[Home](/)/[Resources](/resources)/FCC Part 15 Class B United StatesFCCPart 15 # FCC Part 15 Class B: Emission Limits, Required Statements and the SDoC Route FCC Part 15 Class B is the emissions class for digital devices marketed for residential use (47 CFR § 15.3(i)). Before it can be marketed in the US, a Class B product must stay under the § 15.107(a) conducted limits and the § 15.109(a) radiated limits (40–54 dBµV/m at 3 m). It must also carry the § 15.19 label statement, include the § 15.105(b) statement in its manual, and be authorized, usually by Supplier's Declaration of Conformity (SDoC). This guide gives the exact limits and statement text from the eCFR, explains how the conducted and radiated tests work, and lists the records you need to keep. Not sure which class applies? Start with [FCC Class A vs Class B](/guides/fcc-part-15-class-a-vs-b). Copy Link[Share on WhatsApp](https://wa.me/?text=https%3A%2F%2Fkrono-labs.com%2Fguides%2Fus-fcc-part-15-class-b) At a glance Radiated limit (§ 15.109) 40–54 dBµV/m at 3 m (30 MHz and up) Conducted limit (§ 15.107) 56 dBµV QP / 46 dBµV AVG (0.5–5 MHz) Test method ANSI C63.4-2014 (§ 15.31(a)(4)) Authorization SDoC or Certification; no FCC filing for SDoC ## FCC Part 15 Class B — key technical requirements ### What counts as a Class B digital device (§ 15.3(i)) 47 CFR § 15.3(i) defines a Class B digital device as one "marketed for use in a residential environment notwithstanding use in commercial, business and industrial environments" — personal computers, calculators and similar products sold to the general public are the rule's own examples. Class A (§ 15.3(h)) is the narrow exception: devices marketed only for commercial, industrial or business use, excluding anything marketed to the general public or intended for the home. Classification follows how the product is marketed, not how it is built. A note to § 15.3(i) also lets the FCC classify a device type as Class B regardless of intended use if it repeatedly causes harmful interference, and encourages responsible parties to qualify commercial products to Class B where they can. ### Class B radiated emission limits (§ 15.109(a)) Measured at 3 metres on an open area test site or a calibrated alternative such as a semi-anechoic chamber (§ 15.31(d)). The limits: 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). The tighter limit applies at band edges (§ 15.109(c)). Up to 1000 MHz the limits assume a CISPR quasi-peak detector; a peak detector may be used instead because it always reads equal or higher (§ 15.35(a)). Above 1000 MHz the limits are average values measured with at least 1 MHz resolution bandwidth, with a separate peak limit 20 dB above the average limit (§ 15.35(b)). How high you scan depends on the highest frequency the device generates or uses (§ 15.33(b)): 1 GHz if it is 1.705–108 MHz, 2 GHz for 108–500 MHz, 5 GHz for 500–1000 MHz, and the 5th harmonic or 40 GHz (whichever is lower) above 1 GHz. ### Class B conducted emission limits (§ 15.107(a)) Conducted limits apply from 150 kHz to 30 MHz to equipment that connects to the AC mains, measured between each power line and ground with a 50 µH / 50 Ω LISN. Class B limits: 0.15–0.5 MHz, 66 to 56 dBµV quasi-peak and 56 to 46 dBµV average, both decreasing with the logarithm of frequency; 0.5–5 MHz, 56 dBµV quasi-peak and 46 dBµV average; 5–30 MHz, 60 dBµV quasi-peak and 50 dBµV average. The lower limit applies at band edges. Both detectors must pass. Switch-mode power supplies with a steady switching frequency tend to fail the average limit first, while bursty or spread-spectrum noise is more often a quasi-peak problem. A product that only runs on batteries and cannot operate while connected to mains is exempt from conducted testing, but anything that works while charging, uses an AC adapter, or draws power from a mains-connected host (a USB-powered gadget, for example) must be tested (§ 15.107(d)). ### SDoC — the usual authorization route for Class B Under § 15.101(a), Class B personal computers, peripherals, external switching power supplies and "other Class B digital devices" may be authorized by Supplier's Declaration of Conformity (SDoC) or Certification. Most choose SDoC (§ 2.906): the responsible party has the product tested, keeps the records, and supplies a compliance information statement with the product. Nothing is filed with the FCC, no FCC ID is issued, and SDoC products do not appear in the FCC equipment authorization database. The compliance information statement (§ 2.1077) must give the product name and model, a compliance statement, and the name, address and telephone number or internet contact of a responsible party located in the United States. The FCC SDoC logo is optional (§ 2.1074). ### Test lab and measurement method for SDoC Part 15 unintentional radiators are measured to ANSI C63.4-2014, excluding clauses 4.5.3, 4.6, 6.2.13, 8.2.2, 9 and 13 (§ 15.31(a)(4)). For SDoC, the FCC does not require an accredited or FCC-recognised lab — that requirement in § 2.948(a) applies to Certification. The test site must still be validated to ANSI C63.4-2014 or C63.4a-2017 (§ 2.948(d)), and SDoC testing must not be done at a facility owned or controlled by a prohibited entity as defined in § 2.902 (§ 2.906(a)(1)). In practice most brands still use an ISO/IEC 17025-accredited EMC lab because the report has to stand up if the FCC asks for it. ### Where emissions come from, and why 6 dB margin Three sources cause most Class B trouble. Switch-mode power supply harmonics show up from 150 kHz upward on the mains lead and couple onto cables that then radiate in the 30–200 MHz range. Clock and data harmonics: a 25 MHz crystal puts energy at 75, 125 and 175 MHz; DDR, MIPI and Ethernet lines add more. Cables: USB, HDMI and long DC leads act as antennas for common-mode currents. Labs and design teams usually aim for about 6 dB of margin below the limit at the worst frequency to absorb measurement uncertainty and unit-to-unit variation. A prototype that passes with 1 dB to spare often fails on a production unit with a different cable dress or component lot. ## FCC Part 15 Class B limits at a glance Conducted limits from 47 CFR § 15.107, in dBµV, measured with a 50 µH / 50 Ω LISN on each power line. Class A is shown for comparison. Frequency Class B (§ 15.107(a)) Class A (§ 15.107(b)) 0.15–0.5 MHz 66 → 56 QP / 56 → 46 AVG\* 79 QP / 66 AVG 0.5–5 MHz 56 QP / 46 AVG 73 QP / 60 AVG 5–30 MHz 60 QP / 50 AVG 73 QP / 60 AVG \* Decreases with the logarithm of frequency. QP = quasi-peak, AVG = average. Both must be met. Radiated limits from § 15.109. Class B is specified at 3 m and Class A at 10 m. The last column scales Class A to 3 m (+10.5 dB) so the two can be compared directly. Frequency Class B at 3 m Class A at 10 m Class A scaled to 3 m 30–88 MHz 100 µV/m (40.0 dBµV/m) 90 µV/m (39.1 dBµV/m) ≈ 49.5 dBµV/m 88–216 MHz 150 µV/m (43.5 dBµV/m) 150 µV/m (43.5 dBµV/m) ≈ 54.0 dBµV/m 216–960 MHz 200 µV/m (46.0 dBµV/m) 210 µV/m (46.4 dBµV/m) ≈ 56.9 dBµV/m Above 960 MHz 500 µV/m (54.0 dBµV/m) 300 µV/m (49.5 dBµV/m) ≈ 60.0 dBµV/m Quasi-peak detector up to 1000 MHz; average detector above 1000 MHz, with a peak limit 20 dB higher (§ 15.35). The tighter limit applies at band edges. ## The FCC Class B statements — exact text and what they mean A Class B product needs two different statements. Teams often confuse them, or use the Class A wording by mistake. ### On the product: § 15.19(a)(3) label “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.” What it means: Part 15 devices operate without a licence, so they have no protection from interference and must not cause it. If the product is too small for the statement in 4-point text and has no display for e-labelling, put the statement in the manual _and_ on the packaging or a removable label (§ 15.19(a)(5)). ### In the manual: § 15.105(b) Class B statement “Note: This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a residential installation. This equipment generates, uses and can radiate radio frequency energy and, if not installed and used in accordance with the instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference will not occur in a particular installation. If this equipment does cause harmful interference to radio or television reception, which can be determined by turning the equipment off and on, the user is encouraged to try to correct the interference by one or more of the following measures: —Reorient or relocate the receiving antenna. —Increase the separation between the equipment and receiver. —Connect the equipment into an outlet on a circuit different from that to which the receiver is connected. —Consult the dealer or an experienced radio/TV technician for help.” What it means: the product met the residential limits, which reduce the chance of interference but do not rule it out, and the user is told how to fix it. The rule allows “the following or similar statement”, but most teams use the text word for word. It must sit in a prominent place in the manual, and a PDF or online manual is acceptable (§ 15.105(e)). Add the § 15.21 warning that unapproved changes could void the user's authority to operate the equipment. ### With the product: § 2.1077 compliance information For SDoC products, include the product name and model number, the Part 15 compliance statement, and the name, address and phone number or internet contact of the responsible party, who must be located in the United States. Overseas manufacturers usually name their importer or a US subsidiary. The details of the SDoC route are in our [FCC SDoC guide](/guides/fcc-sdoc-supplier-declaration). ## FCC Part 15 Class B compliance process — step by step 01 Classify the device. If it will be marketed to the general public, sold through retail or online marketplaces, or is intended for use in the home, it is Class B (§ 15.3(h)–(i)). Check § 15.103 for exemptions — for example, digital devices used only in vehicles, only inside appliances, or only as industrial test equipment. 02 Confirm it is a digital device: an unintentional radiator that generates and uses timing signals or pulses above 9,000 per second and uses digital techniques (§ 15.3(k)). Any microcontroller, switch-mode regulator or digital interface qualifies. 03 Pick the authorization route. A product with no transmitter can use SDoC or Certification (§ 15.101(a)). If the product also contains a transmitter such as Wi-Fi or Bluetooth, the radio needs Certification through a TCB and an FCC ID. The digital-device portion can be covered by SDoC or included in the certification. 04 Choose a test lab and agree the test plan: ANSI C63.4-2014 method, AC conducted emissions on each line if the product connects to mains directly or indirectly, radiated emissions from 30 MHz to the upper frequency set by § 15.33(b), and a defined worst-case operating mode. 05 Run pre-compliance scans while the PCB can still change. A near-field probe set and spectrum analyser will find the dominant sources. Conducted emissions in particular can be checked cheaply with a bench LISN. 06 Fix what the scan shows: an input filter or common-mode choke for conducted emissions, ferrites or better cable shield termination for cable-driven radiation, spread-spectrum clocking for clock harmonics, and smaller loop areas on fast switching nodes. 07 Run the formal test with the product in its worst-case configuration: all cables and typical peripherals attached, all ports active, maximum throughput. Test each variant whose electronics or enclosure differ. 08 Prepare the compliance information statement (§ 2.1077): product name and model number, a statement that the product complies with Part 15, and the name, address and phone or web contact of a US-based responsible party. Ship it in the manual or on a separate sheet. Electronic delivery is allowed under § 2.935. 09 Label the product with the § 15.19(a)(3) statement and put the § 15.105(b) Class B statement and the § 15.21 "changes or modifications" warning in the manual. Give the product a unique identifier that cannot be confused with an FCC ID (§ 2.1074). 10 Keep the SDoC records listed in § 2.938(b): dated test report, lab name, test method and equipment, EUT and cable setup, at least two photos of the worst-case conducted and radiated setups, any changes made to pass, signatures, and a signed statement that the product is not produced by an entity on the FCC Covered List. The FCC can request a sample or the data at any time (§ 2.945). ## Common Class B mistakes 01 Using the Class A § 15.105(a) statement on a consumer product. If the product is marketed to the general public, it is Class B whatever the manual says. 02 Leaving out conducted testing on a "battery" product that runs while charging or is powered over USB from a mains-connected host. Section 15.107(d) only exempts products that run purely on battery. 03 Naming an overseas manufacturer as the responsible party. Under § 2.909(b) and § 2.1077, the SDoC responsible party must be located in the United States. 04 Stopping the radiated scan at 1 GHz. A product with a 1.2 GHz processor or DDR clock must be scanned to the 5th harmonic or 40 GHz, whichever is lower (§ 15.33(b)). 05 Testing a lab sample with no cables attached. Cables usually radiate the most, so test with representative cables and peripherals connected. 06 Treating an FCC ID on a radio module as cover for the whole product. The host's digital circuitry still needs its own Part 15B compliance. ## Frequently asked questions ### What are the FCC Part 15 Subpart B classes? Subpart B covers unintentional radiators and splits digital devices into two classes. Class A (§ 15.3(h)) is for devices marketed only for commercial, industrial or business use. Class B (§ 15.3(i)) is for devices marketed for residential use, which in practice means anything sold to the general public. Class B has the tighter limits: at the same 3 m distance, Class B radiated limits are about 6–11 dB lower than Class A, and Class B conducted limits are 13–23 dB lower. Subpart B also covers non-digital unintentional radiators such as receivers, which use the Class B limits. ### What does the FCC Class B Part 15 statement mean? The § 15.105(b) statement in the manual tells the user three things: the product was tested and meets the Class B limits; those limits give reasonable, not absolute, protection against interference in a home; and if it does interfere with radio or TV reception, the user should try reorienting the antenna, moving the equipment further away, using a different outlet circuit, or asking a technician. The separate § 15.19 label statement ("may not cause harmful interference" and "must accept any interference received") means the device has no protection from interference and must not cause it. Neither statement is a certification mark — it is a declaration by the responsible party. ### How is the FCC Class B conducted emissions test done? The product is powered through a 50 µH / 50 Ω LISN, and the lab measures the RF voltage between each power conductor and ground — usually reported as Line 1 (L1, live) and Line 2 (L2 or N, neutral) — from 150 kHz to 30 MHz. Each line is scanned with a peak detector, then the highest points are re-measured with quasi-peak and average detectors and plotted against the § 15.107(a) limits: 66–56 dBµV QP and 56–46 dBµV AVG from 0.15–0.5 MHz, 56/46 dBµV from 0.5–5 MHz and 60/50 dBµV from 5–30 MHz. The test method is ANSI C63.4-2014. Battery-only devices that cannot run while connected to mains are exempt (§ 15.107(d)). ### What is the difference between FCC Part 15 Class A and Class B limits? Class B radiated limits are set at 3 m and Class A at 10 m (§ 15.109). Scaled to the same 3 m distance, Class A allows about 49.5, 54.0, 56.9 and 60.0 dBµV/m in the four bands versus 40, 43.5, 46 and 54 dBµV/m for Class B — 6 to 11 dB more. For conducted emissions (§ 15.107), Class A allows 79/66 dBµV (QP/AVG) below 0.5 MHz and 73/60 dBµV from 0.5–30 MHz, against 56/46 to 60/50 dBµV for Class B above 0.5 MHz. A design that passes Class A comfortably can still fail Class B, and closing that gap usually means hardware changes. ### How can I tell if a product, such as a power inverter, meets FCC Part 15 Class B? Check three things. The product or its packaging should carry the § 15.19 statement. The manual should include the § 15.105(b) Class B statement. A § 2.1077 compliance information statement should name the model and a responsible party with a US address. SDoC products are not listed in the FCC database, so you cannot look them up by FCC ID. If the product has a radio, search its FCC ID in the FCC equipment authorization database (fcc.gov/oet/ea/fccid). For an inverter or other power product, ask the seller for the Part 15 test report: an inverter that switches at tens of kHz is a strong conducted and radiated emitter, and the report is the only real evidence that it passed. ### Does a Class B device need an FCC ID or is SDoC sufficient? For a product with no transmitter (a USB hub, a wired keyboard, an Ethernet-only streaming box), SDoC is sufficient and no FCC ID is issued. Certification is also allowed (§ 2.906(c)) but rarely chosen. An FCC ID only comes from Certification through a TCB, which is required for intentional radiators like Wi-Fi, Bluetooth or cellular modules. A product with a radio carries an FCC ID for the transmitter. Its digital circuitry can be authorized under SDoC or covered in the same certification. ### Does FCC Class B say anything about operating temperature? No. Part 15 Class B is purely an RF emissions classification and sets no operating or storage temperature range. When a datasheet lists "FCC Class B" next to a temperature rating, the two are unrelated: the temperature range is the manufacturer's own specification. Emissions are normally tested at room temperature in typical operating modes. ### How are FCC radiated emission limits measured and at what distance? Class B limits are specified at 3 m. The device sits on a turntable in a semi-anechoic chamber or on an open area test site, rotated through 360 degrees, while the receive antenna is scanned from 1 to 4 m in height in both horizontal and vertical polarisation (ANSI C63.4-2014). The highest reading is compared against § 15.109(a). Measurements may be made at another distance and extrapolated at 20 dB per decade (§ 15.31(f)). Labs normally state their measurement uncertainty in the report, usually calculated to ANSI C63.23. ### What causes devices to fail FCC Part 15 Class B radiated emission testing? Most failures come down to common-mode current on cables. Noise from a switch-mode supply or digital logic gets onto the power cord, USB or HDMI cable, and the cable radiates it, typically between 30 and 300 MHz. Clock harmonics are next: a 100 MHz clock puts energy at 300, 500 and 700 MHz. The usual fixes are input filtering and common-mode chokes, ferrites, proper cable shield termination to chassis, spread-spectrum clocking (which typically takes several dB off the peak harmonic), and tighter PCB return paths. **Disclaimer:** Educational resource only. Regulatory requirements change. Consult a qualified compliance specialist before making 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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