Derating is two multiplications and a separate check, and Texas exam items are built on the order

How the exam asks about derating

Derating is the busiest single idea in this part of the bank. Of the 500 questions we wrote for the Texas journeyman, 68 sit in conductor sizing, and 37 of those 68 hand you an ambient correction factor or a conductor count adjustment factor. Across all 500 the figure is 42, because motor and conduit items borrow the same arithmetic once the conductor question is settled.

The form barely varies. A stem gives you a base ampacity and one or two factors, and the four options are four different things a candidate could have done with those numbers. One applies a single factor and stops, one adds the factors together, one divides where a multiplication was wanted. Reading the option list as a record of four methods is faster than reaching for a formula.

The weighting is published. The PSI content outline gives the journeyman calculations portion 24 scored items, and branch circuit calculations and conductors carries 4 of them. The knowledge portion carries 56 scored items and branch circuits and conductors carries 10. TDLR reported 1,301 passes out of 6,328 attempts on journeyman calculations in fiscal year 2025, a rate of 20.56 percent, against 24.46 percent on the knowledge portion.

Where it sits in the book

Both factors live in NEC 310.15. Ambient correction sits at 310.15(B), which carries the correction tables and the equation behind them, and the rooftop adder sits at 310.15(B)(2). The conductor count adjustment sits at 310.15(C)(1) with the conditions that switch it off. Table 310.16 is the ampacity table for conductors in a raceway, in a cable or directly buried, and the note under it names 310.15(B) as the correction source. Follow that pointer every time, because Article 310 carries two correction tables built on different base temperatures and each one yields a believable answer.

Two further sections finish the problem. 110.14(C) sets the termination limit, which is a second test run on an untouched number. 240.4(D) caps the overcurrent device on 14, 12 and 10 AWG, and its opening sentence places that cap after correction and adjustment have run.

One 2026 move matters for your tab set. 310.15(E), which decides whether the neutral joins your conductor count, was rebuilt into a positive list at (E)(1) and an exclusion at (E)(2) reaching only what the list left behind. Texas adopted the 2026 edition on 1 September 2026 under 16 TAC 73.100, and PSI referenced the examinations to the same edition from that date.

The distinction that decides it

Almost every mark lost here comes from treating the termination check as the end of the multiplication chain. It sits outside the chain. Correction and adjustment are applied to the ampacity read in the column matching the conductor's own insulation rating, and they multiply each other. The termination limit is read from a different column, chosen by the equipment under 110.14(C), with both factors kept off it. Two numbers come out and the conductor is held to the smaller one.

Order binds at the end as well. The small conductor cap runs last, which keeps it a cap on the device and leaves the conductor ampacity where the arithmetic put it. Between the two multiplications themselves there is no order at all, because multiplication commutes, and our bank offers an invented sequencing rule as one of the four options to see who bites.

Which wrong answer people pick

Each row below comes out of the trap lines attached to the bank questions.

That last row is the expensive one, because the answer it produces looks like work. One stem puts a 78 ampere continuous load in a raceway with a 0.80 adjustment. The load rises to 97.5 amperes on the continuous multiplier, and the target is then divided by 0.80, giving 122 amperes. Multiplying 97.5 by 0.80 gives 78, a conductor asked to carry less than the load puts on it.

The backward version, worked

A large share of the derating items in our bank run the calculation in reverse, and that direction is where preparation shows. The forward question gives you a conductor and asks what it delivers, while the backward one gives you the delivery and asks which conductor makes it.

One sentence keeps them apart. A factor cuts what a conductor delivers, so finding the conductor that still delivers a target means dividing the target by the factor, while a continuous load raises the target and takes a multiplication by 1.25 before the factors are touched. A bank item puts both on one stem: 40 amperes of noncontinuous load with 96 amperes of continuous load, an adjustment of 0.80 and a correction of 0.91. The load side comes to 160 amperes, the factors combine to 0.728, and 160 divided by 0.728 is 220 amperes of base ampacity. Applying the 125 percent to the whole 136 amperes yields 234 amperes, and that option sits in the list waiting for it.

A third variant asks what the factors will tolerate. A conductor with a base ampacity of 175 that has to deliver 130 amperes after a 0.87 correction stands at 152.25 amperes, so the smallest adjustment factor it survives is 130 divided by 152.25, which is 0.854. The first band above three current-carrying conductors is 0.80, which falls under that, so the run is held to three current-carrying conductors or the conductor gets larger. Reaching that answer means settling which conductors go into the count before any table is opened.

Where the adjustment stops applying

310.15(C)(1) carries four conditions that switch the conductor count adjustment off, and two of them are worth holding by their trigger. A raceway of 24 inches or less is one, since the bundle is too short to build heat. Underground conductors entering or leaving an outdoor trench are another, subject to the wiring method used for protection, a protected length of 10 feet at most and a ceiling of four conductors. The other two are narrower, and one attaches four qualifying tests to Type AC and Type MC cable, so read that pair in your own book.

The 24 inch case has a partner in Chapter 9. The note permitting a nipple of that length to be filled to 60 percent says in the same breath that the 310.15(C)(1) adjustment factors need not apply to it. Our bank hands you a 20 inch length of conduit between two enclosures carrying ten current-carrying conductors and asks which rule applies. Both do, and their conditions differ: the fill allowance wants the length together with installation between enclosures, while the adjustment relief turns on length alone.

The rooftop rule runs the other way and adds heat. Where a raceway or cable sits in direct sunlight on or above a roof and the gap from the roof surface to the bottom of the raceway falls under three quarters of an inch, 33 degrees Celsius goes onto the outdoor ambient and the correction factor is read at that higher figure. Three quarters of an inch of standoff removes the adder, which is why raceway sits on strut over flat roofs, and Type XHHW-2 is carved out by an exception printed under the subsection.

What it costs to get wrong

On the paper the cost is bounded. Each portion passes at 70 percent, the two are scored separately, and a retake runs $78 whether you lost one portion or both. With 24 scored calculation items in front of you, three derating slips is 12.5 percent of that portion gone, which is most of the margin between a pass and a rebooking.

On the job the cost arrives later. A conductor that satisfies the derating and misses the termination check fails at inspection, and the repair is a larger conductor or different equipment, since a hotter insulation letter on the same size of wire moves nothing at the lugs. A conductor that cleared the terminations and missed the adjustment goes in and stays in, running warm in a full raceway where nothing in the circuit will ever report it.

The cheapest repair on a crowded run is usually a second raceway. Our bank works that one through: twelve current-carrying conductors at a 0.50 adjustment leave 65 amperes of a 130 ampere conductor, and splitting them into two raceways of six raises the factor to 0.80 and takes the same conductor to 104 amperes. That is 60 percent more current out of wire already on the reel, against the price of a second raceway.

What to do before you sit

• Write the sequence down once and use that same order every time: table ampacity from the insulation column, ambient correction, conductor count adjustment, then the termination column read clean under 110.14(C), then the small conductor cap on the device.
• Drill the backward direction on its own. Hand yourself a load, a factor and a target, and divide. Ten of those in one sitting does more for the calculations portion than fifty forward problems.
• Check the base temperature printed at the top of the correction table before you write any factor down. It costs four seconds and it is your only defense against an error that produces a plausible number.
• Tab 310.15, Table 310.16 and 240.4 in your own softbound copy. Publisher tabs only, since homemade ones are turned away at the door, and all your marking goes in before the session starts.

One conflict is worth carrying with you. The PSI bulletin printed in capitals that the examinations are referenced to the 2026 NEC from 1 September 2026, and as read on 24 August 2026 the reference material list inside that same bulletin still named the 2023 edition, as did TDLR's own examination page. The dated line is the one tied to your appointment, so the edition question is worth settling with PSI directly when your date sits near the line. The worked arithmetic for every step above lives in the question set.

Questions people ask

How many Texas journeyman questions involve derating?

In our bank of 500, 68 questions sit in conductor sizing and 37 of those put an ambient correction factor or a conductor count adjustment factor in front of you. A handful outside that area borrow the same arithmetic for motor and conduit problems, taking the figure to 42 across the full bank. On the published PSI content outline, branch circuit calculations and conductors carries 4 of the 24 scored items on the journeyman calculations portion, and branch circuits and conductors carries 10 of the 56 scored knowledge items. The material earns study time on both halves of the paper.

Do the ambient and bundling factors both apply at once?

Yes, whenever both conditions hold, and they multiply each other. A bank item gives a base ampacity of 100 amperes in the 90 degree column, an ambient correction of 0.87 and an adjustment of 0.70, and the answer is 61 amperes, since 100 times 0.87 is 87 and 87 times 0.70 is 60.9. Both 87 and 70 appear in the option list, one for a candidate who applied the ambient correction alone and one for a candidate who applied the count adjustment alone. A fourth option of 157 comes from adding the two factors, which lands above the base ampacity.

Which column do I start in when the lugs are rated lower?

Start in the column matching the conductor's own insulation rating. 110.14(C) permits a conductor with a higher temperature rating to serve as the starting point for correction, for adjustment or for both, and that is the whole practical value of a 90 degree insulation. The derated result is then measured against the column the equipment sets, with no factors applied to that column. One bank stem describes a candidate who applies both factors to a 75 degree figure of 115 amperes and reports 75. The factors belong on the 90 degree value, and the 115 amperes is a ceiling checked at the end.

What happens when a correction factor is above 1.00?

You apply it exactly like any other factor. Correction factors rise above 1.00 below the temperature the ampacity table was built on, because cooler surroundings carry heat away faster and the conductor can carry more. A bank item gives a base ampacity of 125 amperes, a correction of 1.04 for a cold space and an adjustment of 0.80 for six current-carrying conductors, and the answer is 104 amperes. Two distractors record how candidates mishandle it: one drops the 1.04 because a factor above one looks like a misprint, and one reads it as a four percent cut and uses 0.96.

When does the conductor count adjustment stop applying?

310.15(C)(1) names four conditions. A raceway of 24 inches or less takes no adjustment, and underground conductors entering or leaving an outdoor trench take none where the physical protection is rigid metal conduit, intermediate metal conduit, PVC or RTRC, the protected length is 10 feet at most and there are four conductors or fewer. The remaining two are narrower, and one attaches four separate qualifying tests to Type AC and Type MC cable. Chapter 9 carries the partner to the 24 inch case: a nipple of that length may be filled to 60 percent, and the adjustment factors need not apply to it.

Does derating decide the overcurrent device as well?

It decides the number the device is chosen against, since the device protects the conductor as installed. Three rules then act on that number. 240.4(B) permits rounding up to the next standard rating where all three of its conditions hold, among them a ceiling of 800 amperes. 240.4(C) reverses the permission above 800 amperes, so the conductor ampacity has to meet or beat the device. 240.4(D) caps the device on 14, 12 and 10 AWG after correction and adjustment have run, and a derated figure above that cap leaves the cap exactly where it stands.