Tandem, quads avoiding same leg

Reply

  #1  
Old 12-28-17, 06:41 AM
Member
Thread Starter
Join Date: Jun 2014
Location: Canada, Quebec
Posts: 1,560
Received 0 Votes on 0 Posts
Tandem, quads avoiding same leg

I keep reading about avoiding breakers on the same leg but I'm still not sure how it's possible to do.
A quad is 2 single phase circuits so would never be joined and the middle of the quad splits over 2 legs.
With a tandem, they're usually 2 separate 120 circuits also.
Is it the shared neutrals that are an issue rather than the hots?
 
Sponsored Links
  #2  
Old 12-28-17, 07:29 AM
Member
Join Date: Nov 2015
Posts: 1,221
Received 17 Votes on 16 Posts
Avoiding breakers on the same leg does not mean you should not use tandem or quad breakers.
What this means is evenly distribute the loads (not just breakers) as much as possible to avoid imbalanced load.
You have to consider how much load will be placed on each circuit when balancing the load.
For example, if you have 2 120V baseboard heaters on separate circuit each circuit should be on different leg.
 
  #3  
Old 12-28-17, 07:47 AM
Handyone's Avatar
Member
Join Date: Aug 2000
Location: U.S.
Posts: 5,451
Received 0 Votes on 0 Posts
Is it the shared neutrals that are an issue rather than the hots?
A multi-wire branch circuit must be on separate legs. The mwbc shares a neutral and is fed from a double pole breaker.
You are splitting the two hot wires into 2 separate 120V circuits and there are rules on how to split at the first box (neutral must be pigtailed). The advice you are reading probably refers to a split circuit only and is causing confusion.

It sounds like you are not running a MWBC.
 
  #4  
Old 12-28-17, 09:35 AM
Member
Join Date: Jun 2012
Location: USA
Posts: 1,416
Received 12 Votes on 11 Posts
Avoiding breakers on the same leg is usually not an issue. Some panels will state a maximum amperage for a buss stab when using tandems. So in practice don't use high amperage quads across from any high amperage breakers in your panel.
 
  #5  
Old 12-28-17, 11:45 AM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,490
Received 32 Votes on 24 Posts
There are certain General Electric panels that use a thin circuit breaker and in these panels it IS possible to install a two-pole, or two single pole, CB in a manner that puts both poles on the same leg.
 
  #6  
Old 12-28-17, 11:57 AM
Member
Join Date: Nov 2015
Posts: 1,221
Received 17 Votes on 16 Posts
In case of MWBC or GE 2 pole thin breaker same tab, it is something that should be NEVER done not just avoid. In case of MWBC, it is dangerous because it will overload neutral and in case of GE, it just won't work.
 
  #7  
Old 12-28-17, 09:50 PM
Member
Thread Starter
Join Date: Jun 2014
Location: Canada, Quebec
Posts: 1,560
Received 0 Votes on 0 Posts
When you talk about overloading the neutral... How much is overloading. Can't the neutral take the same as the hot?
Eg on a 100a leg, the neutral could have 100 load?
 
  #8  
Old 12-28-17, 10:03 PM
Member
Join Date: Jun 2012
Location: USA
Posts: 1,416
Received 12 Votes on 11 Posts
I forgot about those GEs. If you use them on a 240volt load you will know pretty quick if they are in the wrong position. With a MWBC or subpanel feed it might be a while. If both legs of the breaker on the same stub/phase then the neutral could see 2x the rated load - when both legs on the same phase then the neutral sees the sum of the currents, normally the legs are on different phases so the neutral on a MWBC sees the difference in currents of the two legs,

This is for standard split phase 120/240 volt service. Three phase 120/208 has additional issues.
 
  #9  
Old 12-28-17, 11:28 PM
Member
Join Date: Nov 2015
Posts: 1,221
Received 17 Votes on 16 Posts
When you talk about overloading the neutral... How much is overloading. Can't the neutral take the same as the hot?
In MWBC, a single neutral is shared between hot lines. If they are correctly wired and each hot lines are on different legs, neutral will never see more than the current rating of the breaker.
This is because each legs have phase difference of 180 degrees. This means when one leg is at +120V, the other leg is at -120V. The same applies for the current because it is proportional to the voltage applied. Neutral is a center tap from 240V transformer and also in MWBC, neutral is in the middle between 2 loads. Therefore neutral wire will see the sum between 2 legs, but since they are 180 degrees out of phase, the sum ends up being current difference between 2 legs. If loads on both legs have exactly same load, you will have 0 current on neutral. But because the load will never be equal between 2 circuits, there will be a imbalance and current will flow on neutral.

If both hot lines are on the same leg in MWBC, now you end up with 0 degree phase difference between each legs, and the current will a sum between 2 legs, thus overloading neutral.
 
  #10  
Old 12-29-17, 03:50 AM
Member
Thread Starter
Join Date: Jun 2014
Location: Canada, Quebec
Posts: 1,560
Received 0 Votes on 0 Posts
In practice though, nothing is ever running simultaneously and equally on both legs so you always have an imbalance. So, how much can the imbalance get to.... the breaker rating? The neutral is what guage?
I guess this only affects circuits with neutrals - usually the 120v circuits?
 
  #11  
Old 12-29-17, 06:21 AM
Member
Join Date: Nov 2015
Posts: 1,221
Received 17 Votes on 16 Posts
I assume you did not understand my explanation.

To just simply put, neutral will not be overloaded as long as each leg of MWBC is on different leg.
Putting MWBC on same leg, although technically using larger neutral will work safely, is illegal no matter what size neutral you use.

Neutral is only used in 120V circuit (in the US). How the feeder cable on the breaker panel is wired is basically the same way MWBC is wired.

You don't have MWBC anyway so it doesn't apply to your case anyway.
 
  #12  
Old 12-29-17, 11:05 AM
Member
Thread Starter
Join Date: Jun 2014
Location: Canada, Quebec
Posts: 1,560
Received 0 Votes on 0 Posts
Without mwbc, you still need to balance but you can't overload the wire?
I think I understand now, on a #14 mwbc you could get more than 15A rubbing through the wire because both split circuits might be in use.
On a normal circuit if the hot is by itself and the neutral is not shared then it doesn't matter? You just balance the legs approximately for the same amps?
 
  #13  
Old 12-29-17, 11:13 AM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,943
Received 30 Votes on 26 Posts
There is no way for a two wire circuit to have more neutral current than the hot current.
 
  #14  
Old 12-29-17, 12:57 PM
Member
Join Date: Nov 2015
Posts: 1,221
Received 17 Votes on 16 Posts
Without mwbc, you still need to balance but you can't overload the wire?
Yes. MWBC it self has nothing to do with balancing the load. You want to balance the load as much as possible on the panel. This is not because of the safety issue, but for maximum utilization.
With 200A service, you can have 200A of 120V on each leg, making total of 400A of 120V circuits. If you have all the loads on only one side, you only get 200A.

I think I understand now, on a #14 mwbc you could get more than 15A rubbing through the wire because both split circuits might be in use.
Only if the MWBC circuit is wired incorrectly (both breakers on the same leg). This condition should never occur.

On a normal circuit if the hot is by itself and the neutral is not shared then it doesn't matter? You just balance the legs approximately for the same amps?
As stated above, balancing the load has nothing to do with MWBC.
The balance I was describing was just describing how MWBC circuit works and why neutral can be overloaded if not wired correctly.
 
  #15  
Old 12-29-17, 01:13 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,943
Received 30 Votes on 26 Posts
Loads in a residential panel are transient and shift during the day. Attempting to balance the panel would be a constant juggling act. Balancing panels is important with multiphase panels and constant usage patterns.
 
  #16  
Old 12-29-17, 07:48 PM
Member
Join Date: Aug 2005
Location: USA
Posts: 995
Received 0 Votes on 0 Posts
There are certain General Electric panels that use a thin circuit breaker and in these panels it IS possible to install a two-pole...CB in a manner that puts both poles on the same leg.
What style? In a modern THQL/THQP panel, you'd have to break something in the panel or on the breakers to get that to happen.
 
  #17  
Old 12-29-17, 08:49 PM
Member
Join Date: Nov 2015
Posts: 1,221
Received 17 Votes on 16 Posts
In a modern THQL/THQP panel, you'd have to break something in the panel or on the breakers to get that to happen.
I haven't used THQP 2 pole breakers in very long time, but come to think of it, you are right.
Because tab for full size (THQL) breaker will be in the middle, it is not possible to fit 2 pole THQP on a single leg.
 
  #18  
Old 12-29-17, 09:50 PM
Member
Join Date: Jun 2012
Location: USA
Posts: 1,416
Received 12 Votes on 11 Posts
it is not possible to fit 2 pole THQP on a single leg
Nothing a determined hack can't screw up. Instead use two single pole THQPs with a handle tie to feed a MWBC. In that case do need to pay attention and put each breaker on opposite legs.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: