tom kanzler
Member
- Location
- New York
I have a question about multiwire branch circuits that I can?t find any specific information on, here or elsewhere.
For a single-phase, 120/240V MWBC serving both 120V and 240V loads, does the OCPD have to automatically and simultaneously open both ungrounded conductors?
From 2002 NEC:
210.4 Multiwire Branch Circuits.
(C) Line-to-Neutral Loads Multiwire branch circuits shall supply only line-to-neutral loads.
Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.
From a later NEC, lifted from a thread on this forum, originally posted by Charlie Eldridge (my apologies if it violates protocol)
http://www.mikeholt.com/code_forum/showthread.php?t=62569&page=2&highlight=multiwire
240.20(B) Circuit Breaker as Overcurrent Device. Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 240.20(B)(1), (B)(2), and (B)(3).
(2) Grounded Single-Phase and 3-Wire dc Circuits. In grounded systems, individual single-pole circuit breakers with identified handle ties shall be permitted as the protection for each ungrounded conductor for line-to-line connected loads for single-phase circuits or 3-wire, direct-current circuits
The confusion comes in where, while it seems 240.20(B)(2) allows approved (or identified) handle ties for single-pole breakers serving 240V loads, 210.4(C) exception 2 seems to require that, for MWBC's specifically, the OCPD automatically open both ungrounded conductors. That sounds like automatic, as well as manual, though it doesn't say that. Does 210.4(C) ex 2 supercede 240.20(B)(2), or am I reading it wrong? Handle ties won't necessarily open the other breaker, especially with free-trip mechanisms.
The problem, as I see it, with not opening both ungrounded conductors when both line-to-neutral and line-to-line loads are being served is that the 240V load(s) could backfeed into the 120V loads that were supposedly disconnected when the OP for those line-to-neutral loads opened.
Can anyone shed any light on this. I?m trying to clarify exactly when handle ties are allowed, and when they are not.
p.s. I know, it's time to upgrade my Code books.
For a single-phase, 120/240V MWBC serving both 120V and 240V loads, does the OCPD have to automatically and simultaneously open both ungrounded conductors?
From 2002 NEC:
210.4 Multiwire Branch Circuits.
(C) Line-to-Neutral Loads Multiwire branch circuits shall supply only line-to-neutral loads.
Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.
From a later NEC, lifted from a thread on this forum, originally posted by Charlie Eldridge (my apologies if it violates protocol)
http://www.mikeholt.com/code_forum/showthread.php?t=62569&page=2&highlight=multiwire
240.20(B) Circuit Breaker as Overcurrent Device. Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 240.20(B)(1), (B)(2), and (B)(3).
(2) Grounded Single-Phase and 3-Wire dc Circuits. In grounded systems, individual single-pole circuit breakers with identified handle ties shall be permitted as the protection for each ungrounded conductor for line-to-line connected loads for single-phase circuits or 3-wire, direct-current circuits
The confusion comes in where, while it seems 240.20(B)(2) allows approved (or identified) handle ties for single-pole breakers serving 240V loads, 210.4(C) exception 2 seems to require that, for MWBC's specifically, the OCPD automatically open both ungrounded conductors. That sounds like automatic, as well as manual, though it doesn't say that. Does 210.4(C) ex 2 supercede 240.20(B)(2), or am I reading it wrong? Handle ties won't necessarily open the other breaker, especially with free-trip mechanisms.
The problem, as I see it, with not opening both ungrounded conductors when both line-to-neutral and line-to-line loads are being served is that the 240V load(s) could backfeed into the 120V loads that were supposedly disconnected when the OP for those line-to-neutral loads opened.
Can anyone shed any light on this. I?m trying to clarify exactly when handle ties are allowed, and when they are not.
p.s. I know, it's time to upgrade my Code books.