Unbalanced Alternating Current

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Michael15956

Senior Member
Location
NE Ohio
I understand the principles of current canceling out in the neutral and only the unbalanced current is returned to the transformer. Well, maybe I think I understand. So, in circuits without a lot of harmonics the neutral can be reduced in size proportional with the phase conductors. Now, I'm trying to imagine this current flow. I believe I understand the current flow in a single phase 240v circuit with just two phase conductors and no neutral. The AC comes from one phase conductor and returns through the other phase conductor and then reverses itself 60 times per second. I imagine this as current starting out in one phase conductor, I will name L1, going through a load and then back to the transformer through L2. Then going back to the load through L2 and returning back to the transformer through L1, and then reversing itself again 60 times a second.

Now, I was also told that this is exactly what happens in a 120V load. The same reversing at 60 times per second, but in this circuit it is the neutral conductor not another phase conductor. So in a 120V unbalanced load situation with two or more circuit branches is the cancellation occurring in the neutral where only the unbalanced load returns back to the transformer and the rest of the current returns back to the load through the neutral conductor?

Second question: If the above is correct is this the reason why in T250.66 the code allows the neutral to be reduced in size proportionally to the phase conductors?

TIA
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
You have the basics down right, especially in your description of alternating current passing through a 240 volt load that has no neutral wire.

Consider, now, a single load, powered by Phase A from a 120/240 volt single phase panel. The current paths are exactly as you described. During one half cycle, current goes from the panel, along L1, through the load, and back to the panel via N1 (the neutral wire for this load). During the other half cycle, current goes from the source to the load via N1, and returns to the panel via L1.

Next, consider a second, identical load, this time powered by Phase B. The current patterns are the same as above, but lets call the conductors L2 and N2.

First, let?s consider these two as being a Multi-Wire Branch Circuit (i.e., they share a neutral ? N1 and N2 are the same wire). What happens is that when the Phase A load is on its first half (i.e., current leaves on L1 and returns on N1), the Phase B load is on the opposite half cycle (i.e., current leaves on N2 and returns on L2). As a result, the currents in N1 and N2 (actually, these are the same wire, for this example), cancel each other out. So for all the world, it looks like current is leaving the panel on L1, passing through each of the two loads in series, and then returning to the panel on L2. A half cycle later, current leaves the panel on L2, passes through the two loads, in series but in the other order, and returns to the panel on L1.

Next, let one of the two loads be higher than the other, so that the two load currents are not the same. Currents in N1 and N2 are still opposite, but not identical. Thus, there will be a net current in the neutral. It is in this sense that the neutral wire is only carrying the unbalanced current.

Finally, let?s separate the two circuits, by giving each its own neutral wire. As far as each separate branch circuit is concerned, its neutral wire carries the same current as its phase conductor. The currents in N1 and N2 do not cancel each other out, because they are separate wires, and each carries its own current. However, look at it from the perspective of the panel. It sees current coming in from N1 at the same time that it sees current leaving on N2. So the neutral wire within the feeder to the panel views these two currents as having cancelled each other out. That is why, when we calculate the conductor size for a panel feeder, we sometimes get to size the neutral wire smaller than the phase conductors.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
That many of our loads are line-to-line is what really allows reduced neutral sizing. Those are the only loads that can 'guarantee' that they would never contribute to neutral loading (except during a fault, of course.)

If every circuit of a service was line-to-neutral only, even shared-neutral circuits (MWBC's) could just so happen to have only loads on one phase energized at a given moment, and should have a full-sized neutral.

Here's a little ditty I posted a while back; see if it helps: http://forums.mikeholt.com/showpost.php?p=708650&postcount=4
 

Michael15956

Senior Member
Location
NE Ohio
Thanks Charlie & Larry,

Now I have a good image of how this current flows which leads to another question.

T250.66 for reduced neutrals.

Putting harmonics aside for this question, I have a hypothetical question. A single phase installation with mostly 120V loads and no MWBC. Can this table be used for the sizing of the neutral? Also, would this be a good install?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Michael15956 said:
T250.66 for reduced neutrals.

Putting harmonics aside for this question, I have a hypothetical question. A single phase installation with mostly 120V loads and no MWBC. Can this table be used for the sizing of the neutral? Also, would this be a good install?

Table 250.66 is for sizing GEC's as well as neutral minimums, but does not take minimum neutral-load sizing requirements into consideration. And the reference to 250.24(B) that refers to the utility neutral brought to the disconnect is for the all-important fault-current return pathway.

You should peruse 220.22 ('02 NEC, at least). The neutral load demand must be calculated by determining the maximum possible line-to-neutral current from either line conductor. Pretend you energize every 120v load on whichever line has the greater connected load, and none on the other.
 

Michael15956

Senior Member
Location
NE Ohio
LarryFine said:
You should peruse 220.22 ('02 NEC, at least). The neutral load demand must be calculated by determining the maximum possible line-to-neutral current from either line conductor. Pretend you energize every 120v load on whichever line has the greater connected load, and none on the other.

I think I got this?

Say, if for example there is 120amps possible for one side of the panel line-to-neutral current and the service supply neutral is only rated for 100amps then it is undersized for that possibility. Line L1 has 6 CB's each rated 20amps and there is a possibility of all of them drawing 20amps and the neutral is only sized at 100amps.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Michael15956 said:
I think I got this?

Say, if for example there is 120amps possible for one side of the panel line-to-neutral current and the service supply neutral is only rated for 100amps then it is undersized for that possibility. Line L1 has 6 CB's each rated 20amps and there is a possibility of all of them drawing 20amps and the neutral is only sized at 100amps.
The breaker handle total, and even the possible current total, may well be higher than the service rating. If the load actually gets that high, the main breaker will trip, which is what it's there for.

As long as the service conductors are protected by the main, and the neutral is likewise so rated, all is well. The neutral is protected by the main breaker, since it carries, at most, the same current.
 

Michael15956

Senior Member
Location
NE Ohio
Larry,

Hypothetically here: Say a service is 100% single phase with all circuits installed as 240V, two wire only, there would be no need for any neutral, right?

Now, wouldn't this install be OK too? Single phase residential service 200amps 2/0 copper phase feeders with #4 copper neutral if there were only five 20amps line-to-neutral circuits.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Michael15956 said:
Hypothetically here: Say a service is 100% single phase with all circuits installed as 240V, two wire only, there would be no need for any neutral, right?
Correct, beyond the main disconnect. The supply system grounded conductor must still be brought to the service equipment, so that the service grounding system can be bonded to it.

Now, wouldn't this install be OK too? Single phase residential service 200amps 2/0 copper phase feeders with #4 copper neutral if there were only five 20amps line-to-neutral circuits.
I'd have to say yes, that would be compliant. The minimum size is based on 250.66 and the calculated possible neutral current. But, future additions could easily make it not so.


I recently did a multiple-service mixed-use building, and could have used #2 cu neutral conductors, but since the driven rods were the only electrode, I opted to use 1/0 cu instead.
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
LarryFine said:
Table 250.66 is for sizing GEC's as well as neutral minimums,

I saw this in another thread. Where does this table or it's notes refer to the grounded conductor?
 

Michael15956

Senior Member
Location
NE Ohio
chris kennedy said:
It's 250.24(C)(1)

You could easily be censored for posting false code referances. Is that what you want Larry? Is it?

















:grin::grin::grin:

Hey, quit picking on Larry! He has been a big help for me!:grin:
 

Michael15956

Senior Member
Location
NE Ohio
LarryFine said:
Correct, beyond the main disconnect. The supply system grounded conductor must still be brought to the service equipment, so that the service grounding system can be bonded to it.

I'd have to say yes, that would be compliant. The minimum size is based on 250.66 and the calculated possible neutral current. But, future additions could easily make it not so.


I recently did a multiple-service mixed-use building, and could have used #2 cu neutral conductors, but since the driven rods were the only electrode, I opted to use 1/0 cu instead.

I understand Larry, just wanted to have a firm understanding of T250.66. Thanks
 
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