Two-throw disconnects

13 years ago

I need three new heat pump disconnects (single phase). I would like them to be double-throw to select between two power sources if that is practical. The only double-throw disconnects that I see are pretty expensive by my standards. Am I overlooking something? Some info on what the manual says about the power supply: 208/230 V, max fuse size, 20 A (time delay), min circuit ampacity 15 A .

Details:

I am having three mini-split heat pumps units installed at my home soon. I have an electrician ready to come as soon as the AC contractor specifies location for the outdoor units. I anticipate installing backup power in the near future either associated with a solar PV install or generator or both. Automatic transfer will be part of that. I want to be able to choose between which is the default power source for the individual mini-spit compressors "normal" power or backed up.

Why do I want to do this? Because there may or may not be enough energy available to run all three heat pumps and I want to be able to choose between them depending on the weather and the state of my batteries and fuel supply.

I suppose I could install two large, manual transfer switches that allow me to divert the whole main service panel to backup power when I am home and able to control the load. One would go between the main service panel(s) and the meter. The other between the backup power and the ATS panel in one position, and the backup power and main panels on the other.

Why am I asking here since I will have one master electrician there soon and another ME that is a also an EE to discuss it with? I see evidence of centuries of human hours of experience here so it seems worth it to pick your memories about available equipment and strategy.

Comments (7)

brickeyee
13 years ago
Good luck.

There is not a lot of market for what you are looking for.
wayne440
13 years ago
If you must have 3 separate switches, this is as close as you will probably get.

A better solution might be to feed all three of the new HVAC from a small subpanel and use a single X-fer switch for said panel.

Here is a link that might be useful: small transfer switch
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ionized_gw
Original Author
13 years ago
Thanks, Brickeye. It looks like economies of scale are working against me in this case as I expected.

Interesting idea, wayne, thanks.
ontariojer
13 years ago
I don't know what your Price point is, but hubbell dpdt 20a switches are around $100 each. That doesn't seem like much considering the overall cost of 3 split systems and backup generation. Just my 2cents.
ontariojer
13 years ago
I guess I should have pointed out that the disc. Should be separate and go after the dpdt switches for each unit, in case that wasn't obvious.
brickeyee
13 years ago
"I guess I should have pointed out that the disc. Should be separate and go after the dpdt switches for each unit, in case that wasn't obvious."

Unless you can find a 'center off' type switch, then it can be used as a disconnect.

Since you are only on a 20 amp circuit there are toggle and snap switches available, you have to look for a horsepower rating also, not just current.

Opening a switch on an inductive load like a compressor results in some higher than normal voltages from the magnetic field of the motor collapsing.

A higher clearance switch (more distance between open contacts and faster opening action) is required.
ionized_gw
Original Author
13 years ago
I may have been too hasty with the economies of scale comment. I suppose a simple disconnect is built for occasional use, whereas a DT switch is probably built for a lot more cycles.

The idea of using a small toggle or snap switch has occurred to me. I suppose a traditional disconnect downstream from something like that might be a good idea, even though it might not be required, so no one can be confused. I would also take the load off the (possibly) less-capable contacts on the little switch.

Thanks, Wayne, I have been bouncing around some alternatives similar to that. I might be able to use a small subpanel with one of those cute interlock kits. I don't know if they really come small enough to be practical. I am trying to keep from making the house look like a light industrial building and it is a raised house so the panel will probably have to be outside. I already have two full-size panels at the meter.

Ontariojer, thanks for the information. That is hundreds lower than the prices I have seen for DT disconnects. Yes, that would be reasonable. Yes, I am a cheapskate!

Yes, brickeyee, I was looking for "center-off". It has been a long time since Kinderphysics. I have forgotten all that right-hand and left-hand rule stuff, but I do remember the magnitude of power available from field collapses. One just has to think about transformers and ignition coils. Your tips might be useful for searching with my friend Google, "motor" and "HP".

Now, how about a electrical code/technical question? DPDT was mentioned, but I would like to know, even though my previously-mentioned experts will be doing any installing. Since I will be switching between two panels, main and sub or two subs, do the neutral and grounds need to be switched too? I need to know if three or four contacts are needed and not just two.

You guys are great!