Inverter placement-travel trailer

[bigjim]

I am wondering about the best placement for a 1000w PSW inverter in a travel trailer. I run 2 batteries on the tongue and have run 3 in the past. I only used a small local inverter occaisionally. I obviously want it inside out of the weather. In the current trailer I think I would be looking at a 6-10 ft. minimum run from the batteries at the shortest. I am thinking about routing wire underneath and coming up through the floor. Even though I am considering my current trailer I would still like to hear thoughts about having to use a longer run for future reference.

 

I currently have on hand about 50ft. of number 4 welding cable in good condition.

[Alie&Jim's Carrilite]

The general rule of thumb is as close to the batteries as possible. Check your inverter instructions for the maximum cable size their terminals will accept. #4 is kinda light in my opinion for a 1000 watt 20' (10foot each for Pos and Neg).

[Kirk W]

There are several reasons for locating as closely as possible to the batteries. The current draw is 10 times greater for the 12V that it is on the 120V lines and current loss is caused by the amperage of current and not from voltage. That is the reason that transmission lines operate at very high voltages and low current(relatively speaking). Since power is amps times volts, it is far more efficient to make high current runs as short as possible and it is a major part of the reason for using 120V to power things. It is also more efficient to use alternating current than it is direct but the explanation of that gets deeper into theory than is just justified here.

 

Obviously it would be best to keep your inverter well protected from weather but don't forget it also needs ventilation as they do produce heat. The wire size needed will depend upon the current draw involved as well as the length of the run. An inverter with a 1000 watt output will draw about 85A from the 12V side if you allow a little bit for power loss in conversion, and it will be sending out 8a on the 120V side, when operating at peak output. A 200 watt model will draw about 17a @ 12V and supply about 1.5a output.

 

Of course the 1000 watt inverter would also deplete your three batteries very quickly so I highly doubt that you have anything of that sort in mid but the two examples are just to give you some feel for what we are talking about. An easy rule of thumb is to figure out how many watts of power you are looking to get and then divide that by 12 to get the approximate current draw from your batteries.

[oldjohnt]

Good Question Jim, FWIW I agree with Jim and as I'm sure you and everyone else here already know THE CLOSER TO THE BATTERIES THE BETTER to reduce line voltage drop and to reduce I Squared R heat energy losses plus for safety concerns.

 

Heres how an old rusty retired electrical engineer would approach the problem, but NO WARRANTY ITS CORRECT, I'm NOT an expert by any means, its been too long since I practiced..............Lets look at some charts and numbers:

 

A full loaded 1000 Watt Inverter could "in theory" draw somewhere around (depends on actual load, efficiency and temperature and ratings we don't have) 1000/12 = 83 amps from your battery bank: NOTE 1 that's if the battery voltage were 12 volts, although it may be 12.6 fully charged, yet remember its ONLY 12 at near 50% discharged, but I don't like to draw mine down much over 30% ..LETS JUST USE 12 VOLTS AS AN APPROXIMATION ONLY NOTE 2 Since no Inverter is 100% efficient it would have to draw slightly more from the batteries to yield 1000 watts output power. To get 1000 watts OUT requires slightly more then 1000 Watts IN

 

If you look at the chart linked for 12 VDC distribution,,,,,,,,,,,,,and figure lets say 20 total feet of wire (if Inverter is 10 ft from batteries),,,,,,,,,,,and you don't want to drop over 3% voltage,,,,,,,,,,,,,,,,IF YOU WERE DRAWING 70 AMPS ACCORDING TO THE CHART LINKED FOR 12 VDC, 4 GAUGE WIRE SUFFICES..

 

http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

 

HOWEVER IF YOU WERE ACTUALLY DRAWING 80+ AMPS YOU MAY FALL SLIGHTLY OUT OF THE RECOMMENDED WIRE GAUGE FOR VOLTAGE DROP CONCERNS

 

Since you likely wont often or in any continuous manner be drawing the full 80+ amps,,,,,,,,, and if the length was slightly shorter then 20 total feet,,,,,,,,,,,, your proposed 4 Gauge will "work". NOTE I said it will "work" thats NOT to say its perfect or I am recommending it, that's for the experts and your own call. If the wire is adequately protected I wouldn't loose too much sleep if the total wire length was a tad less then 20 feet and you werent drawing too much over 70 amps for any long continuous time period.

 

Soooooooooo that's my best answer, albeit rusty, now see what others have to say, look at the charts yourself, run some calculations AND DECIDE FOR YOUR OWN SAFETY. If I had that much welding cable on hand its something Id use if it was adequately protected for thermal overload, that way if you draw too much current for the 4 gauge wire you have a safety back up.

 

IE use the 4 Gauge wire fused at 70 amps and that's okay in "my book" BUT ITS STILL YOUR DECISION NONE OF OURS

 

NOTE Its what voltage drop you (or the Inverter) are willing to accept moreso then the ampacity I'm talking about when I speak of protecting the circuit with a 70 amp breaker. That's so if you exceed 70 amps, In which case line voltage drop may be more then 3%, you could stop operation NOT because you're concerned so much about ampacity and heat. BUT UNLESS YOURE RUNNING A HEAVY LOAD ON THE 120 VAC SIDE you may never exceed 70 amps DC input anyway.

 

John T Toooooooooo long retired Electrical Engineer so no warranty on any of this, just my best short answer

[skp51443]

You could always double up the #4 cable and cut your losses in half.

 

Doing estimates at 12 volts is reasonable and it makes conversions between 12 and 120 volts easy but your real world battery voltage can be a lot lower. A resting battery has a voltage that can be easily converted to a charge level but once you start drawing power from it the relationship between voltage and charge level becomes complex. Under heavy load a battery with a nearly full charge can drop to 11 volts or less, not because it is low on charge but because your power draw is greater than the chemical reaction inside the battery can supply power. A common cut-off voltage for inverters is 10.5 volts, scary if you are thinking of resting voltages where that would be a fully discharged and damaging level but under heavy load the battery at this voltage point could still be above 75% in charge level.

 

I carefully calculated everything in my design to properly size every component and piece of wire, then I got a bit of advice and made a small and inexpensive change. The advice was you pay for wire once but losses happen every day. By bumping my wire size up a bit I cut my power losses for a very small price increase. In your case you'd not have more cost, just use up more of your spare wire.

[bigjim]

I'm not tied to the #4, it is just something I have on hand. On the current rig going under and up through the floor I am guess-timating the total distance from the battery to the inverter at 10 ft. maximum. I have a kilowatt meter and calculate based on amp draw now a maximum of 4 amp AC start up and a run of about 1.5 AC maximum. There won't be a constant draw. I have calculated that a 600 watt inverter would do the job but I kind of fall into the catagory of go a little bigger the first time so you don't have to rebuy to move up. I once bought and still use a Trace 30A (Xantrex now) charge controller that is sufficeint but a 40amp would have been better in case I wanted to step up. One thing I don't quite get is you can use larger wire in a lot of cases until you get to the device like charge controller but the terminals won't accomodate the larger wire. On one application for something else I ran some larger wire for most of the distance but then spliced about 8 inches of one size smaller to fit the terminals. I didn't like it but I did it. Is there some better way to do this for something that has a long run and needs large enough guage of wire to avoid excess voltage drop?

[oldjohnt]

Jim, electrical supply houses have all sorts of unions and lugs and splices and terminals etc which may enable you to adapt 4 Gauge Cable to a size that will mate with your inverter, take a piece of your cable and the inverter and see what they may be able to come up with??? I know what you're talking about, I have seen even 2000 watt inverters with such small input terminals you would think they were designed for 10 gauge wire grrrrrrrrrrr

 

With the new numbers you posted above, assuming that's the max load placed on your inverter, I don't envision any problem using 4 Gauge Cable and I'm sure even less would "work" despite my more cautious advice above at which time I was planning for the worse and hedging my bets lol

 

FWIW (to be on safe side and allow for expansion) I bought a 2000 Watt PSW to operate my teeny tiny 120 VAC dorm style fridge which requires like 3 to 6 amp surge then runs not much over 1 amp, and I'm not admitting to anything lol, but there may be less then 4 gauge cable to it!! but its within 2 feet of my battery bank. I have four golf cart batteries in series parallel with 460 Amp Hours and run my fridge via the inverter 24/7 even when dry camped for days on end, but I have 400 watts of rooftop solar. FWIW NOT my advice, my buddy has an older fridge that sucks more energy then mine which he runs off a 600 watt inverter fed with 6 gauge cable.

 

GO FOR IT

 

Best wishes and God Bless all here

 

John T

[Yarome]

All of the above is good advice. Not that what you are considering isn't completely doable, but given your particular situation.. and if it were my own rig.. I might take a little different approach than just "doable". For me, building in a 3% ampacity loss into my "backbone" wouldn't be acceptable. A 10' run (20' round trip) would equate to a .4v loss from source to inverter input. With a 1000w inverter, you don't boondock much, don't run lengthy heavy loads, and have an ample battery bank... that really won't cramp your style much. However:

 

Getting to the point. You have all that pretty #4 laying around not doing much. There is no reason you couldn't purchase dual conductor lugs for each leg and effectively cut your ampacity losses in half while also allowing a little "room to grow" (without having to rewire) if you were to choose to upgrade your inverter in the future. I would call that a win-win with very minimal expense.

 

Not that all ampacity charts on the internet are inaccurate, but they should be taken with a grain of salt. Using a calculator with your specific values would be preferable. A quickie search pulled up this one. Using known values it appears to be accurate, simple to use, and is capable of doing multiple conductor calculations.

 

Just thinking out load.

[oldjohnt]

Hey Yarome,

 

Indeed there are hundreds of on line voltage drop charts and calculators and some (not the one you posted) are for AC ONLY which many don't realize. I did NOT take the time to dust off my old outdated NEC Handbook, but back in my day we were NOT permitted to use parallel conductors except for certain amperages and conductor sizes (higher currents and larger conductors), but that was regarding AC transmission. It does make a person sleep better to know he's NOT operating anywhere near the limits, but of course, if that small fridge he mentioned were his max load he will get by fine BUT IF HE WANTS TO ALLOW FOR EXPANSION (always a good idea) THEN HE MAY NEED SLIGHTLY BIGGER WIRE (or parallel those No 4's where permissible)

 

PS when you're only dealing with 12 volts to start with, you cant afford to loose too many across the line !!!!!!!!!!!!!!!!

 

Good chattin at ya

 

Are you thinking and typing out loud also lol

 

John T

[bigjim]

In the past I have boondocked a fair amount on 2 100W panels and 3 12v deepcycles, in another case with 2 6V& 1 12v wire up for 12v, and for the last 2 year with just 2 6v without much problem. The reason I went with the mismatched configuration was free deepcycle batteries. The ones I have now are about gone and are 2 6v. I would go for 4 6v if I could but I just can't.

 

John, your post #9 made me go back and read my posts to see if I had mentioned a small fridge. If I did I couldn't find it. But that was a good call based on the information I furnished.

 

(20' round trip) Yarome , good jog of my thinking vs my thinking of only 10ft.

[Jack Mayer]

The voltage drop calculator that Yarome posted is the one I use and recommend. It is accurate. And it covers all variants of usage for RV/solar/electrical projects.

 

PLEASE use the calculator when designing your systems. I have seen SO MANY systems miswired that it brings tears to my eyes....

[Yarome]

John, your post #9 made me go back and read my posts to see if I had mentioned a small fridge. If I did I couldn't find it.

 

"maximum of 4 amp AC start up and a run of about 1.5 AC maximum"

 

Translated means: "small residential reefer".

 

PLEASE use the calculator when designing your systems. I have seen SO MANY systems miswired that it brings tears to my eyes....

 

Amen!

 

And don't forget to check the max amp transmission ratings of the wire you intend to use. Ie., generally speaking... #4 has a 60a rating. Granted.. that's a very conservative rating, but it shouldn't be ignored.

[oldjohnt]

All this talk about wire gauge and ampacity and charts and tables and calculators and which ones are most accurate or better etc etc brings to mind an NEC course I attended years ago when the instructor (it may have been Joe McPartland or Mike Holt I forget) asked what the ampacity of a certain gauge of wire was so an eager student raised his hand and quoted from a particular table in the NEC and shouted out a number..........To which the instructor asked:

 

What temperature,,,,,,,,,,,Wires in free space,,,,,,,,,, or buried,,,,,,,,,,,,,if so direct burial or in conduit or encased or not,,,,,,,,,,,or in conduit,,,,,,,,,,, and if so how many and what size,,,,,,,,,,,,,,,, or jacketed,,,,,,,,,,,,,,,,and class and type of insulation,,,,,,,,,,,,,,and about 5 or 6 other questions lol.

 

The point being when someone or a chart or table or calculator says for example 12 Gauge wire is rated fro 20 amps THAT MAY OR MAY NOT BE THE WHOLE STORY

 

Moral of the story to insure complete accuracy look at all the relevant data and only then can you derive the correct ampacity, which may or may not be what one certain table or chart has to say. It has to do with how many amps a certain wire size and class of insulation and number of conductors in a certain location at x temperature can pass without degrading the integrity of the insulation and dissipate the heat generated.

 

If a person wants to be on the safe side and conservative he could use the worse case scenario when deciding what size wire to use.

 

Speaking of No 4 Copper wire, look at the table below for an example, 70 amps or 85 amps or 95 amps?????????? If you figure 60 or 70 you would be on the safe side even if its overkill AND THATS A GOOD WAY TO APPROACH THE PROBLEM IN MY OPINION

 

Wire Size and Amp Ratings

 

Copper Aluminum 60°C (140°F) 75°C (167°F) 90°C (194°F) 75°C (167°F) 90°C (194°F) Wire Gauge Size NM-B, UF-B THW, THWN,
SE, USE, XHHW

THWN-2, THHN,
XHHW-2, USE-2

THW, THWN,
SE, USE, XHHW

XHHW-2, THHN,
THWN-2

14

15

15

15

---

---

12

20

20

20

15

15

10

30

30

30

25

25

8

40

50

55

40

45

6

55

65

75

50

55

4

70

85

95

65

75

3

85

100

115

75

85

2

95

115

130

90

100

1

---

130

145

100

115

 

 

 

Hope this sheds some light on the subject of wire size and rating, it sure opened my eyes when I took that class !!!!!!!!!!!!

NOTE Of course, Voltage Drop is still I x R that's another issue besides ampacity as discussed above WELL DUH and as a conservative designer I often just used the LOWEST rating across those types of charts and tables............

 

 

Fun chattin with yall, best wishes and God Bless

 

John T Too long retired Electrical Engineer and no expert but I enjoy the topics

 

 

OOPS the chart didn't copy and paste correctly, but for 4 gauge copper the ampacity was 70 or 85 or 95 depending on temperature and insulation and other factors grrrrrrrrrrrrrrrrrrrrr

 

LOOK HERE http://cerrowire.com/ampacity-charts