Why cannot tie neutural wires together?

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  #1  
Old 04-03-06, 10:17 AM
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Why cannot tie neutural wires together?

I notice at the forum that a number of people mentioned that we should not do not tie neutural wires together.

Do not quite understand why. I saw quite a lot boxes having white wires tied together. Can anyone explain in detail?
 
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Old 04-03-06, 10:35 AM
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> Do not quite understand why.

There are many reasons.

> I saw quite a lot boxes having white wires tied together.
> Can anyone explain in detail?

The details are numerous.

Unlike ECGs which can always be tied together and do not carry current, neutrals are current-carrying conductors.

Neutral conductors must be tied together only where necessary to complete a circuit.

It's the same rule as for hot (ungrounded) conductors. All have limited ampacities.

Current-carrying conductors must be tied together only where necessary to complete a circuit.

Neutral conductors are more likely to be seen tied in groups of three or more than hots are because we do active work like switches all on the hot side.

But if you could see the big picture, if there are three neutrals tied together, there are three hots tied together somewhere too.

Neutrals are the drain where current goes after we are done with it.

Again, the rule is:
Neutral (grounded) conductors must be tied together only where necessary to complete a circuit and no where else.
 
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Old 04-03-06, 10:49 AM
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Thank you for your quick response.

Neutral conductors must be tied together only where necessary to complete a circuit.
Do you mean that white should be connected only to a fixture to form a loop, such as a ceiling light?
But when you install a bunch of receptacles around the room, you do connect white to white, black to black.... while there is not a load yet. One of the receptacle may have a switch attached. So that box will have 3 white wires tied together.
 
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Old 04-03-06, 10:54 AM
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You are confused. Neutral wires from a single circuit are tied together. Neutral wires from different circuits are NOT tied together except at the neutral buss in the panel.

Neutral wires carry current. If you tie neutral wires together from different circuits then you run the risk of overloading the neutral wire, which could cause a fire.
 
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Old 04-03-06, 11:10 AM
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> Do you mean that white should be connected only to a
> fixture to form a loop, such as a ceiling light?

I don't know what you mean.


> when you install a bunch of receptacles around the room,
> you do connect white to white, black to black.... while
> there is not a load yet.

It's safer if you have the breaker off.


> One of the receptacle may have a switch attached.
> So that box will have 3 white wires tied together.

It has three black wires tied together too.
 
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Old 04-03-06, 11:30 AM
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I unserstand what you are saying, but am wondering in what wiring configuration someone would even try to combine neutrals from different circuits. Can you give me an example of when not to tie the neutrals together? Thanks.
 
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Old 04-03-06, 11:42 AM
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See the topic "2 circuits sharing a neutral "

I got confused at first. I understand now that we can tie together neutral along the single circuit, but not mix cirtuits.

Now I have a follow up question. If you accidentally tied two circuits together in one place, (not in the service panel.) does that mean your loading capacity be doubled, or even worse.
 
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Old 04-03-06, 11:42 AM
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Don't tie the neutrals when it is inappropriate for the current in each neutral to be combined.
 
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Old 04-03-06, 12:26 PM
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Originally Posted by Willg54
Can you give me an example of when not to tie the neutrals together? Thanks.
Neutrals from different circuits are never tied together outside of the panel box.

Now I have a follow up question. If you accidentally tied two circuits together in one place, (not in the service panel.) does that mean your loading capacity be doubled, or even worse.
Yes. Suppose you have two 20A circuits with space heaters on each one drawing 16A. If someone errantly ties the neutrals you have the potential for 32A to be flowing on the shared portion of the neutral wire which is only rated for 20A. This presents a fire hazard.

am wondering in what wiring configuration someone would even try to combine neutrals from different circuits.
There are dozens of possibilities. For example in a multi-gang switch box that controls lights in two adjacent rooms. It is entirely possible that each room's lighting is on its own circuit, yet both are present in the same box. It would be wrong to tie neutrals there. Another common example would be in a kitchen where the garbage disposal switch often shares a box with the countertop receptacles or sink light switch; all of which may be on different circuits and should not have neutrals tied. Additionally, people are always confused by 3-way switch configurations and tie white wires together willy-nilly, even when it may be incorrect to do so.

I saw quite a lot boxes having white wires tied together.
Beware also as not all white wires are neutrals. Especially when involving switches white wires can be hot wires. Nowadays, white wires used as hot are supposed to be marked but almost never are in existing construction.
 
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Old 04-03-06, 02:34 PM
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Yes. Suppose you have two 20A circuits with space heaters on each one drawing 16A. If someone errantly ties the neutrals you have the potential for 32A to be flowing on the shared portion of the neutral wire which is only rated for 20A. This presents a fire hazard.
This makes a lot of sense. Usually, you can tell if one wire is thicker than the other. Thanks a lot, for all the replies.
 
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Old 04-04-06, 08:47 PM
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Now I am going to throw into this a situation that few homeowners will ever see.

208/120 volt three-phase (four-wire) supply circuits often feed three different 120 volt single-phase loads and use only the one neutral and in this case there ARE three load neutrals connected together to one supply neutral.

This is not uncommon in commercial/industrial locations.
 
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Old 04-04-06, 09:55 PM
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Originally Posted by furd
208/120 volt three-phase (four-wire) supply circuits often feed three different 120 volt single-phase loads and use only the one neutral and in this case there ARE three load neutrals connected together to one supply neutral.
This is fine because the neutral carries only the unbalanced current between the three which is always less than or equal to the current carried by the UGC with the most current (exception: triplen harmonics etc.).
 

Last edited by bolide; 04-04-06 at 10:27 PM.
  #13  
Old 04-04-06, 10:27 PM
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208/120 volt three-phase (four-wire) supply circuits often feed three different 120 volt single-phase loads and use only the one neutral and in this case there ARE three load neutrals connected together to one supply neutral.
yeah but the code is little tricky with this one you have to use the same wire size as hot conductors [ the code do not allow to have reduced netural wire size at all see Bolide's comment with this one

Ferd : 208/120 AND 480/277 volt ARE common in larger commercal and industrail area
even super large house use 208/120 system even i heard one or two house have 480/277 system then down step transformer and have tons of subpanels as need it


Merci , Marc
 
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