Wire Size Calculator

Table 310.16 is the table you will open most on the exam. It gives the ampacity of insulated conductors with no more than three current-carrying conductors in a raceway or cable, at a 30°C (86°F) ambient.

Where to look
Article 310, section 310.16, Table 310.16. The left half is copper; the right half is aluminum or copper-clad aluminum. Each half has three columns: 60°C, 75°C and 90°C. The insulation types belonging to each column are printed at the top of the column. If an insulation has different ratings for wet and dry locations (for example XHHW or THWN-2), Table 310.4(1) gives the rating for each.

Procedure

1. Copper unless the question says aluminum.
2. Find the insulation type's temperature column.
3. Go down to the conductor size row and read the ampacity.
4. Reverse problem (size for a load): go down the right column until the first ampacity that is at least the load.

Worked example
What is the ampacity of 1/0 AWG THWN copper?

• THWN appears at the top of the copper 75°C column.
• Row 1/0 AWG, 75°C column: 150 A.

Most common mistake
Reading the wrong column. THHN is a 90°C insulation, THWN is 75°C, and TW is 60°C. Reading the aluminum half by accident is the second classic mistake. Read the column header every time. (Whether the final ampacity may use that column also depends on the terminations, which is the next skill.)

Table 310.16 assumes the air around the conductors is 30°C (86°F). Hotter air means the conductor cannot shed heat as well, so its ampacity must be reduced. Cooler air allows a little more.

Where to look
Article 310, 310.15(B), Table 310.15(B)(1)(1), "based on 30°C". It is the one to use with Table 310.16. Rows are ambient temperature ranges, in °C on the left and °F on the right. The columns are the conductor's temperature rating: 60°C, 75°C, 90°C. (The second table, based on 40°C, goes with Tables 310.18 to 310.21 and is not used with 310.16.)

Procedure

1. Get the ampacity from Table 310.16 in the column for the insulation.
2. In Table 310.15(B)(1)(1), find the row for the ambient. Use the °F scale if the ambient is given in °F; do not convert.
3. Read the factor in the SAME temperature column you used in step 1.
4. Multiply: corrected ampacity = table ampacity × factor.

Worked example
4 AWG THHN copper in a 42°C room.

• Table 310.16, 90°C column: 95 A.
• Table 310.15(B)(1)(1), row 41–45°C, 90°C column: 0.87.
• 95 × 0.87 = 82.7 A.

Most common mistake
Mixing columns, for example taking the 90°C ampacity and multiplying it by the 75°C factor. The factor column must match the ampacity column. Dividing by the factor instead of multiplying is the other common slip; a factor below 1 must make the ampacity smaller.

Table 310.16 assumes no more than three current-carrying conductors (CCCs) together. When more share a raceway or cable for more than 24 in., each one heats the others, so every conductor's ampacity is reduced by a percentage.

Where to look
Article 310, 310.15(C)(1), Table 310.15(C)(1). The left column is the number of conductors (in bands: 4–6, 7–9, 10–20 and so on); the right column is the percentage of the Table 310.16 value. Read the footnote: spares are counted, and the count follows 310.15(E) and (F) (the previous skill).

Procedure

1. Count the CCCs correctly (neutrals only when they count, never EGCs, include spares).
2. Find the band for that count in Table 310.15(C)(1) and turn the percentage into a decimal (80 percent = 0.80).
3. Get the ampacity from Table 310.16 for the insulation column.
4. Multiply: adjusted ampacity = table ampacity × decimal.
5. To size a conductor: divide the required ampacity by the decimal, then find the first table value at least that large.

Worked example
Six current-carrying 10 AWG THHN copper conductors in one conduit.

• 6 falls in the 4–6 band; Table 310.15(C)(1) gives 80 percent.
• Table 310.16, 10 AWG copper, 90°C: 40 A.
• 40 × 0.80 = 32 A per conductor.

Most common mistake
Landing in the wrong band. Four conductors is already "more than three", so 4 gets the first reduction; do not skip it. Also, the percentage multiplies the ampacity; it is never subtracted from it.

Real questions combine a hot room, a full conduit, and terminals that limit the column. 310.15(A) allows the correction and adjustment factors to be applied to the ampacity for the conductor's own temperature rating, as long as the result does not exceed the termination ampacity under 110.14(C). That is why 90°C wire is useful even on 75°C or 60°C terminals.

Procedure

1. Start with the Table 310.16 ampacity in the INSULATION column (90°C for THHN).
2. Multiply by the ambient factor from Table 310.15(B)(1)(1), same column.
3. Multiply by the adjustment percentage from Table 310.15(C)(1).
4. Separately, look up the TERMINATION-column ampacity (110.14(C): 60°C for 100 A or less unless marked; 75°C over 100 A).
5. The allowable ampacity is the SMALLER of step 3 and step 4.

Worked example
Six current-carrying 8 AWG THHN copper conductors, 40 A circuits, ambient 38°C, unmarked terminals.

• Table 310.16, 8 AWG copper, 90°C: 55 A.
• Ambient row 36–40°C, 90°C column: 0.91.
• 6 conductors → 80 percent.
• 55 × 0.91 × 0.80 = 40.04 A.
• Terminals: 40 A circuit, unmarked → 60°C → 8 AWG = 40 A.
• Allowable = smaller of 40.04 A and 40 A = 40 A.

Most common mistake
Starting the derating from the termination column (for example 75°C) instead of the insulation column, which throws away the extra capacity of 90°C wire. The opposite error, stopping at the derated 90°C value without checking the terminal limit, is just as common.