1 Phase Cicuit Breakers on 3 Phase Motors

[fifty60]

If I would want to use a circuit breaker as an OCPD for a 3 phase motor, does the NEC require that I use a 3 phase disconnect, or can I combine 3 single phase disconnects (see Eaton's WMZT line for an example)? The specific wording I am trying to find in the NEC is that all 3 ungrounded conductors must be disconnected at the same time. I know it is a good idea to have them all trip in order to prevent single phasing, but if I already have a Phase monitor then there is no real advantage there.

 

[kwired]

If I would want to use a circuit breaker as an OCPD for a 3 phase motor, does the NEC require that I use a 3 phase disconnect, or can I combine 3 single phase disconnects (see Eaton's WMZT line for an example)? The specific wording I am trying to find in the NEC is that all 3 ungrounded conductors must be disconnected at the same time. I know it is a good idea to have them all trip in order to prevent single phasing, but if I already have a Phase monitor then there is no real advantage there.

Generally speaking multiwire line to neutral loads can utilize handle ties but line to line loads, both two and three pole, typically will require a common trip breaker.

 

[Smart $]

Assuming we're talking about one or more branch circuit breakers for a single motor, 430.56 says the protective devices must comply with 240.15.

240.15(B) says such breaker must open all ungrounded conductors, both manually and automatically unless otherwise permitted [thereunder]... and (B)(3) is the only one which may apply for a 3? motor.

(3) 3-Phase and 2-Phase Systems. For line-to-line loads
in 4-wire, 3-phase systems or 5-wire, 2-phase systems, individual
single-pole circuit breakers rated 120/240 volts ac
with identified handle ties shall be permitted as the protection
for each ungrounded conductor, if the systems have a
grounded neutral point and the voltage to ground does not
exceed 120 volts.

 

[wchild1520]

And yet I have motor control panels that use fuses instead of breakers and I know from experience that all three fuses don't go at the same time.

 

[GoldDigger]

And yet I have motor control panels that use fuses instead of breakers and I know from experience that all three fuses don't go at the same time.

And for that reason, large expensive motors that are protected by fuses often have separate phase loss detection downstream of the fuses that opens the rest of the conductors anyway.

 

[tom baker]

Single pole breakers with handle ties are not allowed on 277 V breakers per the UL listing and this is now in the 2011 NEC

 

[kwired]

Single pole breakers with handle ties are not allowed on 277 V breakers per the UL listing and this is now in the 2011 NEC

Are you referring to 240.15(B)(2)?

240.15 Ungrounded Conductors.

(A) Overcurrent Device Required. A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit.

Informational Note: For motor circuits, see Parts III, IV, V, and XI of Article 430.

(B) Circuit Breaker as Overcurrent Device. Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 240.15(B)(1), (B)(2), (B)(3), and (B)(4). See related UL
(1) Multiwire Branch Circuits. Individual single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multiwire branch circuits that serve only single-phase line-to-neutral loads.


(2) Grounded Single-Phase Alternating-Current Circuits. In grounded systems, individual single-pole circuit breakers rated 120/240 volts ac, with identified handle ties, shall be permitted as the protection for each ungrounded conductor for line-to-line connected loads for single-phase circuits.

If so it sure appears to me that is talking about single pole breakers rated 120/240 volts being permitted to supply line to line loads if used with handle ties, where before you would have had to use a two pole breaker.

Unless there is something elsewhere I think (B)(1) above would allow 277 volt single pole breakers to supply MWBC's as long as handle ties are used. They would not be allowed to be handle tied and supply 480 volt loads, first they are not rated for 480 volts, second they would need to be a common trip unit.