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NEC Table 310.16 — temperature correction, bundling, terminal limits, NM-B rule, continuous loads
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Wire, clamps, and verification tools
After you size conductors, verify loads in the field and strip terminations cleanly.
Klein Tools CL800 Digital AC/DC Clamp Meter
- Measure running current without breaking the circuit
- AC/DC and True RMS for nonlinear loads
- CAT III 600 V — panel and feeder friendly
Fluke 117 Electricians True RMS Multimeter
- Voltage and continuity checks at devices
- Non-contact voltage detection
- Pair with ampacity results before energizing
Southwire 10 AWG Stranded THHN (500 ft)
- Common 30 A branch sizing reference
- THHN/THWN-2 — aligns with 90°C start column math
- Stranded for pulls in conduit
Gardner Bender GESP-70 Wire Stripper
- 10–20 AWG stripping slots
- ArmorEdge blades for clean strips
- Built-in gauge reference on the handle
Klein Tools MM400 Multimeter
- Manual-ranging DMM for circuits and devices
- Temp probe useful for ambient checks
- Budget-friendly second meter in the bag
As an Amazon Associate, TestTalkHQ earns from qualifying purchases. Prices and availability subject to change.
Rules this calculator applies
NM-B / Romex: NEC 334.80 limits final ampacity to the 60°C column even though individual conductors may be 90°C rated. The tool still starts derating from the correct base column per insulation selection, then caps at 60°C for NM-B.
Terminals: NEC 110.14(C) caps the usable ampacity at the terminal temperature column unless equipment is listed and marked for a higher temperature.
Both factors: Ambient correction (Table 310.15(B)(1) style factors embedded as live) and bundling (Table 310.15(C)(1) style) multiply together before the terminal/NM-B cap.
Continuous loads: NEC 210.19(A) — when continuous is selected, required ampacity is 125% of load for sizing in find mode; in check mode the “max continuous load” line divides final ampacity by 1.25.
CCC count Equipment grounding conductors do not count. Neutrals on 2-wire circuits count; MWBC neutrals that only carry unbalanced current typically do not — when in doubt, ask your AHJ.
Step-by-step sizing (overview)
- Step 1: Material and insulation type.
- Step 2: Base ampacity from the column the live tool uses for math (90°C for THHN/NM-B start, 75°C for THW, 60°C for TW).
- Step 3: Multiply by ambient correction.
- Step 4: Multiply by bundling factor.
- Step 5: Cap at terminal column (and NM-B 60°C overall cap).
- Step 6: Apply 125% when continuous is selected.
Frequently asked questions
What size wire for a 30 A circuit?
Non-continuous 30 A with 75°C terminals often lands on 10 AWG copper at 35 A base in the 75°C column. Continuous 30 A needs 37.5 A minimum after derating chain — the calculator’s Mode B walks sizes until the final derated ampacity meets that requirement. NM-B uses the 60°C column cap per 334.80.
Why does NM-B use 60°C for the final answer?
NEC 334.80 requires NM and NMC ampacity not exceed the 60°C column. You may still start derating math from 90°C for THWN-2 conductors inside cable, but the cap is enforced in this tool the same way as live.
Do equipment grounding conductors count for bundling?
No. EGCs are not current-carrying for 310.15(C)(1) adjustment.