> >
>

# 30A circuit for Mini Split

#1
10-23-18, 11:55 AM
Member
Join Date: Aug 2008
Location: Northeast
Posts: 9
30A circuit for Mini Split

I am putting in a 30A circuit for a mini split that will be installed next spring. I have bought and run 10/2 wire at the request of the HVAC guy who will install the unit. I understand that the black and white wires will be the two hot wires and ground will go to the ground bus bar. I realize in a split phase system each leg will be opposite the other, alternating between 0 and 120V.
My question is why doesn't a 240V circuit require a neutral to return current to the panel. Are there never current corrections on a 240V circuit?

#2
10-23-18, 12:41 PM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,944
In a pure 240V circuit, the "return path" for each hot leg is through the other hot leg. You simply are not using the center-tap on the transformer which is commonly called the "neutral". It is not necessary if you do not need 120V, as the only purpose of the center-tap leg is to derive 120V from the 240V source.

#3
10-24-18, 03:42 AM
Member
Join Date: Aug 2008
Location: Northeast
Posts: 9
Thanks ibpooks. That is also what I have come across as an explanation elsewhere. I do understand the concept at the basic level but the last bit of confusion lies in the return being to a hot leg. In a panel or subpanel for a 120V circuit, the return goes to the neutral bus which ultimately is bonded in the main panel to ground right? In a 240V circuit if return comes back via the other hot leg of the circuit what happens to the returned current? Does it just go back to the "pool" and get reused vs. path to ground in the case of a 120V and a neutral path. Basically what happens to the returned current on a 240V circuit.

#4
10-24-18, 12:12 PM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,944

In all circuits current flows in a loop from the source, through the equipment and back to the source. A center-tapped transformer, like the kind the power company uses to feed your home, has three possible loops on the secondary (residential) side. One of those loops operates at 240V, and two of them operate at 120V. You can use them all simultaneously in an overlapping manner which allows for a mixture of 240V and 120V loads to run in the house at the same time.

In this diagram, the 240V loop is pictured in blue. When a device is connected between the red and black hot legs, and completes the 240V loop, you now have a pure 240V circuit. A device connected in this way sees the full 240V output of the transformer. The neutral tap is irrelevant when using the system in this way.

The green loop represents one of the two possible 120V loops. The green one is formed between the black hot leg and the center-tapped neutral, and an identical 120V loop exists between the red hot leg and the white neutral, but isn't pictured in my diagram. A device which completes the green loop sees half the potential of the transformer, 120V, because it literally is tapped into the center of the winding, effectively cutting it in half. A pure 120V device needs only one hot and the neutral, and the other hot is irrelevant.

You also will encounter devices like electric cloths dryers that have both 240V heating elements and 120V drive motor, alarm bell, etc. These type of devices connect with a four prong plug so they have access to both hots and the neutral allowing them to power both 240V and 120V loads simultaneously, but these are called 240V/120V because both sources are required to operate properly.

#5
10-24-18, 03:02 PM
Member
Join Date: Nov 2015
Posts: 1,441
l for a 120V circuit, the return goes to the neutral bus which ultimately is bonded in the main panel to ground right?
While neutral is bonded to ground, the current does not flow into the ground. Neutral is bonded to ground to set a reference point for the transformer. Electricity still works and will have 120V to neutral without neutral bonded to the ground. You will just have some wild voltages in reference to ground, which is potentially dangerous to a human being.

Return path continues ultimately is to the generators at the power plant.
However, technically there really isn't a return path with AC system because it basically is just a pulse pushing and pulling current back and forth.

#6
10-29-18, 09:12 PM
Member
Join Date: Feb 2017
Location: U.S.A.
Posts: 60
Received 1 Vote on 1 Post
More on the theory;

I picked this image up from an earlier forum posting. This diagram shows the voltages of both phases of a typical 120/240 volt system. In a typical situation, about one half of the 120 volt circuit loads will be on the “black” phase, and the other half of the loads will be on the “red” phase. In any one of these circuits (red or black), the neutral wire (white wire) is at zero voltage. (The fact that I am calling it a red phase, has nothing to do with the color of the wire used to carry the load. In these circuits the hot phase is usually on a black wire and the neutral on a white wire.)

If you have a pure 240 volt load, the load is connected between the red and black phases. The current passes between them, and no current passes through a neutral wire. That being the case, there is no need for a neutral wire. In those cases, you can use a two conductor cable (e.g. NM-B 10-2) but, as these cables only come with black and white insulated wires, the white wire should be “marked” with tape, paint, or ink.

In the case of a 120/240 sub-panel, or an appliance that needs both 120 and 240 volt supply, three current carrying wires are needed (typically black, red, and white). In this case, “black” phase loads conduct current between the black and white wires. Similarly “red” phase loads conduct current between the red and white wires. 240 volt loads conduct current between the red and black wires.

Note that in no case in any current supposed to be carried over the ground wire, whether bare or green.

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
Pingbacks are Off
Refbacks are Off