About WireRef
WireRef is an independent NEC electrical reference. Every ampacity value shows the full derating chain that most calculators skip: base ampacity temperature correction bundling adjustment 110.14(C) termination limit.
The site exists because the official NEC tables are behind a paywall and most online tools give you a single number without showing how they got it. WireRef shows every step so you can verify the math against the codebook.
How we calculate ampacity
Every ampacity value follows a 4-step chain derived from NFPA 70 (the National Electrical Code). Each step cites the specific NEC section. This is the process shown on every wire gauge page:
Step 1
Base ampacity from NEC Table 310.16 at the insulation temperature column. THHN at 90°C gives the highest starting value. This is what most simplified charts show - but it’s not the final answer.
Step 2
Temperature correction per Table 310.15(B)(1)(1). When ambient exceeds 30°C, the base ampacity is multiplied by a factor less than 1.0. The correction uses the insulation temperature rating, not the termination rating.
Step 3
Bundling adjustment per Table 310.15(C)(1). More than 3 current-carrying conductors in a raceway reduce ampacity (80% for 4-6, 70% for 7-9, 50% for 10-20). This is multiplicative with Step 2.
Step 4
Termination limit per 110.14(C). This is the step most calculators skip. For circuits ≤100A, the final ampacity can’t exceed the 75°C column value regardless of insulation rating. This “usable ampacity” is what actually matters for your installation.
The
ampacity calculator implements all 4 steps interactively. Every
ampacity page shows the derating matrix: a grid of all temperature × bundling combinations with the Step 4 cap applied.
Data sources
| Data | Source | NEC Section |
|---|
| Base ampacity | NFPA 70-2023 | Table 310.16 |
| Temperature correction | NFPA 70-2023 | Table 310.15(B)(1)(1) |
| Bundling adjustment | NFPA 70-2023 | Table 310.15(C)(1) |
| Termination limits | NFPA 70-2023 | Section 110.14(C) |
| Small conductor OCP | NFPA 70-2023 | Section 240.4(D) |
| Conductor resistance | NFPA 70-2023 | Chapter 9, Tables 8 & 9 |
| GFCI locations | NFPA 70-2023 | Article 210.8 |
| AFCI requirements | NFPA 70-2023 | Article 210.12 |
NFPA provides free read-only access to the current NEC at link.nfpa.org. We encourage you to verify any value against the official text.
How errors are prevented
Values are computed programmatically from NEC table data - not transcribed by hand. The calculation engine applies the same rule chain consistently across all gauges, materials, and insulation types. This eliminates the transcription errors common in hand-built ampacity charts.
The site is reviewed against the official NEC text when new editions are adopted. See NEC 2026 changes for the latest code cycle updates.
Independence and disclosure
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Not affiliated with NFPA, any wire manufacturer, or any electrical contractor. No sponsored content.
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