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Old 06-03-2008, 09:10 AM   #16
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Wall switch for Dishwasher


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Originally Posted by BigJimmy View Post
Neither DW or disposal are considered continuous loads.



Please be careful about the wording here. One of my peeves is the way people seem to use tandem and double pole breaker interchangeably; they are not. A tandem is a two-breaker device that occupies one full position in a panel and both load side terminals are connected to the same bus/lug. Therein, "common trip" is meaningless as applied to a tandem and would be stupid since the circuits would be separate. That is, if one circuit tripped, it would be nothing but a nuissance to trip the other. On the other hand, a double-pole breaker occupies two adjacent single spaces and provides connections to both busses of the split phase system (L1, L2) yielding 240V AC across its terminals (a volt meter reading across the load terminals of a tandem would be zero). These breakers have a tie between handles so that a trip always results in disconnection of both ungrounded conductors.

Now I'm no Speedy (although I aspire to be in this action/comedy/drama that I'm starring in), but I believe that in many instances, electricians will run a 3/c MWBC to the JB under a sink in anticipation of powering both disposal and DW. According to the 210.4(B) of the 2005 NEC,


(B) Devices or Equipment.
Where a multiwire branch circuit supplies more than one device or equipment on the same yoke, a means shall be provided to disconnect simultaneously all ungrounded conductors supplying those devices or equipment at the point where the branch circuit originates. (copied directly)

Here's where the "common trip" comes into play. And BTW, double pole breakers are not manufactured any other way. Speedy-please correct me if I'm off here.
The reason I said tandem is because I thought he said he only has room for tandems.

This may all be moot. I am not sure if he ran a 14/3 or 12/3, OR ran 2 unique home runs. Perhaps one of the experts can confirm:

A) Each circuit of a MWBC should be on separate buses?

B) Both circuits of a MWBC should commonly trip?

C) Regular 1" tandems by nature can't be on separate phases, but quads can (since quads take up 2 1" spaces and use the 2 outters and inners).

D) If you have 2 separate circuits and they are connected to the same outlet but the tab is broken, do these need to be common trip?

Thanks for the clarification.

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Last edited by pcampbell; 06-03-2008 at 09:12 AM.
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Old 06-03-2008, 11:32 AM   #17
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Wall switch for Dishwasher


On the DW+Dishwasher on same circuit trust me I hope you're right. My quick look didn't show anything that outlets under sink must be dedicated which makes me happy. Let's talk about 210.21(B)(2) which talks about receptacles and outlets. Here it is exactly (NEC 2005)

"Total Cord-and-Plug-Connected Load. Where connected to a branch circuit supplying two receptacles or outlets, a receptacle shall not supply a total cord-and-plug-connected load in excess of the maximum specified in Table 210.21(b)(2)." and the table says for a 20A it's 16.

I think I've understood this wrong I've understood it to apply to the circuit as in you can't plug in more than 16A into the circuit and one will have a hard time finding a DW + Disposal <= 16A. Now I see it's saying it applies only to the receptacle, as in you can't plug in a load greater than 16A into a receptacle but, if the DW & Disposal plug into different recepts as long as either appliance isn't over 16A alone you're good to go. Have I got that?

*EDIT* Local code may stipulate they be seperate circuits.

Last edited by Piedmont; 06-03-2008 at 11:40 AM.
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Old 06-03-2008, 12:09 PM   #18
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Wall switch for Dishwasher


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Originally Posted by pcampbell View Post
The reason I said tandem is because I thought he said he only has room for tandems.
Well, if there is room for two tandems, there is room for a 2-p breaker.

Quote:
Originally Posted by pcampbell View Post
A) Each circuit of a MWBC should be on separate buses?
Each conductor must be from separate busses, other wise you have two hots with 0V difference. If you wired this way, using a 3-wire cable (or 3 separate wires), your neutral would carry the sum of both circuits which could result in overload.

Quote:
Originally Posted by pcampbell View Post
B) Both circuits of a MWBC should commonly trip?
Yes.

Quote:
Originally Posted by pcampbell View Post
C) Regular 1" tandems by nature can't be on separate phases, but quads can (since quads take up 2 1" spaces and use the 2 outters and inners).
I'll take your word for it but I'm not sure how this is relevant. I don't deal with tandems normally.

Quote:
Originally Posted by pcampbell View Post
D) If you have 2 separate circuits and they are connected to the same outlet but the tab is broken, do these need to be common trip?
Well, let's think about this for a moment. If you ran a MWBC to a receptacle and didn't remove the tab, you'd have some difficulty resetting the circuit breaker. So you remove the tab to effectively split the duplex receptacle. Assuming that this is served by a MWBC, see "B." You could split the receptacle and feed it with two hots from the same side of the panel as long as you break the tab on the neutral side and provide a dedicated neutral for each. However, this is no longer a MWBC and remember how the idea is to reduce the overall number of conductors?
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Old 06-03-2008, 12:09 PM   #19
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Wall switch for Dishwasher


Quote:
Originally Posted by Piedmont View Post
On the DW+Dishwasher on same circuit trust me I hope you're right. My quick look didn't show anything that outlets under sink must be dedicated which makes me happy. Let's talk about 210.21(B)(2) which talks about receptacles and outlets. Here it is exactly (NEC 2005)

"Total Cord-and-Plug-Connected Load. Where connected to a branch circuit supplying two receptacles or outlets, a receptacle shall not supply a total cord-and-plug-connected load in excess of the maximum specified in Table 210.21(b)(2)." and the table says for a 20A it's 16.

I think I've understood this wrong I've understood it to apply to the circuit as in you can't plug in more than 16A into the circuit and one will have a hard time finding a DW + Disposal <= 16A. Now I see it's saying it applies only to the receptacle, as in you can't plug in a load greater than 16A into a receptacle but, if the DW & Disposal plug into different recepts as long as either appliance isn't over 16A alone you're good to go. Have I got that?

*EDIT* Local code may stipulate they be seperate circuits.
80% only applies to continous loads. These are NOT continous.

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