Dishwasher/Disposal 3-wire circuit to duplex

[e57]

E57,

This is a two part question:

1) First my setup: Both the dishwasher and disposal are cord connected on a duplex receptacle with the tab broken sharing the neut. from 12/3 feed from panel with 2 pole 20A OCPD.

Dishwasher: Eurotech = 12A
Disposal: Insinkerator = 8.1A

When trying to figure the loads for this setup I am somewhat confused from the earlier posts on this thread. If 12/3 has been ran as stated above, the two above amperages are not additive making it 20.1A but on separate 20A circuits with that 2 pole circuit breaker.

~~

2) ~Is the two pole breaker acting like two separate 20A breakers for two separate circuits?

Thanks,

Mka

(1)Typical to all MWBC's (Muilti-Wire Branch Circuits) the ? (hot) conductors are on different legs - either opposite eachother on a split phase system 240/120 (typical to most dwellings), or 120 degrees from each other in the case of 3 phase 208/120 power in many multi-family or high rise buildings. And so the current on the neutral will be the imbalanced portion of the circuit. (In a perfect world) If you had 10A on one leg, and 10A on the other of the circuit (240/120) there would be ZERO amps on the neutral. In your case, of 8.1, and 12A - the difference is 3.9A, and that is what the neutral amperage would be when both units are operating at full load.

However, when only one item is on - the neutral current would match whatever that item is. So the neutral current when ONLY the garbage disposal is on by itself would be 8.1... (as it would be in a 2-wire circuit)

(2)Yes the breaker acts as two seperate breakers, and the handle tie ensures that both are disconnected at the same time.

 

[e57]

Also all appliances or equipment requiring over 12 amps would have a 20 amp cord, not a 15 amp, this is why it would be ok with the NEC for these two items to be placed on there own 15 amp circuit. whether it be a multi-wire or single run circuits both would be code.

It is very likely that an appliance requiring 12A - that the manufacturer would have specified a 20A circuit for it in the instructions for the unit. Since most manufactuers follow a 125% OCP sizing 'rule of thumb' - So... Most manufactures would be 80% of the recommended circuit size or less... 12A load is on the border for a 15A for 125% circuit sizing - most would require a 20A OCP and circuit size.

 

[mka]

Neut. clarification

Neut. clarification

Man this site is awesome! I am learning so much from this site...probably more than school. Thanks LarryFine, E57 and Hurk27!

Mka,

Lu 595, apprentice

 

[LarryFine]

So for example, if I take my voltmeter and apply both leads to an energized 120V receptacle I will read 120V. How much of that 120V is returning to the source on the neutral and when?

First of all, voltage doesn't pass through conductors; current does. The voltage between two points (any two points, whether one is grounded or not) is what drives that current.

Loads do not "suck" current; they allow it to pass through, as do any conductive pathways. Breakers don't trip because the load impedes current; a short permits too much current.

If the meter is the only thing connected between the line and neutral on that circuit, then the only current will be the miliamps that the meter passes while reading the voltage.

In any two-wire circuit with a single load, which is a simple series circuit, the current is the same everywhere in the circuit. Both conductors supplying loads see all the current.

Also, in that earlier post you said,

"Originally Posted by Dennisc
... blah ... blah ... blah ..."

In the case of the 240V appliance with no neutral, how do the electrons flow back to the source?

Think back to the two-battery source. All three wires are "the source." Current will flow on any pathway that interconnects any two points with a voltage difference between them.

With a 240v-only load, the current flows from one end terminal to the other. A voltmeter connected between the mid-point of the heater and the source neutral would show no voltage.

Since there's no voltage difference between those two points (and none could be created since the heater element is a single, non-tapped load), there's no need for a neutral conductor.

 

[LarryFine]

And, to add after reading the next few posts above: the neutral conductor conducts current in its effort to keep the source neutral point and the load neutral point at the same voltage.

Note that this occurs whether or not the system neutral is grounded. It matters not if the neutral is at zero volts to earth, only that there's no difference between the source and load neutrals.

In fact, you could say that the whole point of us making good, reliable, low-impedance connections is to keep source terminals and their corresponding load terminals each at the same voltage.

 

[rbwsparky]

we wire all fixed appliances with 12ga around here, including vent hoods. boss always concerned with future upgrades, easier to pull 12ga once than to have to upgrade wiring again. we work on a lot of older homes tgat are brutal to fish

 

[electricmanscott]

we wire all fixed appliances with 12ga around here, including vent hoods. boss always concerned with future upgrades, easier to pull 12ga once than to have to upgrade wiring again. we work on a lot of older homes tgat are brutal to fish

Upgrades to what?

 

[sparkalot]

not here

not here

sorry no 14 in san antonio, check your local AHJ

 

[quogueelectric]

Hello,

When installing th duplex receptacle for a dishwasher/disposal,running a 12/3 and sharing the neutral at the plug and of course breaking the tab on the hot terminal,I NEED to use a double pole 20A breaker for OCP as opposed to 2 single pole 20A breakers for each circuit? I need 240V potential between the 2 circuits I'm thinking otherwise I could overload neutral?

Thanks

Wvamp

Good thinking multiwire branch circuits must not be on the same phase otherwise you run the risk of overloading the neutral with no overload circuit protection on an overloaded neutral carrying 40 amps on a 12 wire.

 

[quogueelectric]

Going 'green' is the next big thing. Look at people buying furnanes these days. You think they'll happily accept an 80% unit? No, they want at 96 or 97%.
Everyone looks at the effeciency labels these days. And the more expensive an appliance is, the more effecient it is.

Not true my dishwasher is a pricey energywaster with big electric elements to dry the dishes faster. It is an energy PIG.