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#68022 - 07/27/06 02:22 PM Calculating neutral loads.
Trick440 Offline
Member
Registered: 08/15/05
Posts: 247
Loc: Waterford, MI, USA
Whats the formula to find the amps a neutral is carrying?

And if 220 doesn't need a nuetral to work,.. is there current on the nuetral?

So I need some schooling an nuetrals. Any and all help is appreciated.
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#68023 - 07/27/06 06:15 PM Re: Calculating neutral loads.
Roger Offline
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Registered: 05/18/02
Posts: 1716
Loc: N.C.
Trick,
Quote:
Whats the formula to find the amps a neutral is carrying?
are you asking about three phase or single phase?

Quote:
And if 220 doesn't need a neutral to work,.. is there current on the nuetral?
Since a neutral is not needed in this voltage (not true of all world voltage systems)source, it doesn't exist to have current flowing on it.

Roger
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#68024 - 07/27/06 06:55 PM Re: Calculating neutral loads.
Trick440 Offline
Member
Registered: 08/15/05
Posts: 247
Loc: Waterford, MI, USA
Single phase.

Three phase wouldn't hurt either.
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#68025 - 07/28/06 04:14 AM Re: Calculating neutral loads.
Dnkldorf Offline
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Registered: 12/12/04
Posts: 1064
Loc: nowhere usa
For a wye system the nuetral current is calculated by:

N= Sq root of (A^+B^+C^)-(AB+AC+CB)

^=squared

For split phase(single phase) the nuetral current is calculated by:

A-B or B-A.

Does this make sense?
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#68026 - 07/28/06 11:37 AM Re: Calculating neutral loads.
Trick440 Offline
Member
Registered: 08/15/05
Posts: 247
Loc: Waterford, MI, USA
nope it don't make sense.

The Wye formula I see and understand.. except for, What is A, and B?

The single phase. Same situation, what is the A and B refering to. Volts, Watts, amps?

And with this slash (-) are you tring to say A, minus B?
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#68027 - 07/28/06 12:55 PM Re: Calculating neutral loads.
Dnkldorf Offline
Member
Registered: 12/12/04
Posts: 1064
Loc: nowhere usa
Trick, the A,B,C are the currents in phase A, Phase B, and Phase C.

So for a split(single)phase service, the nuetral current is the unbalnced current flowing between phase A and phase B.

So if 30 amps were flowing on phase A, and 15 amps were flowing on Phase B, the nuetral current would be: Yes it is a minus...

A-B=N
30-15=15

On a wye service, if we had 150amps on A, 110amps on B, and 75amps on C.

N= Sq root of (A^+B^+C^)-(AB+AC+CB)
N=Sq root of (150^+110^+75^)-((150*110)+(150*75)+(75*110)
N=Sq root of
(22500+12100+5625)-((16500)+(11250)+(8250))
N=Sq root of
(40225)-(36000)
N=Sq root of
(4225)
N=65 amps


How does that look?



[This message has been edited by Dnkldorf (edited 07-28-2006).]
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#68028 - 07/28/06 02:13 PM Re: Calculating neutral loads.
Roger Offline
Member
Registered: 05/18/02
Posts: 1716
Loc: N.C.
Trick, to go along with Dnk's posts here are some graphics for help in visualizing the single phase neutral

BTW, I like to refer to the ends of single phase windings as legs, not phases.

So with this said, A and B in Dnk's post would relate to legs 1 and 2







Roger

[This message has been edited by Roger (edited 07-28-2006).]
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#68029 - 07/30/06 01:55 PM Re: Calculating neutral loads.
Trick440 Offline
Member
Registered: 08/15/05
Posts: 247
Loc: Waterford, MI, USA
Thats great guys. I see now. Thank you.
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#68030 - 07/30/06 02:44 PM Re: Calculating neutral loads.
Dnkldorf Offline
Member
Registered: 12/12/04
Posts: 1064
Loc: nowhere usa
Anytime.......
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#68031 - 07/30/06 05:12 PM Re: Calculating neutral loads.
wa2ise Offline
Member
Registered: 11/29/02
Posts: 782
Loc: Oradell NJ USA
All the above assumes that all loads are pure resistors, like incandescent light bulbs. If we talk about a computer server farm fed by 3 phase 120V, things get more complicated. Computer power supplies draw large spikes of current at the very peak of voltage of the phase they are connected to. When the voltage on phase A peaks, computers on phase A draw their spikes of current and return them to the neutral. Later, 120 degrees of the 60Hz cycle, phase B peaks, and computers on phase B draw their current spikes, and return them to the neutral. Likewise, 120 degrees later for phase C. No overlap of these current spikes (in terms of time). Spikes last about 5% of the cycle time. Note that the neutral will see 3 times as much spike current than any one phase (assuming equal phase loading). Thus you'd need a thicker wire for the neutral.

[This message has been edited by wa2ise (edited 07-30-2006).]
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