SIS Wire Ampacity Chart
Ampacity for SIS switchboard wire by AWG size, per NEC Table 310.16, with the rule for which temperature column applies. SIS is the 600V, 90°C XLPE-insulated wire used inside switchboards, panelboards and switchgear.
SIS Ampacity by Size (NEC Table 310.16, Copper, 30°C Ambient)
| Size | 60°C Column | 75°C Column | 90°C Column (SIS) | Max. Overcurrent Protection* |
|---|---|---|---|---|
| 18 AWG | — | — | 14 A | 7 A |
| 16 AWG | — | — | 18 A | 10 A |
| 14 AWG | 15 A | 20 A | 25 A | 15 A |
| 12 AWG | 20 A | 25 A | 30 A | 20 A |
| 10 AWG | 30 A | 35 A | 40 A | 30 A |
| 8 AWG | 40 A | 50 A | 55 A | — |
| 6 AWG | 55 A | 65 A | 75 A | — |
| 4 AWG | 70 A | 85 A | 95 A | — |
| 2 AWG | 95 A | 115 A | 130 A | — |
| 1 AWG | 110 A | 130 A | 145 A | — |
| 1/0 AWG | 125 A | 150 A | 170 A | — |
| 2/0 AWG | 145 A | 175 A | 195 A | — |
| 3/0 AWG | 165 A | 200 A | 225 A | — |
| 4/0 AWG | 195 A | 230 A | 260 A | — |
Ampacities per NFPA 70 (NEC) Table 310.16 for insulated copper conductors, not more than three current-carrying conductors, at 30°C (86°F) ambient. SIS is listed in the 90°C column. *NEC 240.4(D) limits overcurrent protection on small copper conductors regardless of the ampacity column: 7 A for 18 AWG and 10 A for 16 AWG (under the conditions in that section), 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG. Wiring inside a listed switchboard or switchgear assembly is sized by its manufacturer to the equipment standard (UL 891 for switchboards); this table is the NEC reference for checking a conductor size. Confirm against the NEC edition your jurisdiction enforces. This chart is for reference only; consult a licensed electrical engineer for your equipment.
Which Column to Use
SIS insulation is rated 90°C, but the connection decides how much current the circuit may carry. NEC 110.14(C)(1) sets the rule:
- Circuits rated 100 A or less, or terminals marked for 14 AWG through 1 AWG: use the 60°C column, unless the equipment is listed and marked for 75°C conductors.
- Circuits rated over 100 A, or terminals marked for conductors larger than 1 AWG: use the 75°C column.
- The 90°C column is the starting point for derating. The derated result is then compared with the 60°C or 75°C figure, and the lower number governs.
Derating for Bundles and Heat
This is where the 90°C rating earns its keep. Switchboard wiring is bundled and laced, and the air inside the gear runs warm.
| Current-Carrying Conductors Together | Multiply By | Ambient Temperature | Multiply 90°C Value By |
|---|---|---|---|
| 4 to 6 | 0.80 | 35°C (95°F) | 0.96 |
| 7 to 9 | 0.70 | 40°C (104°F) | 0.91 |
| 10 to 20 | 0.50 | 45°C (113°F) | 0.87 |
| 50°C (122°F) | 0.82 |
Adjustment factors per NEC 310.15(C)(1) and ambient correction factors per NEC Table 310.15(B)(1) for a 90°C conductor.
Example: eight 12 AWG SIS conductors carrying current in one bundle, in 40°C air. Start at 30 A (90°C column), multiply by 0.70 for the bundle and 0.91 for the heat: 19 A. A 60°C wire in the same bundle would start at 20 A and end up far lower.
SIS vs MTW vs THHN
| SIS | MTW | THHN | |
|---|---|---|---|
| Insulation | XLPE (thermoset) | PVC (thermoplastic) | PVC / nylon (thermoplastic) |
| Temperature rating | 90°C dry | 60°C to 90°C | 90°C dry |
| Typical use | Switchboards, switchgear | Machine tool panels (NFPA 79) | Building branch and feeder in conduit |
| Flexibility | Good (stranded, often tinned) | Very good (fine strand options) | Fair (coarser strand) |
Thermoplastic insulation softens and melts when it overheats; thermoset XLPE chars in place and keeps its shape. That is why switchboard standards call for SIS-type wire. For machine panels, see the MTW wire ampacity chart, which uses UL 508A and NFPA 79 values.
What Is SIS Wire?
SIS is a 600V single-conductor wire with cross-linked polyethylene (XLPE) insulation, listed under UL 44 and rated 90°C in dry locations with a VW-1 flame rating. The name comes from its original designation: switchboard wire, insulated, stranded. The conductor is stranded copper, commonly tinned. Our SIS wire guide covers where the NEC and UL 891 call for it, and the SIS product page lists what we carry.
Frequently Asked Questions
What is the ampacity of 12 AWG SIS wire?
30 amps in the 90°C column of NEC Table 310.16, 25 amps at 75°C and 20 amps at 60°C. NEC 240.4(D) also limits overcurrent protection on 12 AWG copper to 20 amps, so 20 amps is the working figure for most circuits.
What is the ampacity of 10 AWG SIS wire?
40 amps in the 90°C column, 35 amps at 75°C and 30 amps at 60°C, with overcurrent protection limited to 30 amps by NEC 240.4(D).
Can I use the 90°C column for SIS wire?
Use it as the starting point for derating, not as the final answer. SIS insulation is rated 90°C, but NEC 110.14(C) holds the circuit to the temperature rating of its terminations, which is 60°C or 75°C on most devices. The 90°C rating is what lets a bundled or hot conductor still carry its load after the derating factors are applied.
Is SIS wire the same as MTW or THHN?
No. SIS has thermoset XLPE insulation that chars in place rather than melting, and it belongs in switchboards and switchgear. MTW has thermoplastic PVC insulation and belongs in industrial machine panels. THHN is PVC with a nylon jacket and belongs in the building's raceways. See the SIS wire guide.
Does SIS wire have to be tinned?
No, but tinned copper is the common choice for switchboard work because it resists corrosion and solders cleanly. Tell us tinned or bare when you order.