46 CFR 143.565: Overcurrent protection for power and lighting circuits
Last reviewed: October 14, 2026
This section requires suitable overcurrent protection on towing vessel power and lighting circuits. It sets how cable and wiring protection is rated, how devices are installed and coordinated, and covers transformers. It also bars essential and non-essential systems from sharing a circuit or protective device, except on excepted vessels.
Applies toPower and lighting circuits on towing vessels.
What it says
Power and lighting circuits on towing vessels must be protected by suitable overcurrent protection.
46 CFR 143.565(a)“Power and lighting circuits on towing vessels must be protected by suitable overcurrent protection.”
Overcurrent protection for cable and wiring in power and lighting circuits must open the circuit at the standard setting closest to 80 percent of the manufacturer's listed ampacity.
46 CFR 143.565(b)(1)“must have overcurrent protection that opens the circuit at the standard setting closest to 80 percent of the manufacturer's listed ampacity.”
Setting exceptions allowed by NFPA's National Electrical Code (NEC), Article 240, may be used.
46 CFR 143.565(b)(1)“Overcurrent protection setting exceptions allowed by NFPA's National Electrical Code (NEC), Article 240”
If the manufacturer's listed ampacity is not known, the tables referenced in Article 310.15(B) of the NEC must be used, assuming a temperature rating of 75 °C and an assumed temperature of 50 °C for machinery spaces and 40 °C for other spaces.
46 CFR 143.565(b)(2)“assuming a temperature rating of 75 °C and an assumed temperature of 50 °C for machinery spaces and 40 °C for other spaces.”
Overcurrent protection devices must be installed so they do not open the path to ground in a circuit, and only ungrounded conductors must be protected.
46 CFR 143.565(b)(3)“Overcurrent protection devices must be installed in a manner that will not open the path to ground in a circuit; only ungrounded conductors must be protected.”
Overcurrent protection must be coordinated so the circuit breaker or fuse nearest to the fault clears an overcurrent situation.
46 CFR 143.565(b)(3)“Overcurrent protection must be coordinated such that an overcurrent situation is cleared by the circuit breaker or fuse nearest to the fault.”
Each transformer must have overcurrent protection that meets Article 450 of the NEC.
46 CFR 143.565(b)(4)“Each transformer must have protection against overcurrent that meets Article 450 of the NEC”
On a towing vessel other than an excepted vessel as defined in § 136.110 of this subchapter, essential and non-essential systems must not be on the same circuit or share the same overcurrent protective device.
46 CFR 143.565(b)(5)“essential systems and non-essential systems must not be on the same circuit or share the same overcurrent protective device.”
Common questions
What setting must overcurrent protection on towing vessel wiring have?
It must open the circuit at the standard setting closest to 80 percent of the manufacturer's listed ampacity. Exceptions allowed by NEC Article 240 may be used.
“must have overcurrent protection that opens the circuit at the standard setting closest to 80 percent of the manufacturer's listed ampacity.”
What if the manufacturer's listed ampacity is unknown?
The tables referenced in Article 310.15(B) of the NEC must be used. They assume a temperature rating of 75 °C, with 50 °C for machinery spaces and 40 °C for other spaces.
“assuming a temperature rating of 75 °C and an assumed temperature of 50 °C for machinery spaces and 40 °C for other spaces.”
Can essential and non-essential systems share a circuit on a towing vessel?
No, except on an excepted vessel as defined in § 136.110 of this subchapter. Otherwise they must not be on the same circuit or share the same overcurrent protective device.
“essential systems and non-essential systems must not be on the same circuit or share the same overcurrent protective device.”
The text of 46 CFR 143.565
Electronic Code of Federal Regulations (eCFR), current as of April 16, 2026 · Read it on eCFR
Electrical overcurrent protection other than generators and motors
(a) General requirement. Power and lighting circuits on towing vessels must be protected by suitable overcurrent protection.
(b) Specific requirements. (1) Cable and wiring used in power and lighting circuits must have overcurrent protection that opens the circuit at the standard setting closest to 80 percent of the manufacturer's listed ampacity. Overcurrent protection setting exceptions allowed by NFPA's National Electrical Code (NEC), Article 240 (incorporated by reference, see § 136.112 of this subchapter) may be employed.
(2) If the manufacturer's listed ampacity is not known, tables referenced in Article 310.15(B) of the NEC (incorporated by reference, see § 136.112 of this subchapter) must be used, assuming a temperature rating of 75 °C and an assumed temperature of 50 °C for machinery spaces and 40 °C for other spaces.
(3) Overcurrent protection devices must be installed in a manner that will not open the path to ground in a circuit; only ungrounded conductors must be protected. Overcurrent protection must be coordinated such that an overcurrent situation is cleared by the circuit breaker or fuse nearest to the fault.
(4) Each transformer must have protection against overcurrent that meets Article 450 of the NEC (incorporated by reference, see § 136.112 of this subchapter).
(5) On a towing vessel, other than an excepted vessel as defined in § 136.110 of this subchapter, essential systems and non-essential systems must not be on the same circuit or share the same overcurrent protective device.
Sections it refers to
The plain-English explanation on this page was written with AI from the regulation text above and checked automatically before publishing: every quote appears in the text, every number comes from it, and a second model confirmed each statement adds nothing the text doesn't say. Regulation text current as of April 16, 2026. The regulation text governs. RegKnot is not affiliated with the U.S. Coast Guard.