Resi. 240v center tapped transformer 2

[WhyDoYouAsk]

Would you say that,
if the 120v on one side of the neutral connection increases by any amount,
because of a sudden 120v load (let's say 10A) on the other side of the neutral,
the neutral connection is necessarily bad (high resistance connection)?

 

[ScottyUK]

Sounds like a good bet.


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If we learn from our mistakes I'm getting a great education!

 

[opmgr1]

Does this 120V load has a ground that is tied into the neutral? is this ground ok? if this is the case then there shouldn't be a significant increase in the voltage across the load. While you are clearing corroded joints on the neutal it pays to also look at the grounding at the affected equipment.

 

[WhyDoYouAsk]

Let's say no grounds anywhere. That's for a later question! Baby steps for now.

 

[opmgr1]

No grounds anywhere and neutral joints are corroded, then 240V will drop across these 120V loads (equipment failure).

 

[waross]

This is a classic case of voltage rise. It is caused by an unbalanced load and a long neutral. The neutral current causes a voltage drop in the neutral. But this voltage drop must be added to the line voltage vectorily.
With no load on a 120/240 V single phase three wire circuit the voltages may be 120 volts and 120 volts.
The line conductors are the same size or gage as the neutral conductor.
Now put a load on one side of the circuit. For the sake of example say that the load results in a 10 volt voltage drop and the voltage across the load is now 110 volts. This 10 volt drop will be 5 Volts on the line conductor and 5 Volts on the neutral conductor. The 5 Volts on the neutral willadd to the voltage on the other side rather than subtracting. The load voltage will be 110 volts as stated and the open circuit voltage on the other line willbe 125 volts.
Scotty, The neutral designation in North America is not the same as the neutral designation in the UK.
Single phase, we designate the center tap as the neutral. This situation is common in long residential circuits in North America, but it is not often noticed. In Canada, split receptacles were required in kitchens for many years. These were a receptacle fed from a 2 pole breaker with a common neutral. Each individual plug of the duplex receptacle was the normal 120 volts, and there was a commo neutral.
If the kitchen was a long way from the panel and a lamp was plugged into one plug, and then a heavy load such as a toaster or electric kettle was plugged into the other side, the lamp would get brighter. Nothing is wrong but the effect is not often noticed.
Now if the poster is from the UK, I am all wrong.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[opmgr1]

waross,
You are right and on a lot of occassions the Voltage rise on the otherside of the center tap is not noticable. However the scenario that you painted would not be alltogether applicable in this region as for a house-hold the 240V cct. is split. The outlets are on one "leg"and lightning on the other. The effect of plugging in a large load will result in the lights dimming. If you lose the neutral connection and your local ground is poor, this is the time that the lights glow brighter.

 

[WhyDoYouAsk]

Here's my spreadsheet on this. I'm from MD.

Ia, Ib are the currents circulated due to V1, V2, respectively
They both go through Rn, the current through Rn being Ia - Ib.
120.0 ent V1, volts
120.0 ent V2, volts
0.1 ent Rn, ohms
40.0 ent R1, ohms
20.0 ent R2, ohms
20.1 calc'd A= Rn+R2=
40.1 calc'd B=R1+Rn=
4,824.0 calc'd Num1= V1*Rn+B*V2
806.0 calc'd Denom1= (A*B)-(Rn^2)
6.0 calc'd Ib= Num1/Denom1, amps
120.6 calc'd Num2= C*Rn + V1
40.1 calc'd Denom2= B
3.0 calc'd Ia= Num2/Denom2, amps

1,079.1 calc'd total power in, (V1*Ia) + (V2*Ib), watts
0.9 calc'd power dissipated in Rn, [(Ia-Ib)^2]*Rn, watts
361.8 calc'd power dissipated in R1, [Ia^2]*R1, watts
716.4 calc'd power dissipated in R2, [Ib^2]*R2, watts
1,079.1 calc'd total power dissipated, watts

0.3 calc'd voltage across Rn, volts
120.3 calc'd voltage across R1, volts
119.7 calc'd voltage across R2, volts
0.6 calc'd voltage difference, R1-R2, volts

 

[waross]

MD, that would be Moldova, welcome. (This is an international forum.)
You are on the right track.
However you have neglected the resistance of the line conductors. With equal resistance lines and neutrals, one load has to be more than twice the current of the other for the voltage to rise.
For a real world exercise assume one load to be 10 amps, (1200 Watt heater) and 1 amp.
You have about 60 feet of #14 three conductor cable back to the panel.
Approxmate resistance of 60 feet of # 14 copper = 0.1884 Ohms.
Voltage drop on line #1 @ 10 amps = 1.884 volts.
Voltage drop on the neutral @ 9 amps = 1.696 volts.
Voltage drop on line 2 @ 1 amp = 0.1884 volts.
Voltage at 10 amp load = 120 V - 1.884 V - 1.696 V = 116.4 Volts.
Voltage at 1 amp load = 120 V - 0.1884 V + 1.696 V = 121.5 Volts.
If the neutral is undersized the voltage rise will be greater.
If there is a long neutral from the panel back to the transformer the voltage rise may be greater.
The resistance value is based on DC resistance for 7 strand wire. The actual voltage drops and rises may be greater in an AC circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi Bill,

I had guessed that the installation was as you described. I'm getting used to the funny things you guys do to your electricity. As far as I can see there is no significant difference in behaviour between a sound but excessively long neutral and a shorter neutral with a bad joint going high resistance. Do I need to go back to sleep for another hour?


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

The original questions says "increases by any amount". A voltage increase will be caused by impedance of the neutral but further info is needed to determine if the neutral is bad or not.

 

[waross]

Very well stated Lionel.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[opmgr1]

For the voltage increase to be visually noticable there must be a neutral problem, given that the equations highlighted above is not a reflection of real-world situation.

 

[waross]

For the voltage increase to be visually noticable there must be a neutral problem, given that the equations highlighted above is not a reflection of real-world situation.

Voltage rise on a neutral is a common SITUATION in Canada where three wire circuits (120:240V, shared neutral) were mandated for kitchen receptacles by the code for years.
It is normal, but not often noticed, despite the fact that it is happening in millions of Canadian kitchens every morning when the coffee maker or toaster is plugged in.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[WhyDoYouAsk]

"Voltage rise on a neutral is a common SITUATION in Canada where three wire circuits (120:240V, shared neutral) were mandated for kitchen receptacles by the code for years.
It is normal, but not often noticed, despite the fact that it is happening in millions of Canadian kitchens every morning when the coffee maker or toaster is plugged in."

How much voltage rise do you get, due a ?10A load? I hope to get to go/nogo specs for this rise, for U.S. residences.

 

[opmgr1]

Please note I am not saying that voltage rise on neutral is not common. What I am saying is that the very simple equation illustrated above is not a reflection of real world situation. In real world situation the voltage rise due to load unbalance is not noticeable and is not of the magnitude to cause damage to equipment or any other concern.

 

[bacon4life]

The IEEE 141 flicker limits suggest that a 1.5% voltage rise would be noticeable only if switched more than 5 times per hour and only begin to be irritating at more than 2 times per minute. Since voltage drop should be limited to 3% on the phase + neutral, I don't see why you would need a separate specification to limit the rise on the neutral to something even less noticeable.

Waross - Is lighting allowed on Canadian kitchen receptacles? Without incandescent lamps there would not be much to notice.

 

[WhyDoYouAsk]

Loose neutrals come up all the time on Internet Forums. I'm looking for a go/nogo check with some kind reasoning in back of it.

 

[waross]

A voltage rise on the neutral is common in Canada but not noticeable or troublesome. Many years ago lighting circuits and receptacles were sometimes mixed on three wire circuits with a common neutral. Receptacle circuits and lighting circuits are no longer mixed in Canada.
BUT in some older installations with a long service drop of fairly light gauge conductor, it is possible for a voltage drop on the service neutral to be noticeable on lighting circuits.
But, open neutrals are much more noticeable, and cause damage.
For a Go-Nogo test, consider using a test load and a voltmeter. Measure the nominal 120 volts at the receptacle or point under consideration and measure the nominal 240 volts at the same point. Now energize the test load and measure both voltages again. The voltage drop (unloaded voltage minus the loaded voltage) on the 240 volt circuit should be about 1/2 of the voltage drop on the 120 volt circuit. This is a normal reading. Very much difference indicates a high resistance somewhere in the circuit. If the difference is more than 50% suspect a neutral problem. If the difference is less than 50% suspect a problem on the hot conductor.

Example.
No load voltages;
120 Volts
240 Volts
Loaded voltages
118 Volts, 2 volts drop.
239 Volts, 1 volt drop.
This is normal.
If there is a slight difference it probably indicates a section of wiring between the test point and the transformer where the neutral is reduced in size below the size of the hot conductors. An open neutral or corroded neutral will show much more difference.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Why don't you do a simple test? Measure the actual 120V voltage on a known good circuit. Plug a portable heater in and turn it on. Measure the voltage again. This will give you some insight as to an acceptable voltage drop. If you do this on a split receptacle then you can also check for any possible voltage rise on the other leg.

I would expect a few volts, maybe up to 5V, for a typical household installation.