Voltage Drop for a Multiwire Branch Circuit

[barone]

Hello all, and thanks for looking in on this.

I have a 3 phase-4wire multi wire branch circuit that originates in a 120/208V-WYE panelboard. There are several of these and they are all quite long. I have been calculating voltage drop for the conductors and have been doing so based on the circuit being a 3-phase circuit, unbalanced. Is this correct? Or should I be calculating Vd for each individual 120V circuit as though they were all 120V single phase loads. I assume that the formula for 3-phase, 4-wire balanced circuits is out of the question, but my gut is telling me that since all 3 phases are in a single cable assembly with the shared neutral (12/4), I can treat this as a 3-phase circuit and use the 3-phase Vd formula.

My brain stopped working about 10 mins ago so forgive me if the question is a stupid one.

 

[charlie b]

If the unbalance is severe, then that is a problem in itself, and should be corrected. If the unbalance is not severe, then disregard it. Treat it as a 3-phase balanced load, and go from there.

 

[barone]

Thanks so much, that's what I thought also.

 

[LarryFine]

Basically, any neutral current will be current not seen by one (or more) line conductors, and the same applies to any neutral voltage drop.

 

[Smart $]

... I can treat this as a 3-phase circuit and use the 3-phase Vd formula.

...

Depends. As mentioned, if balanced, yes you can treat it as a 3? circuit. However, in doing so, you are assuming (rather we are assuming, you have determined) the loads are always coincident (when operating normally)... meaning the 120V loads are either all on or all off. Yet if this is the normal condition of operation, you could actually treat the three circuits as one-wire "circuits"... meaning calculate as though single phase but do not multiply the one-way length times two. Your circuit voltage drop is only from the breaker to the load end neutral junction.

Click on this link, scroll down the page, and use the calculator entering both 208V 3? 3W and 120/208 3? 4W (one at a time, of course) with all other conditions the same. The former Vd will be 1.732 higher than the latter...

 

[steve66]

I agree with Charlie unless these are receptacle loads.

If they are receptacle loads, then the unbalance will be unpredictable, & I would tend to treat them as single phase loads. (Although I'm not sure I would insist on limiting the drop to 3%).

 

[Smart $]

I agree with Charlie unless these are receptacle loads.

I agree concerning receptacle loads...

But if we have three dedicated circuits with single-phase loads on a mwbc, we are concerned with voltage drop across the line-to-neutral-connected loads. Calculating as if a 3? circuit would be determining the voltage drop across all the loads.