220v Question

mmcwick · Joined: 01 Apr 2011 Posts: 3 Location: Berkeley, CA

I have a basic question about how 220v works: if you disconnect one of the two hot lines to a 220v receptacle, will that receptacle lose all power or will it still be energized at 110v? I assume it losses all power because there is no neutral wire connected to the receptacle, but wanted to confirm this.

Thanks in advance.

**kal** · Posted: Sat Apr 02, 2011 12:44 am Post subject:

I assume you mean 240V.

A typical 240V circuit will have two HOTs and one NEUTRAL.

Across either HOT and a NEUTRAL you have 120V.
Across both HOTs you have 240V.

If you disconnect one HOT, you can still get 120V across the other HOT and NEUTRAL.

For more info, see my page on Supply Power here: http://www.theelectricbrewery.com/control-panel-part-2?page=3

mmcwick · Joined: 01 Apr 2011 Posts: 3 Location: Berkeley, CA

Kal - Thanks for the response. I have read that section a few times and appreciate your explanation.

My original post was based on specific question I had to your wiring diagram for the heating elements. If you close the Power In Relay (via the main key switch) and then close the HLT Relay (via the three-way switch), won't the HOT B line be hot all the way to the HLT receptacle? If that's true and if, for example, the SSR is not supplying power through the HOT A line, will the HLT receptacle and therefore the HLT element be receiving 120v? I'm assuming not, since it would contradict the whole concept of controlling the elements with a PID. I'm looking for an explanation as to why this is. If the HOT B line is not hot at the receptacle in this case, then perhaps I have a fundamental misunderstanding of how the DPDT switches work.

I'm referring to the diagram on this page:
http://theelectricbrewery.com/control-panel-part-2?page=13

rosenjm · Joined: 21 Dec 2010 Posts: 249 Location: Ballston Spa, NY

I'm not really sure I understand what your asking, let's see. If you look at the digram you linked to it appears that only one of the hot lines is "switched" via the SSR. This leaves the other hot live all the time regardless of the SSR position. I assume your question is why the element does not heat up?

When the SSR is "shut" you have 120 V on one side (black) and 120 V on the other side (red) of the element. Each of these lines, or phases is shifted 120 degrees. When you measure from black to red you get 240V (120+120), since the SSR is "shut" you have a path for current to flow, so it does and the element gets hot. With the SSR open you have 120 V on one side (red) and nothing on the other (black), but the SSR is open, so there is no path for current flow so no heat.

In a 120 V circuit, one side (red) is 120 and the other side is ground or neutral at 0 V, giving you 120 V (120+0). Since there is no neutral lead on the element, a single 120V line will not cause them to heat up.

As a side note, when we refer to the line voltage (240 V or 120 V) we are actually talking about the RMS, or root mean square value of the sine wave. The peak voltages are actually higher (0.7 times). If you look at 2 sine waves 120 degrees out of phase, the difference in voltage between them changes constantly. A voltmeter can't respond quickly enough to read those differences, so we take a weighted average, which is about 120V to ground.

Hope that answers your question. Mug

**kal** · Posted: Sat Apr 02, 2011 2:30 am Post subject:

mmcwick · Joined: 01 Apr 2011 Posts: 3 Location: Berkeley, CA

Thank you both - that's exactly what I needed to know. I'm 90% done with my CP and when reviewing my wiring diagram, had an 'oh sh*t' moment in regards to the HOT B wire going to my element receptacle. Turns out the answer is a fundamental of electricity that I was forgetting. Cheers!