- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
We recently had a thread where we mentioned that a neutral must be sized at 173% when harmonics are present. I understand that this is based on 1.73 that we use for 3 phase but I am curious what are the factors that go into determining what the load on that neutral is going to be.
For instance, we have a 3 phase load and all the circuits of the MWBC are loaded to 15 amps. We use the formula to calculate neutral current. In excel I would use this formula =SQRT(((A5*A5)+(B5*B5)+(C5*C5))-((A5*B5)+(B5*C5)+(A5*C5))). Now do we use 173% for the neutral on the branch circuits as well. I have never seen this done before and normal if the circuit is balanced the load on the neutral is 0. With a 16 amp load on all 3 phases the worst scenario is 16 amps on the neutral if two circuits are turned off. This would mean 27+ amps on the neutral. Is this correct or is this dependent on the ballast pf or some other factors.
Certainly with the 2008 code we have art. 210.4 that virtually will require all circuits to be deenergized at once. So is this really an issue for branch circuits? Have any of you installed #10 neutral on these circuits with #12 ungrounded conductors-- talking lighting not receptacle loads.
For instance, we have a 3 phase load and all the circuits of the MWBC are loaded to 15 amps. We use the formula to calculate neutral current. In excel I would use this formula =SQRT(((A5*A5)+(B5*B5)+(C5*C5))-((A5*B5)+(B5*C5)+(A5*C5))). Now do we use 173% for the neutral on the branch circuits as well. I have never seen this done before and normal if the circuit is balanced the load on the neutral is 0. With a 16 amp load on all 3 phases the worst scenario is 16 amps on the neutral if two circuits are turned off. This would mean 27+ amps on the neutral. Is this correct or is this dependent on the ballast pf or some other factors.
Certainly with the 2008 code we have art. 210.4 that virtually will require all circuits to be deenergized at once. So is this really an issue for branch circuits? Have any of you installed #10 neutral on these circuits with #12 ungrounded conductors-- talking lighting not receptacle loads.