[Home](/)/[Resources](/resources)/FCC Part 15 Subpart B USFCC # FCC Part 15 Subpart B: EMC Emission Rules for Unintentional Radiators Explained Every digital device sold in the United States must comply with FCC Part 15 Subpart B — whether or not it has a radio. Getting Class A vs Class B classification wrong, or relying on an EU EMC test report that the FCC does not accept, are two of the most common and avoidable compliance failures hardware companies make on their first US market entry. Copy Link[Share on WhatsApp](https://wa.me/?text=https%3A%2F%2Fkrono-labs.com%2Fguides%2Ffcc-part-15-subpart-b-emissions) At a glance Regulation 47 CFR Part 15, Subpart B Classes A (commercial) and B (residential) Route SDoC or Certification Measurement ANSI C63.4 / C63.10 ## Understanding Part 15 Subpart B ### What Subpart B covers — digital devices as unintentional radiators FCC Part 15 Subpart B applies to any digital device — defined as any unintentional radiator that generates and uses timing signals or pulses at a rate above 9 kHz and uses digital techniques. This encompasses virtually every electronic product with a microcontroller, processor, or digital interface: computers, monitors, switching power supplies, IoT sensors, industrial controllers, and embedded systems. The regulation applies regardless of whether the device also contains an intentional radio transmitter. ### Class A vs Class B emission limits — residential use triggers stricter requirements Class B limits (§15.109(b) for radiated, §15.107(b) for conducted) are stricter than Class A (§15.109(a), §15.107(a)) and apply to devices marketed for use in residential environments. A device that will be sold at retail for consumer home use must meet Class B limits. Class A applies to devices marketed exclusively for use in commercial, industrial, or business environments — and must carry a labeling statement warning that it may cause interference in residential installations. Misclassifying a consumer product as Class A is a common and consequential compliance error. ### Radiated and conducted emission tests — limits, frequencies, and measurement distances Radiated emission limits apply from 30 MHz to 40 GHz (upper frequency depends on the highest internal frequency of the device). Measurements are taken at 3 metres for Class B or 10 metres for Class A from the EUT (equipment under test) in a semi-anechoic chamber or open-area test site (OATS). Conducted emission limits apply from 150 kHz to 30 MHz on AC mains power lines, measured using a line impedance stabilisation network (LISN). Both types of limit are expressed in dBµV/m (radiated) or dBµV (conducted) and tighten at higher frequencies. ### Compliance route — SDoC for most digital devices, FCC certification for intentional radiators Most digital-only devices (unintentional radiators without intentional RF transmitters) use the Supplier's Declaration of Conformity (SDoC) route: the manufacturer tests the device, retains the test report, and applies the FCC compliance statement to the label. Products that incorporate intentional radiators (Wi-Fi, Bluetooth, cellular) require FCC certification through a Telecommunications Certification Body (TCB). A product with both intentional and unintentional RF sources needs to satisfy both Subpart B (emissions) and the applicable intentional radiator subpart (Subpart C for Wi-Fi, etc.). ## How to demonstrate Part 15 Subpart B compliance 01 Classify the device as Class A or Class B based on its intended market and likely use environment — if the product will be marketed through retail channels, sold on Amazon, or intended for home use in any way, assume Class B limits apply; Class A classification requires affirmative evidence that the product is sold only into commercial or industrial channels. 02 Engage an FCC-accredited or ANSI C63-accredited test laboratory early and request pre-compliance testing on your first working prototype — pre-compliance testing identifies the dominant emission sources before the design is frozen, when layout changes and filtering modifications are still feasible and low-cost. 03 Conduct radiated emissions testing per ANSI C63.4 in a semi-anechoic chamber or accredited OATS at 3 m (Class B) or 10 m (Class A) test distance, scanning from 30 MHz to at least 1 GHz (or higher if the device's highest internal frequency warrants it per §15.33). 04 Conduct conducted emissions testing per ANSI C63.4 on all AC mains interfaces using a calibrated LISN, measuring from 150 kHz to 30 MHz — common failure frequencies for switching power supplies are below 1 MHz where filter capacitors are most critical. 05 Prepare the SDoC statement referencing 47 CFR §15.107 and §15.109, retain the full test report and supporting technical documentation, apply the required FCC compliance statement to the product label and user manual, and register the responsible party in the FCC's SDoC database if required by the applicable subpart. ## Frequently asked questions ### What is the difference between Class A and Class B in Part 15 Subpart B? Class B limits are 6–10 dB stricter than Class A limits for radiated emissions, and similarly tighter for conducted emissions. Class B applies to devices for residential use; Class A applies to commercial/industrial-only devices. A Class A product sold into residential environments is non-compliant regardless of whether it passes Class A testing — the classification is determined by the intended market, not just the emission performance. Many manufacturers choose to test to Class B voluntarily even for commercial products to avoid channel restrictions and future reclassification risk. ### Does a product with Wi-Fi need both Subpart B and Subpart C compliance? Yes. A product containing a Wi-Fi module is an intentional radiator and must be certified under Subpart C (or use a pre-certified module with a valid FCC ID). The product as a whole must also comply with Subpart B for its unintentional emissions from the digital electronics — clocks, processors, switching regulators — that are separate from the intentional radio transmissions. The FCC requires that both compliance paths are addressed and documented in the technical file. Using a pre-certified Wi-Fi module only satisfies Subpart C; it does not address Subpart B conducted and radiated emissions from the rest of the circuit. ### What test standard does the FCC use for emissions testing? ANSI C63.4 (American National Standard for Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment) is the primary measurement standard for FCC Part 15 Subpart B testing. ANSI C63.10 covers testing of unlicensed wireless devices. The FCC does not accept EN 55032 (the EU standard) test reports directly as evidence of US compliance — the measurement procedures, detector functions, and quasi-peak/average limits differ in ways that can produce different results for the same product. ### Can I use an EU EN 55032 test report for Part 15 Subpart B? Not directly. While EN 55032 and Part 15 Subpart B test similar emission phenomena, they differ in limit values, measurement distances, detector types (quasi-peak vs average), and frequency ranges. A combined test report that captures both EN 55032 and ANSI C63.4 measurements can be produced by a laboratory that is accredited for both standards, which is the most efficient approach for companies launching in both the EU and US markets simultaneously. Some test laboratories offer combined EU/US EMC testing packages for this reason. **Disclaimer:** Educational resource only. Regulatory requirements change and vary by product category. Consult an accredited test laboratory or qualified compliance engineer before making certification decisions. 🇺🇸 US roadmap for your product Every standard, document, and test that applies — free, no account required. See your free roadmap[ Want an expert to take your product through 🇺🇸 US compliance for you? One consultant from Krono's compliance team takes your product from requirements to legal sale, with a fixed quote before any work starts. See compliance services](/services) Learn this properly In-depth course that teaches the full process, not just this one answer. [Start the course — $149](/courses/02-fcc-authorization-for-hardware-startups)[Prefer to read? 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