4.4 Putting It Together: Full Conductor-Sizing Problems

The Complete Sequence

Every conductor-sizing question, no matter how it is dressed up, follows the same six steps:

1. LOAD          Determine the load in amperes
2. CONTINUOUS    Multiply continuous portion by 125% (210.19(A)(1), 215.2(A)(1))
3. SELECT        Pick a conductor size
4. DERATE        Apply correction (310.15(B)(1)) x adjustment (310.15(C)(1))
5. TERMINATE     Cap at the 110.14(C) column ampacity; take the LOWER value
6. PROTECT       Size the OCPD per 240.4, including 240.4(B) and 240.4(D)

Steps 5 and 6 are where marks are lost. Step 5 produces the conductor's allowable ampacity. Step 6 produces the overcurrent device rating. These are different numbers answering different questions.

The Continuous-Load Multiplier

210.19(A)(1) for branch circuits and 215.2(A)(1) for feeders both require the conductor to have an allowable ampacity of not less than the noncontinuous load plus 125 percent of the continuous load. 210.20(A) and 215.3 impose the same requirement on the overcurrent device.

A continuous load (Article 100) is one where the maximum current is expected to continue for three hours or more. Store lighting, most commercial HVAC, and sign circuits are continuous. A dwelling range is not.

Worked Problem 1 — Commercial Lighting Feeder

A feeder supplies 64 A of continuous lighting load and 22 A of noncontinuous receptacle load. THWN-2 copper in EMT, three current-carrying conductors, 30 degrees C ambient. Equipment terminations are marked 75 degrees C. Size the conductor and the overcurrent device.

Step 1–2, required ampacity:

(64 x 1.25) + 22 = 80 + 22 = 102 A

Step 3–4, select and derate: three CCC at 30 degrees C means no correction and no adjustment.

Step 5, termination cap: 75 degrees C column, copper. 2 AWG = 115 A; 3 AWG = 100 A. 100 A is less than 102 A, so 3 AWG fails. 2 AWG copper at 115 A is the answer.

Step 6, overcurrent device: 210.20(A)/215.3 require the device to be at least 102 A. The next standard size from 240.6(A) is 110 A. Check 240.4: the conductor ampacity is 115 A, so a 110 A device protects it. 2 AWG copper on a 110 A device.

Worked Problem 2 — Derating Governs

Six current-carrying 4 AWG THHN copper conductors in one raceway, ambient 40 degrees C, terminations 75 degrees C. What is the allowable ampacity and the largest standard overcurrent device?

Base, 90 C column, 4 AWG Cu ................ 95 A
Correction, 36-40 C, 90 C column ........... x 0.91  = 86.45 A
Adjustment, 4-6 CCC ........................ x 0.80  = 69.16 A
Termination cap, 75 C column, 4 AWG Cu ..... 85 A
Final allowable ampacity ................... 69.16 A  (the lower)

Overcurrent device: 69.16 A does not correspond to a standard size. Under 240.4(B) the next higher standard device is permitted if the device is 800 A or less, the conductors do not supply multioutlet branch circuits for cord-and-plug-connected portable loads, and the ampacity does not correspond to a standard rating. All three conditions are met, so 70 A is permitted.

Worked Problem 3 — Small Conductors and 240.4(D)

A 12 AWG copper THHN conductor, six current-carrying in a raceway, 30 degrees C. What is the maximum overcurrent device?

Base, 90 C column, 12 AWG Cu ............... 30 A
Adjustment, 4-6 CCC ........................ x 0.80  = 24 A
Termination cap, 75 C column ............... 25 A
Allowable ampacity ......................... 24 A

Now apply 240.4(D)(5): unless specifically permitted in 240.4(E) or (G), the overcurrent protection for 12 AWG copper shall not exceed 20 A after any correction factors for ambient temperature and number of conductors have been applied. The adjusted ampacity of 24 A exceeds 20 A, so 20 A is the maximum device.

Had the adjustment produced 18 A, the device would be limited to 15 A — the next standard size below, because 240.4(D) is a hard cap and 240.4(B)'s round-up is not available for small conductors that exceed their 240.4(D) limit.

240.4(D) Small-Conductor Limits

240.4(E) lists tap conductors governed instead by 210.19(A)(3) and (A)(4), 240.5(B)(2), 240.21, 368.17(B) and (C), and 430.53(D). 240.4(G) lists specific conductor applications — motor circuits under Article 430, air-conditioning under 440, fire alarm under 760, and others — that follow their own article's protection rules rather than 240.4(B) through (D).

The Round-Up Rule — 240.4(B) and (C)

240.4(B) Overcurrent Devices Rated 800 A or Less. The next higher standard overcurrent device rating above the ampacity of the conductors being protected shall be permitted, provided all of the following are met:

1. The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads;
2. The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or circuit breaker in 240.6 without overload adjustments; and
3. The next higher standard rating does not exceed 800 A.

240.4(C) Overcurrent Devices Rated Over 800 A. Where the overcurrent device exceeds 800 A, the conductor ampacity shall be equal to or greater than the device rating. No rounding up above 800 A.

240.6(A) Standard Ampere Ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000. Additional standard ratings for fuses are 1, 3, 6, 10, and 601.

Worked Problem 4 — Over 800 Amperes

A service feeder's calculated conductor ampacity works out to 1150 A after paralleling. May a 1200 A overcurrent device be used?

No. 240.4(C) applies above 800 A: the conductor ampacity must equal or exceed the device rating. With 1150 A of conductor you may install a 1000 A device, or you must increase the conductor set until its ampacity reaches at least 1200 A.

The One-Sentence Summary

Ampacity answers how much current the conductor may carry. The overcurrent device answers how much current the circuit is allowed to draw before it opens. Below 800 A and outside the small-conductor range, the device may round up past the ampacity; for 14, 12, and 10 AWG it may not exceed the 240.4(D) values; and above 800 A the conductor must always be at least as large as the device.