mstrlucky74
Senior Member
- Location
- NJ
Technically 3 Circuits or 1??
- Thread starter mstrlucky74
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- Location
- Massachusetts
That is one branch circuit.
Yes, a multiwire branch circuit is permitted to be considered multiple circuits, but by default it is one circuit.
- Location
- Massachusetts
We don't know that is a MWBC do we?
Being the circuit is supplied by a 3P30A breaker and the run only has 3 circuit conductors, it cannot be an MWBC.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Since the OP stated that there can be "no more than 3 circuits in a raceway" you could run three of these circuits or 9 CCC's. I would bet that whoever wrote this spec didn't mean it that way.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
You are dealing with an engineer (or other spec writer) who does not pay attention to the words that he or she is putting on paper. The specific example the OP has posted offers a particularly tricky trap, and it is in a large part the fault of the spec writer.
I believe the intent is to limit the number current-carrying conductors in the same conduit to a value that would not create a mismatch between the breaker rating and the wire size. We often see the number 9 used as the limit (or the “magic number”). That works for 20 amp circuits and #12 wire. That also equates to three 3-phase MWBCs, and I believe that to have been the spec writer’s intent.
To belabor what is going to be obvious, the limit of 9 is based on a 70% derating from the #12 AWG’s 90C ampacity of 30, getting a result of 21 amps, which means that a 20 amp breaker still works.
Here’s the problem: The OP’s example uses a 30 amp breaker and #10 wire. For this instance, the “magic number” is 6, not 9. That is based on an 80% derating from the #10 AWG’s 90C ampacity of 40, getting a result of 32 amps, which means that a 30 amp breaker still works.
So if Mr. Lucky had blindly followed the spec’s limit of 3 circuits in a conduit, and if he thereby ran the circuits for three of the dust collection units, each fed by #10 wire and each protected by a 30 amp breaker, he would have had a code violation on his hands.
My company has a standard phrase that goes on power-related floor plans. It grants the contractor free reign to combine multiple circuits into a common conduit. But it imposes on the contractor the responsibility of following the requirements of article 310.
I believe the intent is to limit the number current-carrying conductors in the same conduit to a value that would not create a mismatch between the breaker rating and the wire size. We often see the number 9 used as the limit (or the “magic number”). That works for 20 amp circuits and #12 wire. That also equates to three 3-phase MWBCs, and I believe that to have been the spec writer’s intent.
To belabor what is going to be obvious, the limit of 9 is based on a 70% derating from the #12 AWG’s 90C ampacity of 30, getting a result of 21 amps, which means that a 20 amp breaker still works.
Here’s the problem: The OP’s example uses a 30 amp breaker and #10 wire. For this instance, the “magic number” is 6, not 9. That is based on an 80% derating from the #10 AWG’s 90C ampacity of 40, getting a result of 32 amps, which means that a 30 amp breaker still works.
So if Mr. Lucky had blindly followed the spec’s limit of 3 circuits in a conduit, and if he thereby ran the circuits for three of the dust collection units, each fed by #10 wire and each protected by a 30 amp breaker, he would have had a code violation on his hands.
My company has a standard phrase that goes on power-related floor plans. It grants the contractor free reign to combine multiple circuits into a common conduit. But it imposes on the contractor the responsibility of following the requirements of article 310.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
If the load is 28 amps (40 amps*70%) then he could still go up to the next standard size OCPD which would be 30 amps.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
True, but then the "6" loses its "magic number" quality. It becomes a specific design instance, rather than a general rule.
mstrlucky74
Senior Member
- Location
- NJ
Correct ..I've seen no more than 3 ccc before but this specifically says no more than 3 circuits....which kind of sort of in a way sometimes is the same thing. ...except this instance.
Last edited:
mstrlucky74
Senior Member
- Location
- NJ
....RolandoMir
Member
- Location
- Miami, Fl
Branch Circuit, Multiwire.
A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded con- ductor of the system.
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goldstar
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
This is just my opinion, but when you have a spec that requires a 3-pole breaker it is more than likely a 3-phase system. The chances of it being a MWBC are a shot in a million. It doesn't call for a neutral (current carrying conductor), it calls for a #10 ground wire.
You apparently have not dealt much with commercial lighting systems. MWBC's are commonplace for 277V lighting on a 480Y/277V 3Ø 4W system, or even 120V lighting on 208Y/120V 3Ø 4W system.
goldstar
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
I suppose you may be correct but then wouldn't the spec be different ? The spec in the OP called for a 3-p 30A breaker for a dust collection unit. What are the chances that's a MWBC ?
- Location
- Illinois
- Occupation
- retired electrician
But, at least around here, it would be much more common to see three single pole breakers with handle ties than to see a 3 pole breaker for a multi-wire branch circuit.
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