Complicated Panelboard Grounding Question

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  #1  
Old 05-02-06, 05:52 PM
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Thumbs up Complicated Panelboard Grounding Question

If I have a single phase 3 wire system feeding my single phase panelboard with two hot legs and have a dryer in the house. Is the following true?: My dryer has an instrument panel and takes two hots for the needed 240 volts yielding some unbalanced current on one of the hots I would suspect.

Unlike a 3 phase system where unbalanced currents travel on the 4th grounded nuetral wire, a single phase system with computers, single phase air conditioners, and other capacitive loads will cause unbalanced currents in one of the hots for a single phase system I suspect. The nuetral on the two leg single phase house circuit sees the same current on it since it is merely a collection of the two hots going to ground I suspect. One of the two hots, namely the one most loaded with dryers, ACs, etc. needing 240volts will have more AC current. True.

Also if a wye 3 phase primary on a wye-wye transformer has its nuetral grounded, is there ever a case where you would ground the phase conductors (ungrounded conductors) on the primary also?

What about a delta primary on a delta wye transformer. Would you ground the primary nuetral and all the phase conductors as an application need?


Thanks,
 
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  #2  
Old 05-02-06, 06:51 PM
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> My dryer has an instrument panel and takes two hots for the
> needed 240 volts yielding some unbalanced current on one of
> the hots I would suspect.

That's why it has a neutral. The panel control and motor use 120V.

> Unlike a 3 phase system where unbalanced currents travel on the 4th
> grounded neutral wire,
> a single phase system with computers,
> single phase air conditioners, and other capacitive loads will
> cause unbalanced currents in one of the hots for a single
> phase system I suspect.

You suspect incorrectly.
If you would realize that what we call split single-phase does, in fact, have two legs 180° apart (whereas in three phase they are 120° apart), then you should realize the errors in your reasoning.

> The neutral on the two leg single phase house circuit sees the
> same current on it since it is merely a collection of the two hots
> going to ground I suspect.

Incorrect.
The neutral sees the unbalanced current between the two hots.
If one has 30A and the other has 10A, the neutral carries 20A (disregarding waveform distortion).

> One of the two hots, namely the one most loaded with dryers,
> ACs, etc. needing 240volts will have more AC current.
> True.

False. A 240V load draws current from both hots precisely equally.
A dryer, however, also has 120V loads.


> Also if a wye 3 phase primary on a wye-wye transformer has
> its neutral grounded,
> is there ever a case where you would ground the phase
> conductors (ungrounded conductors) on the primary also?

Think about what you are saying.
An ungrounded conductor is never intentionally grounded.

And no, if you have a grounded neutral, you cannot also ground one of the phase conductors.

You could run a delta with two ungrounded and one grounded conductors.
But the grounded conductor would not be an ungrounded conductor in such a case.

> What about a delta primary on a delta wye transformer.
> Would you ground the primary neutral and all the phase
> conductors as an application need?

What is a primary neutral? I don't see why a delta primary ever needs a neutral.
 

Last edited by bolide; 05-02-06 at 07:27 PM.
  #3  
Old 05-02-06, 09:28 PM
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> What about a delta primary on a delta wye transformer.
> Would you ground the primary neutral and all the phase
> conductors as an application need?


on commercal/ industrail location most delta connection dont genrally use the netural at all but some as i say some delta do have netural in there [ i will leave this matter out for now it get compacted due wild leg etc ]

on delta primary you dont need netrual at all for secondary wye yes you will have netural there but the codes are pretty picky with what we called SDS seprated distubrbed system [sp]

like this 480 primary - 120/208 wye secondary that is a sds


this kind of system is kinda very compated and i genrally will not recomoned the DIY's try to mess around with 3 phase system and majorty of diy's system are single phase but if need more info about 3 phase there are few book expain about the wye and delta system i dont rember which book it have good details in there [ i work on alot of 3 phase stuff everyday ]

Merci, Marc
 
  #4  
Old 05-03-06, 10:04 AM
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My Mistake

I am sorry. I said Delta primary nuetral. I knew that the Delta had no nuetral; this was a typo.

Bolide answered the question that the primaries are not normally grounded for the delta or wye primary.

I was looking at a design and am almost sure the 3 delta primaires comming in to the load break elbow on the pad mounted transformer had a ground strap connected from the primary cables to the grounding loop electrode system (ground rods inserted 10 feet underground).

French-A seperately derived system you indicate going from 480volt to 120/208 volt is something I will study further. I think when a primary Y nuetral is grounded at the transformer, you are creating sepatarately derived system on the secondary of the transformer pointing towards the facility based on my past experience with this matter.
 
  #5  
Old 05-03-06, 10:58 AM
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> I knew that the Delta had no nuetral; this was a typo.

Any reference to nuetral is a typo.


> I was looking at a design and am almost sure the 3 delta primaries
> coming in to the load break elbow on the pad-mounted transformer
> had a ground strap connected from the primary cables to the grounding
> loop electrode system (ground rods inserted 10 feet underground).

That simply is impossible.
More likely the insulation around the conductors was grounded.
Regardless, the ungrounded conductors are not grounded.
An open delta can have at most one grounded conductor.


> I think when a primary wye neutral is grounded at the transformer,
> you are creating separately derived system on the secondary
> of the transformer

Whether it is SDS depends much more on whether the derived system is grounded. If the secondary is ungrounded or if the source is a PV grid, a battery of batteries, centrifugal or superconducting electrical storage, a fuel cell, or a mechanical generator, then it looks like an SDS to me.

Grounding the primary does not make an SDS.
It simply allows for imbalance between the three phases.


> pointing towards the facility
Pointing?
 
  #6  
Old 05-03-06, 04:00 PM
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Concentric Nuetral

I am waiting on a call back from a pad mounted transformer vender. The reason I saw a ground wire comming off of the primary cable is that it was a concentric nuetral, but I believe a concentric nuetral should not have been used for a delta primary, but maybe only for a wye primary indicating the design was wrong.

If I use a concentric nuetral (1/2 or full) for a wye primary, it seems that the primary lugs (load break elbow) connections used for the input to the pad mounted transformer should be 3 and not 4 load break elbows. An electrician once told me you strip the concentric nuetral (located near the outside of the cable jacket and is made of copper) off before attaching to the load break elbow and run it to the grounding electrode. If you do this on a wye, there would be no place for connecting a nuetral to the nuetral load break elbow??
 
  #7  
Old 05-03-06, 04:52 PM
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I really do not think that a DIY forum is the place to be asking questions about primary service to a three-phase distribution transformer.

I have been in the "energy" business for more than thirty years and I can state with a fair amount of authority that a wye-connected primary on a distribution transformer is pretty darned rare.

What you seem to be calling a "concentric nuetral" (and by the way, the word is spelled NEUTRAL) is in actuality a shield conductor and it is use on medium and high voltage wiring systems. Medium voltages being over 600 volts phase-to-ground and high voltage being over 10,000 volts phase-to-ground. In these systems the insulation has a tendency to become "charged" due to the capacitive effect of the conductor and the insulation. The shield of all three individual cables in a three-phase configuration are connected together and grounded to "draw off" this charge. The shield is NOT a current carrying conductor as far as the system operation is concerned but ONLY a safety feature.
 
  #8  
Old 05-03-06, 05:34 PM
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Back to the Unbalanced Current Issue

Thanks Furd, I think you are right. The only DIY application for a 3 phase transformer would be if I were trying to put one in. Now I know the design was right to put in concentric cable for a delta.

Bolide said the neutral subtracts from the two hots on a single phase panelboard if I understand him right. For example, if you have 30 amps on one hot leg and 20 amps on the other hot leg, then 10 amps flows in the return (otherwise called the grounded white wire neutral).

Assuming one of the hot legs has more single phase loads or has some three phase heavy loads on it the other hot leg does not have on it, it makes since that one of the hot legs has 20 amps and the other has 10 amps. Each hot leg is a tap off of the secondary terminal of the single phase pot transformer, then terminating on the loads in the house for example. Maybe a handyman book would explain it pictorially (and I will check it tomorrow), but why do the currents on each leg subtract once they have exited negative terminals of the devices they are powering and meet up with the neutral heading for ground?
 
  #9  
Old 05-03-06, 05:42 PM
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There are no negative terminals. Perhaps that is the reason you don't quite grasp the concept. In an AC system there is no negative.

The current on the two hot legs "subtract" because they are 180 degrees out of phase with each other. (Technically they do not subtract, but rather they add together.)
 
  #10  
Old 05-03-06, 09:35 PM
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They do subtract in that they create a larger potential difference.

When one leg is at, say, +130V (approaching the peak of the wave), the other leg is at -130V (approaching the trough of the wave).

The difference |( ( +130V ) - ( -130V ) )| = ( 260V ).

Arithmetically speaking, subtracting a negative has the same effect as adding a positive.
It looks like addition (130 + 130), but actually it is not.

Voltage is the potential difference (subtraction).
 

Last edited by bolide; 05-04-06 at 08:38 PM.
  #11  
Old 05-04-06, 10:13 AM
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Ok I'll Run it Through My Math Tester

Ok,

You say the amps subtract and the volts add. The math for the volts were proofed by Bolide.

For the amps, I=p/v. For one leg, I1=8,240watts at 0 degrees (for 10 amps on one leg at 0 P.F. for example) /240 volts*cos 90 degrees. I2=p/v at bottom of trough. I2=7,200watts at 0 degrees, 30 amps for other hot leg and 0 P.F. (number used just for illustrative purposes)/240 volts*cos 270 degrees.

Add them together and there is a subtraction effect you say. I will go get my calculator with trig on it from work and see if this is true. Thanks.
 
  #12  
Old 05-04-06, 10:47 AM
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> You say the amps subtract and the volts add.

No, the amps are added, subtracted, or ignored depending on the reference point.


> For the amps, I=p/v.
But are you using an absolute or a negative number for voltage?
Because if you use negative voltage to refer the the split phase that is shifted 180°, then its I is negative, so to get neutral current you add (and then take absolute value).

But if you use absolute value for both, then you subtract.


> For one leg, I1=8,240watts at 0 degrees
> (for 10 amps on one leg at 0 P.F. for example) /240 volts*cos 90 degrees.

That's meaningless.


> I2=p/v at bottom of trough.

No, we don't consider instantaneous current or voltage.
We use the RMS value (nominal 120V) as if it were basically a square wave (with the same area under the curve) rather than sinusoidal.

> I2=7,200watts at 0 degrees,
> 30 amps for other hot leg and 0 P.F./240 volts*cos 270 degrees.

Again, meaningless. This has nothing to do with cosines.
A PF of 0 draws infinite current. So you have 30 ÷ 0 ÷ 240 · 0.
Whatever you call that formula, it sure isn't math.


> Add them together and there is a subtraction effect you say.
How do you add to get a "subtraction effect"? Are you referring to adding negative numbers?


> see if this is true.
Be sure to post all your math.
 

Last edited by bolide; 05-04-06 at 08:41 PM.
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