The answer depends on marketing, not engineering: if your product is marketed for use in a residential environment, FCC Class B limits apply; if it is marketed exclusively for commercial, industrial or business environments, Class A is sufficient. Class B is the stricter tier - roughly 10 dB tighter once measurement distances are normalized - because homes put digital devices within meters of radio and TV receivers. The limits themselves live in 47 CFR Sections 15.107 (conducted) and 15.109 (radiated).

How does the FCC define Class A and Class B?

Section 15.3 of the FCC rules defines the classes by intended market, not by technical character. A Class A digital device is marketed for use in a commercial, industrial or business environment, excluding devices marketed to the general public or intended for home use. A Class B digital device is marketed for use in a residential environment, notwithstanding use in commercial environments - the rule explicitly lists personal computers, calculators and similar consumer devices. The logic: manufacturers do not control where a consumer product ends up, so anything a household might buy must meet the residential limits. When in doubt, the FCC expects Class B.

How do the limits compare?

The radiated field-strength limits of Section 15.109, measured with a quasi-peak detector below 1 GHz per ANSI C63.4:

The numbers look deceptively close, but Class B is measured at 3 m and Class A at 10 m; free-space field strength falls inversely with distance, so at equal distance Class B is significantly tighter across the band. Conducted limits in Section 15.107 follow the same pattern: Class B allows 66-56 dBuV quasi-peak across 150-500 kHz and 60 dBuV at 5-30 MHz, while Class A permits a more relaxed 79/73 dBuV quasi-peak profile. Both are measured through a standard 50 ohm / 50 uH LISN - see what is a LISN for why. Full context on the regulation, including SDoC and certification routes, is on our FCC Part 15 page.

What labeling and user information is required?

Each class carries a mandated statement in the user documentation per Section 15.105. The Class B version states the equipment has been tested and found to comply with the limits for a Class B digital device, that it generates and can radiate radio frequency energy, and lists remedies (reorient antenna, increase separation, different circuit, consult a technician). The Class A version instead warns that operation in a residential area is likely to cause harmful interference, in which case the user will be required to correct the interference at their own expense. In addition, Section 15.19 requires the general Part 15 compliance statement (“This device complies with Part 15 of the FCC Rules…”), and devices authorized under certification must display their FCC ID.

What are the common mistakes?

• Choosing Class A to pass, not because the market is commercial. If the product is sold through consumer channels, Class A is not available regardless of test margins; misclassification risks enforcement and market withdrawal.
• Comparing Class A and B numbers without distance correction. A device with 3 dB margin to Class A at 10 m does not have margin to Class B at 3 m.
• Forgetting the conducted side. Products pass radiated testing and fail Section 15.107 conducted limits surprisingly often, especially switching power supplies at 150-500 kHz where the Class B limit is tightest.
• Copying the wrong Section 15.105 statement. Class A text on a consumer product (or vice versa) is a documentation nonconformity found in routine audits.
• Assuming FCC class equals CISPR class. CISPR 32 also uses Class A/B labels with similar intent, but limits, distances and port requirements differ; a combined test plan must check both, a point covered in CISPR 22 vs CISPR 32.

How should you decide in practice?

Write down where the product will be marketed and sold, not where you hope it will be used. Consumer channel or dual-use: test to Class B. Exclusively industrial or commercial channel with documentation to prove it: Class A is legitimate and buys you roughly 10 dB of headroom. Either way the measurement procedure is ANSI C63.4, described along with the committee behind it on our ANSI C63 page, and pre-compliance scanning per EMC testing basics before the accredited test remains the cheapest insurance available.