AC Theory/ Unbalanced Current

Josh White

I've always been told that all unbalanced current goes to ground, either in the form of the grounded or grounding conductor. But I have also been told that a circuit will only draw as much current as will be consumed by the resistance of that circuit. So why is there an unbalanced current? Does the circuit actually draw more amperage than the devices (or resistors or whatever) can use?

ConcreteGuy

My son is studying electrical engineering. I don't understand him.

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A reasonable expectation becomes an assumption when it goes wrong.

toastermaker

Hello

Someone straighten me out. Is this how it would go?

In a simple 120 volt circuit it would often be accidentally mixing the neutrals from two circuits that causes an imbalance

Basically the amps returning on the neutrals from both circuits would combine, and then split pretty much evenly and return half the total on each neutral.

If you have some lights on one circuit that are drawing 3 amps but the neutral is mixed with another circuit before it gets back to the panel then there would be a 1.5 amp imbalance if the "other circuit" that the neutral was mixed with had no draw on it at the time.
If the "other circuit" had a motor running on it that was drawing 9 amps then you would get a 3 amp imbalance on the light circuit and a 3 amp imbalance on the motor circuit.

I too am trying to understand this. I have tried to apply some of what I have been reading to this question and would like to know if I got it right.

Also, three phase and advanced stuff is greek to me, I am not ready for that myself yet. If you speak of it my brain will overload.

toastermaker

RonH

Most 120 volt circuits are derived form a 240 volt center tapped transformer. At any given time one end of the transformer is 120 volt positive, the other end is 120 volt negative. The center tap is 120 volts from either one. This polarity reverse 60 times a second. If there is 8 amps on the left side, and 10 amps on the right side, and if the neutrals from each circuit are both tied to the center tap, the amount they are equal (8 amps) cancel each other out. The remaining 2 amps from the right side (10 amps) goes through the neutral as the un-balanced current. Look at the power line coming to your home and you will see three wires. The insulated ones are 240 volt from one to the other, and the bare one (messenger support) is the neutral.
Ronh :Thumbs:

shopdust

OK Ron, help out another newbie. Lets say one had a 220 circuit (that two 110v lines) running to his saw which is 220 v. But before the juice reaches the saw, someone where to tap one of those 110 volt legs to run something like like a 5 amp drill? (Assuming the nuetral is the same used for the 220v system.
Any harm to 220v saw? What? Notify next of kin? Thanks in advance!!!!

mdshunk

No harm, and it's normal. One hot leg will carry the load of the saw plus 5 amps, the other hot leg will carry the load of the saw only, and the neutral will carry just the 5 amps.

What you've described is normal for appliances that are 120/240 rated right on the dataplate such as your clothes dryer and your electric range. They both have (should have) 4 prong cords. That's two hots, a neutral, and a ground. In your dryer, for instance, the heating element runs on 240, and the motor runs on 120. In your stove, the bake and broil elements run on 240, and the stove top burners are often only 120 (althought they could be 240). The lights, buzzers, clocks, etc. on the control panel are 120.

Swimming pools are often set up in the manner of the table saw and drill arrangement you described. The electrician may run two hots and a neutral out to the pool. The pool pump often runs on the 240, and a regular 120 volt receptacle is tapped off one pump hot leg and the neutral for the miscellaneous other pool crap.

shopdust

Many thanks!!!

toastermaker

Yes, thank you all very much.