FCC Part 15 Guide: What Hardware Engineers Need to Know

TL;DR -- Part 15 of the FCC rules (47 CFR Part 15) covers nearly every consumer electronic product sold in the US, from laptops and USB hubs (unintentional radiators) to WiFi routers and Bluetooth speakers (intentional radiators). How Part 15 classifies your device determines your authorization path, test requirements, cost, and labeling obligations. Misclassify it, and you'll waste months.

What Part 15 actually covers

Title 47 of the Code of Federal Regulations is divided into numbered parts. Part 15, "Radio Frequency Devices," is the one that matters for unlicensed consumer electronics. If your product contains digital circuitry or deliberately transmits RF energy without a spectrum license, Part 15 applies.

The scope is broad. A switching power supply with no wireless capability falls under Part 15. So does a WiFi 7 router. The difference is which subpart of Part 15 applies and which authorization procedure you follow.

Part 15 does not cover:

• Licensed transmitters (cellular radios fall under Parts 22, 24, and 27)
• Industrial/scientific/medical equipment that uses RF for heat or imaging (Part 18)
• Amateur radio equipment built by licensed operators (Part 97)

If your product has both a licensed radio (cellular) and unlicensed radios (WiFi/BT), the unlicensed portions still fall under Part 15 while the cellular portions are governed by other rule parts. Modern smartphones are certified under five or six rule parts simultaneously.

Intentional vs unintentional: the split that matters

Part 15 divides devices into two categories: intentional radiators and unintentional radiators. This classification, defined in 47 CFR Section 15.3, determines your authorization path, test suite, cost, timeline, and labeling rules.

Unintentional radiator (Section 15.3(z)): A device that generates RF energy internally -- clock oscillators, high-speed data buses, switching regulators -- but isn't designed to emit that energy. The radiation is a side effect of digital electronics doing their job.

Intentional radiator (Section 15.3(o)): A device that deliberately generates and emits RF energy by radiation or induction. WiFi, Bluetooth, Zigbee, NFC, wireless charging, RF remotes -- these all exist because they transmit.

flowchart TD
    A["Your Device"] --> B{"Does any part of it\ndeliberately emit RF?"}
    B -->|No| C["Unintentional Radiator\nSubpart B"]
    B -->|Yes| D{"Which frequency bands\nand technologies?"}
    D --> E["2.4 GHz WiFi / BT / Zigbee\n→ Subpart C §15.247"]
    D --> F["5 GHz / 6 GHz WiFi\n→ Subpart E §15.407"]
    D --> G["UWB ranging\n→ Subpart F"]
    D --> H["Other low-power TX\n→ Subpart C general"]
    C --> I["SDoC\nSelf-declaration"]
    E --> J["Certification\nTCB review + FCC ID"]
    F --> J
    G --> J
    H --> J
    style I fill:#1e3a5f,color:#fff
    style J fill:#1e3a5f,color:#fff

Most real products are composite devices. A laptop with WiFi is both an unintentional radiator (the CPU, display controller, SSD) and an intentional radiator (the WiFi/BT module). Each portion is tested under its own subpart. In practice, the radio is usually a pre-certified module with its own FCC ID, so the laptop OEM only handles the unintentional radiator portion.

Part 15 subparts at a glance

For most hardware teams, the subparts that matter day to day are B, C, and E. Subpart A sets the ground rules for everything else.

Subpart A: The ground rules

Subpart A (Sections 15.1 through 15.38) is not a test standard. It's the shared scaffolding every other subpart relies on. Three sections are worth knowing by heart.

Section 15.5 -- General conditions of operation. Two sentences that define the deal Part 15 devices accept: (1) you may not cause harmful interference, and (2) you must accept any interference received, including interference that causes undesired operation. There is no right to a clean RF environment under Part 15. If a neighbor's legal device wipes out your WiFi signal, that's your problem.

Section 15.19 -- Labeling. Sets the compliance statement requirements. Certified devices display an FCC ID. SDoC devices carry a text statement. Our FCC label requirements guide covers what goes on the device, what goes in the manual, and when electronic labeling is allowed.

Section 15.21 -- User manual warning. Every Part 15 device manual must include a statement warning users that unauthorized modifications void the authority to operate the device. This is not optional boilerplate; it is a regulatory requirement.

Subpart B: Unintentional radiators

Subpart B governs devices that use RF-frequency signals internally but are not designed to radiate. If your product has a microprocessor, a switching regulator, an HDMI port, or any digital bus running above a few MHz, Subpart B applies.

Class A vs Class B

Subpart B divides devices into two classes based on their intended use environment:

Class B is the default for consumer products. If there is any chance your device ends up in someone's home, it needs to meet Class B limits. Marketing a Class A device for residential use is a compliance violation, and retailers like Amazon increasingly enforce this.

The 10 dB gap between Class A and Class B limits is significant in practice. A product that passes Class A with 6 dB of margin will fail Class B. This is one of the most common surprises for teams moving from industrial to consumer markets.

What Subpart B tests measure

The core Subpart B test suite covers:

• Radiated emissions (30 MHz to 1 GHz minimum, extended to 5th harmonic of highest internal frequency or 40 GHz, whichever is lower)
• Conducted emissions on AC mains (150 kHz to 30 MHz), only for devices with a wired power connection

Sounds simple. It isn't. The measurement distance (3 m or 10 m), antenna height scan range, detector type (quasi-peak vs average), and frequency step size are all defined in ANSI C63.4 and referenced by Section 15.31. Get any of these wrong and the test is invalid.

For teams doing early-stage validation before booking a formal test lab, our EMC pre-compliance testing guide covers what equipment you need and which measurements you can approximate in-house.

Subpart C: Intentional radiators

Subpart C (Sections 15.201 through 15.258) covers most consumer wireless products. If your device deliberately transmits RF and doesn't operate in the 5/6 GHz U-NII bands or use ultra-wideband, this is your subpart.

Section 15.209 -- The baseline emission table

Section 15.209 sets default radiated emission limits for intentional radiators across the spectrum, measured as field strength at a specified distance. Think of it as the baseline -- specific sections like 15.247 and 15.249 override these with higher (more permissive) limits for devices that qualify under those rules.

Section 15.247 -- WiFi and Bluetooth

If you're building a consumer wireless product, you'll probably spend most of your time here. Section 15.247 covers spread spectrum and digital transmission systems in three ISM bands:

Section 15.247 permits two system types:

Frequency hopping spread spectrum (FHSS): Must hop across a minimum number of channels (75 or more in 2.4 GHz with channel bandwidth up to 1 MHz). Average dwell time per channel cannot exceed 0.4 seconds in any 30-second window. Maximum conducted output power: 1 W. Bluetooth Classic and BLE are certified under these rules.

Digital transmission systems (DTS): Minimum 6 dB bandwidth of 500 kHz. Maximum conducted output power: 1 W. This covers 802.11b/g/n/ax at 2.4 GHz.

The 1 W conducted power (30 dBm) ceiling is absolute. Most consumer products operate well below it. Your FCC grant specifies the maximum power for each modulation mode, and exceeding it -- even through a firmware update, even by a fraction of a dB -- triggers permissive change requirements or full recertification.

Section 15.249 -- Low-power, non-spread-spectrum devices

Section 15.249 covers devices in the same ISM bands (plus 24 GHz) that do not use spread spectrum. The permitted field strength is much lower than 15.247, so this section applies to simpler devices: short-range remotes, tire pressure monitors, basic telemetry links.

Other notable Subpart C sections

• Section 15.239 -- FM transmitters (88-108 MHz), limited to 250 microvolts/meter at 3 meters. This is why consumer FM transmitters have terrible range.
• Section 15.219 -- AM band operation (510-1705 kHz), for devices like vintage-style AM transmitters for holiday displays.
• Section 15.255 -- 60 GHz band (57-71 GHz), used by WiGig and some short-range high-bandwidth links.

Subpart E: 5 GHz and 6 GHz WiFi (U-NII)

Subpart E (Section 15.407 primarily) governs Unlicensed National Information Infrastructure devices. This is where 5 GHz and 6 GHz WiFi lives.

U-NII band plan

gantt
    title U-NII Frequency Bands
    dateFormat X
    axisFormat %s

    section U-NII-1
    5.150 – 5.250 GHz (indoor, 250 mW) :done, 5150, 5250

    section U-NII-2A
    5.250 – 5.350 GHz (DFS required) :active, 5250, 5350

    section U-NII-2C
    5.470 – 5.725 GHz (DFS + TPC) :active, 5470, 5725

    section U-NII-3
    5.725 – 5.850 GHz (outdoor OK, 1 W) :done, 5725, 5850

    section U-NII-5 to 8
    5.925 – 7.125 GHz (WiFi 6E/7) :crit, 5925, 7125

DFS (Dynamic Frequency Selection) is mandatory in U-NII-2A and U-NII-2C to protect weather and FAA radar. Your device must scan for radar before transmitting and vacate the channel within 10 seconds if radar is detected. DFS certification adds dedicated testing per KDB 905462. Budget an extra $2,000 to $5,000 and 1 to 2 weeks for DFS testing alone.

TPC (Transmit Power Control) is required in U-NII-2C. The device must be capable of operating at least 6 dB below maximum power.

The 6 GHz band (5.925 to 7.125 GHz, WiFi 6E/7) opened for unlicensed use under U-NII-5 through U-NII-8. Standard power outdoor devices need Automated Frequency Coordination (AFC) to protect incumbent services. Low-power indoor devices are exempt from AFC but limited to lower power levels.

Subpart F: Ultra-wideband

Subpart F covers devices with instantaneous bandwidth greater than 500 MHz or fractional bandwidth greater than 0.20. UWB signals spread their energy so thin they sit below the noise floor of conventional receivers, which is how they coexist with licensed services.

You'll find UWB in precision indoor ranging (Apple U1 chip, Samsung SmartTag), automotive passive keyless entry, and ground-penetrating radar. All require Certification, and the spectral density limits are strict -- especially around GPS frequencies near 1.5 GHz, where even small amounts of interference cause problems.

Authorization paths: what Part 15 actually requires

The authorization path follows directly from the device classification:

SDoC means you test at an accredited lab, prepare a declaration of conformity, and keep the records on file. No submission to anyone. No FCC ID. No review. The trade-off: if the FCC audits you and your test report has problems, nobody caught it before you shipped. Full comparison in our SDoC vs Certification guide.

Certification means you test at an accredited lab, then submit a full application package (test reports, internal/external photos, block diagram, operational description, user manual, label artwork) to a Telecommunication Certification Body (TCB). The TCB reviews everything and issues an FCC grant with your FCC ID. Cost and timeline details are in our FCC certification cost breakdown.

Emission limits: the numbers that matter

Unintentional radiators (Subpart B, Class B)

Selected radiated emission limits for Class B digital devices at 3 meters:

Class A limits are approximately 10 dB higher across all bands.

Intentional radiators (Section 15.247)

For devices under Section 15.247, the critical limits are conducted output power and antenna gain:

• Maximum conducted output power: 1 W (30 dBm) for both FHSS and DTS systems
• For point-to-multipoint: antenna gain up to 6 dBi with no power reduction required
• Above 6 dBi antenna gain: conducted power must be reduced by 1 dB for every dB of antenna gain above 6 dBi
• For fixed point-to-point: 3 dB reduction per dB above 6 dBi (more favorable ratio)
• Spurious emissions must meet Section 15.209 limits

Mistakes that actually cost money

These come from TCB rejection data and lab experience. They're listed roughly in order of how often we see them.

Misclassifying the device. Treating an intentional radiator as unintentional sends you down the wrong authorization path. The most common version: a product with an embedded radio module gets submitted as SDoC-only because the team forgot the module needs a Certification grant. Even if you're using a pre-certified module, verify the module's FCC grant covers your specific integration.

Forgetting the host device's unintentional emissions. Teams focus on the radio and forget that the digital host still needs Subpart B compliance. A WiFi-enabled device needs both Subpart C/E Certification for the radio and Subpart B testing for the host. A pre-certified module does not exempt you from Subpart B.

Exceeding granted power levels through firmware. Your FCC grant specifies maximum transmitter power for each mode. A firmware update that bumps TX power beyond the granted level is a compliance violation, even by a fraction of a dB. At minimum this triggers a permissive change filing, and it may require full recertification.

Swapping to an untested antenna. The grant covers specific antenna types and gains. A higher-gain antenna changes the EIRP and can push you over limits, even on devices with standard antenna connectors.

Skipping pre-compliance. About half of consumer electronics fail formal EMC testing on the first try. A first-pass failure adds $5,000 to $30,000 and 4 to 12 weeks once you factor in diagnosis, board redesign, and rebooking the lab. Spending $3,000 to $8,000 on pre-compliance testing drops that failure rate below 10%.

Incomplete documentation. TCBs reject applications for missing photos, vague operational descriptions, and user manuals without required compliance statements. The FCC label requirements guide covers what must appear in the manual and on the device. Getting documentation right on the first submission saves 1 to 4 weeks of back-and-forth.

Related guides

Part 15 is the largest piece of FCC compliance for most products, but it's rarely the only piece. These guides cover the steps that come after (or alongside) Part 15 classification:

• FCC certification overview -- the full authorization process, classification through grant issuance
• SDoC vs Certification -- how to choose between the two paths Part 15 feeds into
• FCC certification cost -- what testing actually costs by device type, including hidden costs
• FCC testing timeline -- phase-by-phase timeline from pre-compliance through grant issuance
• Pre-certified modules -- how modular certification under Section 15.212 lets you skip intentional radiator testing
• EMC pre-compliance testing -- in-house testing to catch Subpart B failures before they cost $10,000 at a formal lab

One note on international markets: if you're selling outside the US, the same product likely needs CE marking under the EU's RED and EMC Directive. The test methods overlap -- CISPR 32 (behind EN 55032) aligns closely with Part 15 Subpart B -- but limits, measurement details, and documentation differ enough that passing one does not mean passing the other.

Where to start

Part 15 is not one rule. It's eight subparts across hundreds of pages. The subpart that applies to your product determines which tests you run, which authorization path you follow, how much you pay, and how long it takes.

Get the classification right -- intentional vs unintentional, Class A vs Class B, Subpart C vs Subpart E -- and the rest of the process is mechanical. Get it wrong, and you're looking at wasted lab time, rejected applications, and a timeline that slips by months.

If you're not sure where your product falls, start with our FCC certification hub or use our requirements tool to map your device to the right rule parts.