Fault Protection Uses Table FLC and Big Percentages

After conductors (125% of FLC) and overload (115%/125% of nameplate), Prov still asks for the branch-circuit short-circuit and ground-fault protective device. That device must:

1. Clear faults before conductors are damaged.
2. Ride through normal starting inrush.

Those goals produce Table 430.52 percentages that look “too big” if you are still thinking like overload. They are supposed to look big.

Tab 430.52, Table 430.52, 240.6(A) standard sizes, and 430.62 for feeders.

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Table 430.52 — Maximum Ratings / Settings

430.52 requires a branch-circuit short-circuit and ground-fault protective device for each motor branch circuit. The device rating or setting shall not exceed the values in Table 430.52 except as permitted by the section’s exceptions.

For the common AC polyphase motors other than wound-rotor (and similar rows you must match exactly), typical maximum percentages of full-load current are:

Always open the table and match the motor type row and device column. Wound-rotor, DC, and Design B energy-efficient motors can differ. The percentages above are the workhorse values for ordinary squirrel-cage / Design B style stems on JW exams — verify before committing.

Current basis: Table FLC from 430.6(A)(1), not nameplate.

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Worked Example H — Inverse-Time Breaker

Motor: 10 hp, 460 V, 3φ. Table 430.250 FLC = 14 A.

Maximum inverse-time breaker before exceptions = 14 × 2.50 = 35 A.

35 A is a standard size in 240.6(A), so the maximum is 35 A without needing Exception No. 1.

Wrong paths:

• 14 × 1.25 = 17.5 A → that is conductor ampacity, not the breaker maximum.
• 14 × 1.25 using nameplate-style thinking → still wrong current/percentage mix.
• 14 × 1.75 = 24.5 A → dual-element fuse path, not inverse-time breaker.

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Worked Example I — Dual-Element Fuse

Same motor, FLC 14 A.

Maximum dual-element (time-delay) fuse = 14 × 1.75 = 24.5 A.

If 24.5 A is not a standard fuse rating, 430.52(C)(1) Exception No. 1 generally allows the next higher standard size. Standard fuse sizes in 240.6(A) include 25 A — so 25 A is the usual next-size result (confirm the exception conditions in your book).

Contrast: an inverse-time calculation that landed on 35 A exactly needed no bump; a dual-element calculation that lands between standard sizes often does.

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Worked Example J — Larger Motor, Next Size Up

Motor: 25 hp, 460 V, 3φ. Table FLC ≈ 34 A.

Inverse-time maximum = 34 × 2.50 = 85 A.

85 A is standard → 85 A breaker maximum under the base table percentage.

Dual-element maximum = 34 × 1.75 = 59.5 A → next higher standard size commonly 60 A under Exception No. 1 when 59.5 is not standard.

Nontime-delay fuse maximum = 34 × 3.00 = 102 A → next higher standard size may be 110 A if 102 is not standard — open 240.6(A) and the exception text; do not invent sizes.

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Exceptions When the Motor Will Not Start

If the values in Table 430.52 are not sufficient for the starting current of the motor, 430.52 exceptions permit higher percentages up to stated ceilings (for example, higher dual-element and inverse-time limits — read the exact exception you need). Exam skill:

1. First compute the table maximum.
2. Only apply a starting-current exception when the stem says the motor will not start or the calculated device is inadequate for starting.
3. Never jump straight to the exception ceiling on a silent stem.

Instantaneous-trip breakers have their own setting rules and often require combination motor controllers — if the stem specifies instantaneous trip, open that column and the related exceptions rather than using 250%.

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Several Motors on a Feeder — 430.62

430.62 sizes the feeder short-circuit/ground-fault device for motor loads:

• Rating not greater than the largest rating of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder plus the sum of the full-load currents of the other motors.

Practical calculation pattern when using inverse-time breakers throughout:

1. Find each motor’s Table FLC.
2. Compute the largest motor’s Table 430.52 maximum (e.g., FLC × 250%).
3. Add the FLCs of the remaining motors (100%, not 250%).
4. Select a standard feeder OCPD not exceeding that sum (or as permitted by the section).

Worked example K — feeder OCPD

Motors at 460 V, 3φ (FLCs verify in Table 430.250):

Feeder maximum ≈ 35 + 7.6 + 4.8 = 47.4 A → next standard size discussion depends on whether you are at a calculated maximum that may use standard ratings under the section — commonly candidates select 45 A if not exceeding 47.4, or evaluate next-size rules carefully. The keyed skill is the formula: largest motor’s SC device allowance + other FLCs — not 250% × (14+7.6+4.8).

Trap: 250% × 26.4 A = 66 A is a common wrong answer that applies the branch percentage to the entire feeder load.

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Relationship to Conductors and Overloads

Keep the three numbers on scratch paper for one motor:

Seeing all three prevents mixing. The breaker can be 35 A while the conductor only needs 17.5 A ampacity — that pairing is normal for motor circuits and is why motor branch conductors are often “breaker larger than wire ampacity” compared with ordinary Article 240 small-conductor intuition. Motor circuits follow Article 430, which modifies the general rules.

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Timed Open-Book Tips for §13.4

• Underline the device type: dual-element vs inverse-time vs nontime-delay vs instantaneous.
• FLC from the table → multiply by the Table 430.52 percentage → compare to 240.6(A).
• Use Exception No. 1 only when the product is not a standard size (or as the exception states).
• Feeder motors: one motor gets the big percentage; others add at FLC.
• If the stem says “overload heater,” you opened the wrong Part — return to §13.3.