Texas Electrician ExamTexas journeyman & master

Derating is two multiplications, and picking the wrong correction table is the classic error

There are two ambient temperature correction tables in Article 310. They look nearly identical. Using the wrong one gives you a wrong answer that looks completely reasonable.

On this page
  1. The shape of the calculation
  2. The trap that costs the most points
  3. The correction factors are not arbitrary
  4. Rooftops add thirty three degrees
  5. The conductor count adjustment
  6. Counting conductors is the actual question
  7. A worked example
  8. Do not forget the small conductor rule
  9. What this page cites

The shape of the calculation

Ambient temperature and conductor bundling are two separate corrections and they multiply Start from the ampacity in the table for the insulation rating. Multiply by the ambient temperature correction factor if the ambient is not thirty degrees Celsius. Multiply again by the adjustment factor if more than three current carrying conductors are bundled together. Then compare the result to the terminal rating. People commonly apply one of the two corrections and stop. tableampacity × ambientcorrection × bundlingadjustment = correctedampacity from the column for the insulation rating only if the ambient is not 30°C only if more than three are bundled now check this against the terminal rating both corrections apply if both conditions are true they are not either-or, and the exam writes questions where both bite
Two knobs, applied one after the other. Skipping the second one is a very common miss. Drawn against NEC 310.15(B) and 310.15(C)(1). Factors are in the tables in your book.

You start with a table ampacity. Then you multiply by an ambient temperature correction factor, and then by an adjustment factor for the number of current-carrying conductors. Two multiplications, in that order.

The order does not change the arithmetic, but the code states the sequence and questions sometimes hand you an already-corrected number and ask for the next step. Knowing which step you are on matters.

The trap that costs the most points

Article 310 contains two ambient temperature correction tables. One is based on thirty degrees Celsius. The other is based on forty. They sit near each other, they have similar titles, and they contain different numbers for the same ambient.

Each one belongs to a specific set of ampacity tables. The note printed beneath each table tells you which. The everyday ampacity table, the one for conductors in a raceway or cable or directly buried, pairs with the thirty degree correction table. The free-air and high-temperature tables pair with the forty degree one.

Read the note under the ampacity table you are using. It names the correction table by number. Then read the note under the correction table, which names the ampacity tables it serves. They point at each other, and following that pointer is the whole defense against this error.

The reason it is so easy to get wrong is that both answers are plausible. Neither produces an absurd number. You get a wrong conductor size and no signal that anything happened.

The correction factors are not arbitrary

The code publishes the equation the tables are built from, in the ambient temperature correction section. It relates the conductor's temperature rating, the new ambient, and the ambient the table was computed at.

You do not need to use the equation. The tables have already run it. But knowing it exists tells you something useful: the correction depends on the conductor's temperature rating, not on its size. A ninety degree conductor and a sixty degree conductor at the same ambient get different factors, and the ninety degree one is always the gentler correction.

That is why starting from the ninety degree column is standard practice even when the terminations limit you to seventy five. You derate from the higher number, then check the result against the termination limit.

Rooftops add thirty three degrees

There is a provision most candidates have not read. Where a raceway or cable is exposed to direct sunlight on or above a rooftop and it sits closer than three quarters of an inch to the roof surface, a temperature adder is applied to the outdoor ambient before you look up the correction factor.

The adder is substantial. It is applied first, then the correction table is consulted at the higher figure.

One insulation type is exempt from the adder by exception. Find that exception in your code book. It is the kind of detail that appears on an exam precisely because it is easy to miss.

The conductor count adjustment

The second multiplication applies when more than three current-carrying conductors run together in a raceway or cable without maintaining spacing, for more than twenty four inches.

The adjustment table gives a percentage by conductor count, in bands. The bands step down as the count rises.

The hard part is not the table. It is the count.

Counting conductors is the actual question

Most adjustment questions are really counting questions with a lookup attached. The rules are specific and they are scattered across several subsections.

That third item is where the nonlinear load qualifier lives, and it is the one people miss. A neutral that would normally be excluded becomes a current-carrying conductor when the load is mostly nonlinear, because harmonic currents do not cancel.

Work the count before you touch the table. A question that gives you a raceway full of conductors and one line about the load type is testing the count, not the multiplication.

A worked example

Twelve current-carrying conductors of the same size and insulation share a raceway. Ambient is forty five degrees Celsius. The conductors are rated ninety degrees.

  1. Look up the ninety degree ampacity for that conductor size in the ampacity table for raceways.
  2. Read the note under that table. It sends you to the thirty degree correction table.
  3. Find the row for a forty five degree ambient and the ninety degree column. That is your first multiplier.
  4. Count the conductors. Twelve falls in one of the adjustment bands. Read the percentage.
  5. Multiply the table ampacity by the correction factor, then by the adjustment percentage.
  6. Compare the result against the termination temperature limit and the overcurrent protection rules for small conductors.

The answer will be substantially below the table value. That is the point of the exercise. Twelve conductors in a hot raceway is a hostile environment and the code says so with two multiplications.

Do not forget the small conductor rule

After all the derating, the overcurrent protection article imposes a separate limit on the smallest conductor sizes regardless of what the ampacity calculation produced.

A derated ampacity higher than that limit does not raise the limit. The limit governs. Questions that walk you through a full derating and then ask for the overcurrent device size are usually testing whether you remember this last step.

What this page cites

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