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electricalist

electricalist

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For branch circuit's like offices 4 plugs 2 3 lamp 2x4 lights and everything is 120 v. Ivebeen reading about derating and Iim trying to see if im learning or getting confused. 3 hots a b c phase & 1 neutral are ideal for plugs . If i got it right this is 3 current carying conductors? The neutral being balanced. . If Iit were 1 hot and 1 neutral it is unbalanced. For derating this is 2 ccc. What if its 2 hots 1 neutral a & b phase. If a full boat is a theoretically better combination. I think for derating. 3 full boats in a conduit will keep me code compliant for a 20 amp circuit #12 wire and 20 amp breaker. Next its the linear and non. Linear being things with controlled voltage at the equipment electronic ballast vfd. Non would be outlets. Non can be a problem do to 3 rd harmonic adding to heat problems on the neutral. Should not mix them . Keep plus and lights seperate.
 
Here's a breakdown of when the neutral is considered a CCC:

Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:

3?- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1?- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
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Rob's information quoted explains it fairly well. One thing it kind of leaves out that is maybe assumed is the fact that when a conductor is carrying current it produces some heat. When neutral is only carrying unbalanced current it is not contributing any more heat to a raceway or cable interior then the other circuit conductors it is balancing are contributing.

Take a simple single phase three wire MWBC with 10 amps on L1 and 10 amps on L2. Neutral is carrying zero - only getting heat from two conductors. Now load same circuit with 10 amps on L1 and 5 amps on L2 - neutral will have 5 amps of current in this case. But heating effects of L2 is split over two conductors so the net heating effect of all conductors in the raceway is still same as for two conductors.
 
So if im trying to make derating work for me and effectively install mwbc @ 20 amps. I should / could run 9 circuits 135 /n 7911/n131517/n keeping each 4 wire set either as plug circuits or 120v lighting but dont mix them when possible. . Is this correct? Thank you very much.
 
electricalist said:
So if im trying to make derating work for me and effectively install mwbc @ 20 amps. I should / could run 9 circuits 135 /n 7911/n131517/n keeping each 4 wire set either as plug circuits or 120v lighting but dont mix them when possible. . Is this correct? Thank you very much.
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The only real issue is whether you have harmonic loads which will make the neutral count as a CCC.
 
Not sure you could justify a balanced load on a multicircuit general type recept loads -- you have no control on what phase is loaded or out of balance which would make the grounded conductor a CCC.
 
mwm1752 said:
Not sure you could justify a balanced load on a multicircuit general type recept loads -- you have no control on what phase is loaded or out of balance which would make the grounded conductor a CCC.
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The whole idea of not counting the neutral in an MWBC is that even at the worst possible imbalance, you would have the neutral carrying full current, but one of the line conductors would then be carrying no current. The end result is that for the extremes and everywhere in between the heating is the same as for two conductors with full load current in a 120/240 and the same as three wires with full load current in a 208Y/120.
 
So im ok within my statement. I pay attention when im going to different jobs. Ive seen many different ways it seems when ive found quality workmanship panels located in a good place labeled in a orderly way and mwbc that arent ovetloaded it was much like I am trying to understand. Its my belief I had to understand it before I can apply it.
 
GoldDigger said:
The whole idea of not counting the neutral in an MWBC is that even at the worst possible imbalance, you would have the neutral carrying full current, but one of the line conductors would then be carrying no current. The end result is that for the extremes and everywhere in between the heating is the same as for two conductors with full load current in a 120/240 and the same as three wires with full load current in a 208Y/120.
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Just sayin -- A-15A, B-3A, C-7A, N-approx 5A = 4 CCC Appliance multi branch circuits Dryer, Range, Ovens, Motors, are more balanced by design -- what defines current carrying? a percentage of max load? minimal current?
 
mwm1752 said:
Just sayin -- A-15A, B-3A, C-7A, N-approx 5A = 4 CCC Appliance multi branch circuits Dryer, Range, Ovens, Motors, are more balanced by design -- what defines current carrying? a percentage of max load? minimal current?
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When you add up the heat created in each of the four conductors, you see that although four conductors actually carry current, the heat produced is no greater than you would get from a maximum load on three conductors.
The count of CCCs to use for derating is not the same as the number of Cs Cing C.
 
For commercial and industrial, I believe it is best to always consider the neutral as current carrying due to inherent imbalances, now or in the future. Never mix lights and receptacles.
 
kingpb said:
For commercial and industrial, I believe it is best to always consider the neutral as current carrying due to inherent imbalances, now or in the future. Never mix lights and receptacles.
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There us a world of difference between trying to downsize a neutral because it will only carry imbalance current (not possible if the load is not well known and controlled or diverse enough for statistics to apply) and reducing the CCC count for an MWBC. The only argument against the latter is the possibility of non linear loads, not simply unbalanced loads.
 
GoldDigger said:
There us a world of difference between trying to downsize a neutral because it will only carry imbalance current (not possible if the load is not well known and controlled or diverse enough for statistics to apply) and reducing the CCC count for an MWBC. The only argument against the latter is the possibility of non linear loads, not simply unbalanced loads.
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As I understand more about wiring methods and their application. I wonder how the orginal electricians knew the load type requirements. If its because new they had this info then that sounds reasonable. Till new tenants move in . Then there is trouble , possibly. What would you say is the best approach for new offices with no requirement . To be rented. A to keep cost reasonable
B to keep labor reasonable
C to keep system versatile . I do understand if you or all pass on any type of answer. I realize this info is hard earned
 
I agree with gold digger. We are talking about simple MWBC for lights and plugs. There need not be any worry about load and balance.

Load up A phase to twenty amps. What is the current on the noodle?
Add a 18A load to B phase and a 2A load to C phase. Is the noodle in danger? No. It only sees the imbalance.
 
electricalist said:
As I understand more about wiring methods and their application. I wonder how the orginal electricians knew the load type requirements. If its because new they had this info then that sounds reasonable. Till new tenants move in . Then there is trouble , possibly. What would you say is the best approach for new offices with no requirement . To be rented. A to keep cost reasonable
B to keep labor reasonable
C to keep system versatile . I do understand if you or all pass on any type of answer. I realize this info is hard earned
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I have not done super large office spaces, but from what I have seen the bigger concern with MWBC has come when a few code cycles ago they wanted common trip or handle ties - nobody wants to have to shut down two or three circuits to work on one in a busy office, and they have gone away from using MWBC because of that rule. Leaving that aside though I have not seen overloading effects of harmonics on neutral conductors in your typical office setting. Maybe you will see it in areas that have continuous high loading on individual circuits but not on general use receptacle circuits that have a big load diversity in general. I would suggest full size neutrals for feeders and services or even oversized neutrals here if high harmonics are expected, but for a typical office I don't see too much trouble with a 12 AWG neutral for a 20 amp MWBC feeding general purpose receptacles. If you are running MWBC's for lighting, you may want to think about it a little harder, but also consider total connected load - just because you have a 20 amp breaker on a particular circuit doesn't mean you actually have 20 amps of load connected to it but if you are heavy on the load you may want to consider harmonic effects and possible larger neutrals.

JMO.
 
Nice. in 98 imo it was about know the needs and downsizing was more common. I personally have never done it. Mostly because I couldnt honestly so it was ok to.
 
GoldDigger said:
There us a world of difference between trying to downsize a neutral because it will only carry imbalance current (not possible if the load is not well known and controlled or diverse enough for statistics to apply) and reducing the CCC count for an MWBC. The only argument against the latter is the possibility of non linear loads, not simply unbalanced loads.
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Well said, :thumbsup: I was thinking non-linear loads but did not write it.:dunce:
 
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