Inverter Manual Transfer Switch Wiriing Questions

[jonw]

Hi everyone -

I have an Optima deep cycle battery that I can hook up (by plugging in a connector) to a CTEK charger when on shore power. Currently the battery is only used for 12v lighting and a ceiling vent fan. It will continue to stay hooked up to my 12v panel after the modifications below.

I've purchased a Harbor Freight 750W inverter to primarily to run a CPAP machine when boondocking. So I'm not having to connect and disconnect device plugs all the time I want to do some rewiring and add some switches as follows:

1) attach the battery wires to a DPDT 20A switch so that the battery will be connected either to the CTEK charger (output) or the inverter (input), so that only one can be used at a time. The switch has a center off detent that allows completely disconnecting both from the battery (so when boondocking I can completely electrically disconnect the inverter when not using it).

Questions about this setup:

- the inverter power source wire is 8GA, and the CTEK output wire is much lighter than that. I'm assuming I'll need to run 8GA wire from the battery to the DPDT switch. Any problems with the charger connecting to the battery through such a large wire? Current loss? Any affect on the charger reading the state of the battery?

- I'm going to look for some ring terminals for the 8GA wire to use to connect to the rather small screw terminals on the back of the DPDT switch. The ones on the 8GA wire are quite large. Any problems using smaller ones that better fit the switch terminals? Heat buildup?

- the inverter has a fan so I assume it needs to be mounted somewhere with good airflow, like NOT in my under-counter storage compartment by the battery. Can these be mounted to side walls so they don't take up counter space?

2) I have a second DPDT 20A switch to control the 120V power source, so that only one can be used at a time. The two inputs would be shore power and the inverter output, and the switch output will be the wiring to my 120V electrical boxes. Any issues? I would think this is a fairly straightforward application.

For the usual scenarios these switches would be set as follows:
Shore power - 1=charger or off, 2=shore
Boondocking, need 120V: 1=inverter, 2=inverter
Boondocking, no 120V: 1=off, 2=off

Thanks in advance for your help and feedback!

[Shadow Catcher]

Are you sure you need 120AC for your CPAP, many do not or have 12v adapter, which means they are really DC appliances. The efficiency of the HF modified sine wave/square wave inverter is not stated but suspect.
I just wired our trailer to use a Morningstar SureSine inverter and used a 30A DPDT switch to switch between shore power and inverter I have however differed in that I have set up the the inverter to plug into the battery and the AV outline using Anderson Power Poles.
The 8 ga wire does not make a lot of sense trying to push it through a 20A switch.

[Dale M.]

The larger the wire gauge (#8) the less voltage drop the less current loss so large wire size is not a issue beyond practicality....

Wire gauge (size) should be pretty much determined on "load" requirements and not arbitrarily on physical size... The main thing to avoid is using to small a wire gauge... Since you are using a switch only rated at 20 amp I am going to guess your load factor is less then 20 amp... A 12 gauge wire should be more than sufficient for voltage/current requirements for a 20 amp load...

I would also try to simplify all your power needs by everything controlled by one switch.... Position 1: Shore power - Center off - Position 2: DC Power (everything AC on inverter)... You may want to consider something like a simple toggle switch controlling a couple of relays or a master cutover switch...

Also take advantage of a lot of the "auto" features on any inverter/converter/chargers you run .. Converters and chargers will only operate when shore power is available and any inverter will only run when DC power is available... All this can be controlled of a single (SPDT- Center off) toggle switch and some relays...

Dale

[jonw]

Are you sure you need 120AC for your CPAP, many do not or have 12v adapter, which means they are really DC appliances. The efficiency of the HF modified sine wave/square wave inverter is not stated but suspect.

---(snip)---

The 8 ga wire does not make a lot of sense trying to push it through a 20A switch.

Unfortunately the DC input for the CPAP is 24v, not 12v.

The 8GA wire came with the inverter, so I'm assuming that's what it requires. The switch is rated at 20A for 120v. Am I wrong in assuming it will be fine for the 12v battery power coming in on 8GA wire?

Thanks for your help.

[jonw]

I would also try to simplify all your power needs by everything controlled by one switch.... Position 1: Shore power - Center off - Position 2: DC Power (everything AC on inverter)... You may want to consider something like a simple toggle switch controlling a couple of relays or a master cutover switch...

Also take advantage of a lot of the "auto" features on any inverter/converter/chargers you run .. Converters and chargers will only operate when shore power is available and any inverter will only run when DC power is available... All this can be controlled of a single (SPDT- Center off) toggle switch and some relays...

Dale

Thanks Dale - good idea on the single switch, I just couldn't figure out how to do it. I'll look into relays, but I don't mind having two switches instead of one.

I think I read in the CTEK manual that it draws a little battery power when off, which is why they have a quick coupler on the battery cord. I'm also wondering if the HV inverter does the same since it's manual recommends disconnecting from the battery when not in use.

[GuitarPhotog]

Your 120VAC switch will not accept 8GA wiring. You will need either a bigger switch, such as a battery transfer switch like this http://tinyurl.com/7o9qkws or you will have to reduce your wire size to match the switch.

<Chas>

[jonw]

Thanks everyone. I think I'm going to go with 12GA wire with crimp-on connectors at the end. Realistically I'll probably never use 750W of power thru the inverter anyway and I've got the trailer set up for max 15A 120v service.