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United StatesFCCPart 15

# FCC Part 15 Class B: Radiated and Conducted Emission Limits for Consumer Electronics

FCC 47 CFR Part 15 Subpart B sets the emission limits that every digital device marketed for residential use in the US must meet. Class B limits are more stringent than Class A, measured at 3 metres, and enforced through the SDoC self-declaration pathway for most unintentional radiators. This guide covers the exact numerical limits, the accredited lab requirement, the common failure modes, and the 6 dB compliance margin strategy engineers rely on.

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At a glance

Radiated limit (above 960 MHz)

500 µV/m (54 dBµV/m) at 3 m

Conducted limit (0.5–30 MHz)

60 dBµV QP / 50 dBµV avg

Measurement distance

3 metres (radiated)

Authorization pathway

SDoC — no FCC filing required

## FCC Part 15 Class B — key technical requirements

### Class A vs Class B — the residential use distinction

FCC 47 CFR § 15.3(h) defines a Class B digital device as one that is marketed for use in a residential environment notwithstanding use in commercial, business, and industrial environments. Class B limits are more stringent than Class A because residential environments have less electromagnetic shielding and other equipment nearby. The classification is determined by the manufacturer's marketing intent — a device marketed for home use is Class B even if it is technically identical to a commercially marketed device. Devices that are marketed only for commercial, industrial, or business use are Class A. Misclassifying a residential product as Class A is a common enforcement finding during FCC market surveillance.

### Class B radiated emission limits (§ 15.109)

Class B radiated emission limits are measured at 3 metres from the device under test in a semi-anechoic chamber or on a calibrated Open Area Test Site (OATS). The limits by frequency band: 30–88 MHz: 100 µV/m (40 dBµV/m); 88–216 MHz: 150 µV/m (43.5 dBµV/m); 216–960 MHz: 200 µV/m (46 dBµV/m); above 960 MHz: 500 µV/m (54 dBµV/m). Measurements use a quasi-peak detector below 1 GHz and a peak detector above 1 GHz, with the antenna scanned for maximum reading at each frequency across both horizontal and vertical polarisations. The device is operated in its worst-case configuration — maximum clock speed, maximum data throughput, all peripheral ports active.

### Class B conducted emission limits (§ 15.107)

Conducted emission limits apply to devices that connect to the AC power mains. Measurements are made using a LISN (Line Impedance Stabilisation Network) at the power input terminals. Class B limits: 0.15–0.5 MHz — 48.5 dBµV quasi-peak / 35.5 dBµV average (the limits tighten linearly from the upper end of this range); 0.5–5 MHz — 60 dBµV quasi-peak / 50 dBµV average; 5–30 MHz — 60 dBµV quasi-peak / 50 dBµV average. Both quasi-peak and average detector readings must comply — products with high peak-to-average emission ratios (switching power supplies with poor average control) can pass quasi-peak but fail average, or vice versa. The average detector is the tighter constraint for most modern switch-mode power supply harmonics.

### SDoC pathway — the default for most Part 15B devices

The Supplier's Declaration of Conformity (SDoC) under 47 CFR § 15.19(a)(3) is the default authorization pathway for most unintentional radiators (Part 15B devices). The manufacturer tests the device at an FCC-recognized accredited laboratory, prepares an SDoC, labels the product per § 15.19, and retains the test data. No FCC filing is required — the SDoC is not submitted to the FCC, and no FCC ID is assigned. The SDoC must include: identification of the responsible party, identification of the device, and the statement of compliance. However, test records must be retained for 10 years and made available to the FCC within 30 days of any request. The FCC's equipment authorization database at fcc.gov/oet/ea/fccid does not contain SDoC records.

### FCC-recognized accredited lab requirement for SDoC

For SDoC, testing must be performed at an accredited testing laboratory recognised by the FCC. Recognised accreditation bodies include A2LA (American Association for Laboratory Accreditation), NVLAP (National Voluntary Laboratory Accreditation Program), and laboratories accredited by ILAC MRA member bodies. The lab must be accredited specifically for the FCC test methods — general ISO/IEC 17025 accreditation is not sufficient without the specific FCC scope. The FCC's list of recognized accreditation bodies and their laboratory databases are maintained at fcc.gov. Labs that perform only part of the test programme (e.g., conducted emissions only) while another lab performs radiated emissions must both be accredited for their respective tests, and both reports are included in the technical file.

### Emission hotspots and the 6 dB compliance margin strategy

The three dominant emission sources in consumer electronics: (1) Switching power supply harmonics — the fundamental switching frequency and its harmonics from 40 kHz to 30 MHz propagate onto the AC line and radiate through the power cable acting as an antenna; (2) Clock harmonics — a 100 MHz system clock generates odd harmonics at 300 MHz, 500 MHz, 700 MHz (all in the most stringent radiated bands); (3) USB data lines — high-speed USB 3.x data lines at 2.5 or 5 Gbps radiate via the connected USB cable acting as an unintentional antenna. Labs routinely target 6 dB below the limit at each frequency to accommodate measurement uncertainty (ANSI C63.4 specifies expanded uncertainty requirements) and production variation between units. A result at the limit from a prototype often fails in production units with slightly different layout or cable routing.

## FCC Part 15 Class B compliance process — step by step

01

Classify the device as Class A or Class B based on the intended use environment and marketing intent. If the product will be sold at retail, marketed for home or home-office use, or described in any marketing materials as suitable for residential environments, it is Class B.

02

Confirm the device contains digital circuitry — defined as any circuit operating at a clock rate of 9 kHz or higher that uses digital techniques (binary, multi-level, or any non-sinusoidal digital waveform) and generates RF energy. Nearly all modern consumer electronics meet this definition.

03

Determine the authorization pathway: SDoC is appropriate for unintentional radiators (Part 15B devices with no intentional transmitter). Products with intentional transmitters (Wi-Fi, Bluetooth, cellular) require FCC equipment authorization via a TCB for the intentional radiator portion — the Part 15B unintentional radiator aspects are typically covered under the same TCB application.

04

Engage an FCC-recognized accredited test laboratory. Verify the lab's accreditation scope includes ANSI C63.4 (radiated emissions, OATS or semi-anechoic chamber method) and ANSI C63.4 or FCC KDB 905462 (conducted emissions). Major labs with FCC recognition include Element, UL, SGS, Intertek, TÜV SÜD, and MET Laboratories.

05

Perform pre-compliance testing before formal compliance testing if the design is not mature. Near-field probing with a spectrum analyser identifies the dominant emission sources. Pre-compliance testing at a suitable lab or in-house semi-anechoic chamber costs a fraction of a formal compliance run and commonly prevents redesign cycles.

06

If pre-compliance testing reveals emissions near or above the limit, identify the source: use a near-field probe set (H-field loop probe for current sources, E-field probe for voltage sources) to localise the emitting structure. Apply remediation: ferrite clamps on cables for conducted emissions propagating as common-mode antenna current, Pi-filters on power supply input, spread-spectrum clock generation to reduce peak harmonic amplitudes, PCB layout improvements to reduce loop areas on high-frequency switching nodes.

07

Run the formal compliance test at the accredited lab. Bring the device in worst-case configuration: all cables attached (cables are major radiated emission antennas), all ports active, running at maximum data throughput. Test all physically distinct configurations if variants exist (different cable lengths, different peripheral combinations).

08

Prepare the SDoC. The SDoC must include: (1) identification of the responsible party (name, address, phone), (2) equipment description (trade name, model number), (3) statement that the equipment complies with Part 15 of the FCC Rules, and (4) identification of the applicable rule part (§ 15.107 and § 15.109 for Class B). The SDoC is typically a one-page document retained internally — it does not need to be physically attached to the product.

09

Affix FCC compliance labelling per § 15.19. For SDoC devices, the label must state: 'This device complies with Part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) This device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.' For products too small to carry the full statement, it may be placed in the user manual with a reference on the product.

10

Retain the test report, SDoC, and all supporting technical documentation for 10 years. The FCC may request these within 30 days during market surveillance. Establish a document control process to track which device variants have which test records — the FCC considers variants with different internal configurations to potentially require separate testing.

## Frequently asked questions

### What's the difference between FCC Part 15 Class A and Class B limits?

Class B limits are more stringent than Class A in every frequency band, reflecting the less electromagnetically robust residential environment. For radiated emissions above 960 MHz, Class B allows 500 µV/m at 3 m, while Class A allows 700 µV/m at 3 m (measured at 10 m for Class A). In the 216–960 MHz band, Class B allows 200 µV/m vs 500 µV/m for Class A (10 m). For conducted emissions, Class B quasi-peak limits in the 0.5–30 MHz range are 60 dBµV vs 79 dBµV for Class A. The practical implication is that a design that comfortably passes Class A may fail Class B by 6–15 dB — a gap that often requires hardware changes, not just retesting.

### Does a Class B device need an FCC ID or is SDoC sufficient?

Most Class B digital devices (unintentional radiators) use the SDoC (Supplier's Declaration of Conformity) pathway and do not receive an FCC ID. An FCC ID is assigned only through the equipment authorization process via a TCB (Telecommunication Certification Body) or direct FCC filing — this is required for intentional transmitters (Wi-Fi, Bluetooth, cellular radios). A product that contains both an intentional transmitter and a digital circuit will have an FCC ID for the intentional radiator portion; the ID typically covers the unintentional radiator aspects within the same grant. For a pure Class B digital device with no intentional transmitter — a laptop with no wireless, a USB hub, a streaming box using only Ethernet — SDoC is sufficient and no FCC ID is issued or required.

### How are FCC radiated emission limits measured and at what distance?

FCC Part 15 Class B radiated emission limits are specified and measured at a 3-metre separation distance between the device under test and the measurement antenna. The test is performed in a semi-anechoic chamber (SAC) or on a calibrated Open Area Test Site (OATS) per ANSI C63.4. The device is placed on a turntable and rotated 360 degrees at each frequency, with the antenna height scanned from 1 to 4 metres, and both horizontal and vertical polarisations measured. The maximum reading across all turntable positions and antenna heights is recorded as the measurement result. Measurement uncertainty must be calculated per ANSI C63.23 and declared in the test report. Class A limits, by contrast, are specified at 10 metres but may be measured at a shorter distance and extrapolated using a 20 dB/decade distance factor.

### What causes devices to fail FCC Part 15 Class B radiated emission testing?

The three most common failure causes: (1) Switching power supply harmonics radiating via the AC power cord — the cord acts as a monopole antenna coupling conducted emissions into radiated emissions, typically in the 30–200 MHz range. This is fixed by improving conducted EMI filtering on the power supply input stage. (2) Clock harmonics — a board running at 100 MHz generates harmonics at 300 MHz, 500 MHz, and 700 MHz, all in frequency bands with tight limits. Spread-spectrum clocking reduces peak amplitudes by 6–10 dB by spreading the energy across ±0.5% of the nominal frequency. (3) USB cable antenna effect — high-speed USB 3.x differential data signals at 5 Gbps and 10 Gbps have spectral content in the 1–5 GHz range; the USB cable becomes an efficient radiating antenna. PCB-level common-mode filtering on the USB differential pairs is the typical fix.

**Disclaimer:** Educational resource only. Regulatory requirements change. Consult a qualified compliance specialist before making decisions.

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