Wiring a resmed 90W DC converter

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Evaret
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Wiring a resmed 90W DC converter

Post by Evaret » Mon Nov 24, 2014 11:49 pm

Hi

I have bought a resmed DC converter to use when we go away in the caravan and do not have access to 240 volt power.

The caravan has a 12 volt house battery and there is a 2 pin 12 volt socket in the caravan that I could use (the 2 pin plug is in a t - one horizontal the other vertical). So the easiest option for me would be to wire up a lead to go from the 2 pin plug to the resmed cigarette lighter

There is lots of good info on the site about the design of battery boxes and so on.

But I could not find any details on the size of the wire I should be using for the cable from the 12 volt plug in the wall to the cigarette lighter connection on the converter. (The wire on the converter seems to be labelled 2x1.3 square mm 16 AWG but it is hard to read).

I would appreciate any advice users can offer.

Cheers

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cnaumann
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Re: Wiring a resmed 90W DC converter

Post by cnaumann » Tue Nov 25, 2014 12:26 am

I would recommend 16 gauge wire and fusing the circuit at 15 Amps.

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palerider
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Re: Wiring a resmed 90W DC converter

Post by palerider » Tue Nov 25, 2014 9:50 am

cnaumann wrote:I would recommend 16 gauge wire and fusing the circuit at 15 Amps.
based on?

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Re: Wiring a resmed 90W DC converter

Post by fdw » Tue Nov 25, 2014 10:11 am

palerider wrote:
cnaumann wrote:I would recommend 16 gauge wire and fusing the circuit at 15 Amps.
based on?

Actually it all dependents on the length but 16 gauge is just fine as long as your run is 10' or less and less than 90 watts. As far as the fuse, 15 amp is alright but to be on the safe side I'd use a 12 amp fast blow just to protect things.

But technically he's right, with a few stipulations as noted above.
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CapnLoki
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Re: Wiring a resmed 90W DC converter

Post by CapnLoki » Tue Nov 25, 2014 10:26 am

cnaumann wrote:I would recommend 16 gauge wire and fusing the circuit at 15 Amps.
16 Gauge should work, especially if the run is short and the load is limited. Current for the CPAP should top at about 8 Amps, which for a 5 foot run of 16 gauge is about a 3% voltage drop. This is the guideline for electronics, especially for continuous use.

In this case, however, I would consider 14 gauge since it could be used in the RV for a variety of loads. If the run is more than a few feet, I would definitely upsize the wire. (In other words, just because ResMed decided 16 gauge is fine for a 5 foot run, doesn't mean its good for 20 feet.) I've put a number of 12V outlets on my boat and use 12-14 gauge so I don't have to worry.

BTW, the Respironics 12V cable has an 8 Amp (I think) fuse in the tip.

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cnaumann
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Re: Wiring a resmed 90W DC converter

Post by cnaumann » Tue Nov 25, 2014 4:12 pm

based on?
You caught me. I don't actually know what standard governs the wiring of medical devices on caravans.

I am flying by the seat of my pants here. I did this calculation (at least a quick version of it) but I didn't figure anyone actually wanted to see it. But since you asked so nicely:

The converter is rated at 90W. It is likely that the actual load is only about 80% of that. It is also likely that the converters efficiency is around 80% so these will likely cancel out an dwe can figure our maximum continous load is 90W. A "12V" circuit are usually a little more than 13V, they can be a little higher still, they can be a little lower, but not usually much lower than 11V. Most switching converters have a negative dynamic resistance, they will actually pull the most current at lowest voltage, so it is good idea to calculate the current based on the lowest voltage.

Figuring 90W at 11V is little over 8A. For thermoplastic insulation rated at 60C and conductors in free air, you could probably get by with some pretty small wire just based on temperature rise.

if you believe some random table on the internet:
http://www.allaboutcircuits.com/vol_1/chpt_12/3.html

60C is a pretty warm wire, so you would probably want to use at least 18 gauge wire based on temperature rise.

However, this is low voltage wiring and voltage drop is usually more important than temperature rise.

So lets try to calculate a reasonable voltage drop. There are a lot of unknowns here, I don't know the minium voltage avaiable at the outlet, I don't know the minium voltage required by the converter, I also don't know the cable length. The outlet voltage is probably nominally 12V, the converter is probably capable of working down to at least 10.8V (-10%) but we don't want to blow the entire voltage drop budget on this one cable.

Lets figure the cable can have up to a 5% voltage drop (0.60V) and operates at 8A. Based on this, the maximum resistance of the cable loop should be no more than 0.075 ohms. If we want two conductors spanning 6 feet (12 feet total), we will need a wire with a resistance of less than 0.075 / 12 = 6.25 milliohms / foot.

Again, trusting some random table on the internet:
http://en.wikipedia.org/wiki/American_wire_gauge

This shows that we could possibly get by with 18 gauge wire based on these assumptions. For good measure, and because we did make a lot of assumptions, it is reasonable to go up to 16 gauge wire.

As noted in a previous comment, it is very reasonable to upsize the wire if the cable need to be very long, say over ten feet. The rule of thumb is to go down three gauges to double the length.

Lastly, there is the fusing. We really only need to protect the cable. The CPAP device should have its own fuse. Hopefully, the outlet is fused. Going back to the first table, we see that 16 gauge wire can be continually loaded with 18 amps and operate below 60C. A 15A fuse should provide plenty of protection. In fact, if it can be verified that the outlet is fused at no more than 20A, I would not even bother with a fuse in the cable. I have no way of knowing if the outlet will be fused and batteries can deliver dangerous amounts of current if shorted, so I highly recommend a fuse.

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Sir NoddinOff
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Re: Wiring a resmed 90W DC converter

Post by Sir NoddinOff » Tue Nov 25, 2014 5:26 pm

For all the posts on DC converters for ResMed or Phillips type this into your internet browser's search window exactly as shown (ie. Google, Duckduckgo, Bing etc). Don't bother using CPAPtalk's search function, it's pretty weak:

site:cpaptalk.com resmed DC converter

or for other folks:
site:cpaptalk.com Phillips DC converter

or maybe:
site:cpaptalk.com RV CPAP DC converter

and so on, and so on as you refine your search... you'll find dozens of good threads... read as many as you can handle.

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palerider
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Re: Wiring a resmed 90W DC converter

Post by palerider » Tue Nov 25, 2014 6:15 pm

cnaumann wrote:
based on?
You caught me. I don't actually know what standard governs the wiring of medical devices on caravans.
apparently, (and I didn't know this until I started looking) the number of conductors also affects the ampacity of wire, if this source is to be believed: http://www.engineeringtoolbox.com/wire- ... d_419.html

based on that, I'd probably go with slighty heavier wire and a good bit smaller fuse, but I'm overprotective (pun intended) like that

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cnaumann
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Re: Wiring a resmed 90W DC converter

Post by cnaumann » Wed Nov 26, 2014 12:00 am

The ampicity of a cable depends on a lot of factors including number of conductors, construction of the cable and the intended use. In the end, it usually comes down to the governing body that regulates said cable.

In this case, you really only need the fuse to protect the cable under short circuit conditions. It is unlikely the the cable would ever be loaded in such a way as to draw a current near the maximum rating of the fuse for a prolonged period of time and get uncomfortably hot.

You have a similar situation in your house, it is perfectly acceptable to plug a lamp with an 18 gauge cord into an outlet fused at 20A. Under normal conditions, the cord will see a current of a couple of amps. If the lamp shorts, the cord could briefly see a very large current before the circuit protection opens. However, is unlike that the lamp cord will see an near 20A load for a long time and become uncomfortable hot. While it is acceptable to have an 18 gauge wire plugged into a 20A outlet, it would be totally unacceptable to have the 20A outlet wired to the main panel with anything smaller than 12 gauge wire.
Last edited by cnaumann on Wed Nov 26, 2014 7:39 am, edited 2 times in total.

Evaret
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Re: Wiring a resmed 90W DC converter

Post by Evaret » Wed Nov 26, 2014 1:53 am

Thanks to all for their comments.

It seems I need to add some more info to help with my question.

Unfortunately the wires in the caravan are not labelled but I would guess the distance from the battery to the 2 pin plug is 5 - 6 meters, allowing for the way the wiring is laid under the floor. The distance from this plug to the resmed converter would be another 2 - 3 metres. If I ran another set of wires from the battery to the converter, the distance would be 3 metres.

All wiring from the battery box has 20 amp fuses.

Cheers

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Re: Wiring a resmed 90W DC converter

Post by Guest » Wed Nov 26, 2014 2:01 am

Evaret wrote:Thanks to all for their comments.

It seems I need to add some more info to help with my question.

Unfortunately the wires in the caravan are not labelled but I would guess the distance from the battery to the 2 pin plug is 5 - 6 meters, allowing for the way the wiring is laid under the floor. The distance from this plug to the resmed converter would be another 2 - 3 metres. If I ran another set of wires from the battery to the converter, the distance would be 3 metres.

All wiring from the battery box has 20 amp fuses.

Cheers
I don't see a question. Does that mean you are all set? Is it working? Or not working?

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CapnLoki
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Re: Wiring a resmed 90W DC converter

Post by CapnLoki » Wed Nov 26, 2014 7:13 am

Evaret wrote:It seems I need to add some more info to help with my question.

Unfortunately the wires in the caravan are not labelled but I would guess the distance from the battery to the 2 pin plug is 5 - 6 meters, allowing for the way the wiring is laid under the floor. The distance from this plug to the resmed converter would be another 2 - 3 metres. If I ran another set of wires from the battery to the converter, the distance would be 3 metres.

All wiring from the battery box has 20 amp fuse.
One thing you don't mention is setting of the CPAP. For instance, if you don't use a humidifier the load can be well under an amp. Refer to:
http://www.resmed.com/assets/documents/ ... lo_eng.pdf.
Remember, a "90 Watt Converter" or power supply does not mean it ever actually has a 90 Watt load.

Here's a guide to the US boat wiring standards. Note that they are based on a 3% voltage drop, not how much heat gets generated. There is roughly a factor of four safety margin comparing the 3% drop to the "ampacity" or total safe amperage of the wire.
http://www.acbsphl.org/Tips_and_hints/ABYC_Wiring.htm
Frankly, its an easy thing to upsize the extension cord if there's any thought that a larger load might be used. The bigger issue is checking out the wiring in the caravan. If the only protection is 20 Amp, hopefully the built-in wiring is not too wimpy.

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cnaumann
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Re: Wiring a resmed 90W DC converter

Post by cnaumann » Wed Nov 26, 2014 12:20 pm

I would still recommend 16 gauge wire (or the metric equivalent). If there is a 20A fuse between the battery and the plug, you do not need an additional fuse for the extension cable. I would use heavy duty-lamp cord or appliance cord for the construction. Make the cable to the length needed, don't add excessive length.

Based on the estimates of the amount of wire between the battery and the plug, there is no way to guarantee such a set up will work. There may be excessive voltage drop between the battery and the plug. If that is the case, there is really nothing you can do about if (up-sizing your wire will likely not help) other than run your own (fused) wire back to the battery. I do not believe there is any significant risk in testing the setup I have described. It will either work or it will not. I do not believe that a converter that is designed to operate on battery power would be damaged by a low voltage or excessive voltage drop. There is no added safety risk from trying the set-up that I can think of.

As was suggested, If it does not work, it may be possible to get it to work by turning off the humidifier.