MC Cable Ampacity Rating | Know What Your Cable Can Carry

MC cable ampacity comes from the insulation rating of the conductors inside, not the metal armor.

You can’t look up “the” ampacity for MC cable because no single universal number exists. The actual MC cable ampacity rating for a specific run depends on the temperature rating of the insulated conductors inside, the termination limits at each end, and derating factors that apply when you bundle cables or run more than three current-carrying conductors together. Getting this wrong is what causes undersized circuits and failed inspections.

How MC Cable Ampacity Is Determined

NEC Article 330 governs Type MC cable, and it points you straight to the standard conductor tables. Per NEC 330.80, you determine ampacity from Table 310.15(B)(16) or 310.17 depending on the conductor type inside the assembly. The armor and jacket do not carry current and do not set the rating.

What you actually need to know is the conductor insulation type—typically THHN/THWN-2 in modern MC cable—and its temperature rating. A product sheet for Encore Wire’s copper MC cable with THHN conductors, for example, lists ampacities directly from NFPA 70 Table 310.15(B)(16).

Here is a typical example of what manufacturer tables show for copper conductors at standard ambient temperatures:

These numbers come straight from published MC cable product sheets and reflect what the conductors can handle based on their insulation rating alone. Always confirm the specific values against the manufacturer’s data sheet for the exact cable you’re buying, since construction differences like steel versus aluminum armor or a PVC jacket can change the listing.

The Termination Limit You Cannot Ignore

Your cable might be rated for 90°C, but the equipment you connect it to likely isn’t. That is the trap electricians fall into when they read the highest number on the table.

NEC 110.14(C) limits conductor ampacity based on the termination temperature rating of the equipment. For circuits rated 100 amps or less, or those marked for 14 AWG through 1 AWG conductors, the rules require you to use the 60°C column when equipment is marked with a 60°C termination rating. Most standard residential breakers and panels fall into this category, which is why 90°C-rated cable frequently gets installed at 75°C or even 60°C ampacity values for the terminations.

The 90°C rating is not wasted—it helps with derating—but the termination column sets the practical ceiling for most standard installations.

When Derating Factors Apply

You must reduce ampacity when conditions stack conductors together. NEC 310.15(C)(1) requires adjustment factors when more than three current-carrying conductors are in the cable assembly or when cables are installed without maintained spacing, such as bundles. A guidance source for MC cable confirms these rules apply to the assembly itself.

The neutral counts here. One manufacturer sheet notes that where the neutral is current-carrying in a 4-conductor cable, ampacity drops by a factor of 0.80. That means a cable that could handle 70A at 75°C is limited to 56A when the neutral carries the unbalanced load in that configuration. Multiconductor sheets point to the same adjustment factor notes, so read the fine print on any cable with a neutral.

Common Mistakes to Avoid

Several errors show up repeatedly when people size MC cable. The most frequent is treating MC cable as if it has one fixed rating per wire size. The next is grabbing the 90°C column without ever checking the termination limits under NEC 110.14(C). Third is skipping derating when you have more than three current-carrying conductors or tightly bundled cables. Finally, do not confuse the armor or jacket construction with ampacity—the conductor insulation is what controls the rating.

When you are ready to buy, our tested 10-2 MC cable recommendations cover the configurations that work for typical branch circuits.

FAQs

Does MC cable have a higher ampacity than NM cable?

The real difference is physical protection, not current capacity. Always check the specific cable listing and the termination temperature ratings before assuming any advantage.

Can I use the 90°C ampacity for a 100-amp breaker?

Usually not. NEC 110.14(C) requires you to use the termination rating of the equipment. Most standard breakers and panels are rated at 60°C or 75°C for termination, so the 90°C conductor rating only helps after derating is applied. The termination limit sets the final usable number in nearly every standard residential installation.

What size MC cable do I need for a 50-amp circuit?

Verify your specific cable’s rating because construction differences matter. You need no derating applied for a typical three-conductor run with a ground, which keeps the installation straightforward.