O.T. Wire gauge

[MrFixit]

I am hooking up my 1500 Watt 12VDC/120VAC Power Inverter in my Truck. I calculated it will require 125 Amperes from the electrical system. The alternator can handle up to 160 Amperes. My question is "How do I determine the wire gauge that will handle 12VDC 125 Amperes safely"?

 

[rbel]

p/v = I 1500 / 120 = 12.5 amps so you better look at #10 at the smallest, and it should be stranded not solid

 

[Dusty MI]

Here's a chart for the amount of current a wire can carry. In the electrical code book the size varies according to the type of insulation of the wire, and how it's used.

http://www.powerstream.com/Wire_Size.htm

Dusty

 

[JDemaris]

A 1500 watt inverter needs:
2 gauge if only a foot of cable
2/0 copper cable if no longer then 5 feet
4/0 copper cable to a max of 10 feet.

You never wire an inverter directly to the alternator. It needs a buffered power source. You hook it to the battery (or battery bank), and the battery gets recharged by the alternator.

You didn't mention actual specs on the inverter. 1500 surge watts or 1500 constant duty watts? They all have two ratings. Most 1500 watt full-time use inverters are rated at surges up to 2500 watts. So, the wire gets sized by the surge rating.

Wire size is determined by wattage AND length of run AND allowable voltage drop.

1500 watts with a wire 4 feet long requires #2 gauge copper wire with a 2% voltage drop.

I've got inverters all over the place if you want some info on them. My house runs on 240VAC from inverters. My truck has three inverters -one 1500 watt and two 3000 watt inverters.

Many inverters on the market can't handle near what they are advertised at - so you have be very careful what you buy.

 

[John T]

MR Fixit, Im taking you at your word 125 amps is the max continuous current required (the genny can have all sorts of continuous or peak etc ratings) so based on that I WOULD USE NO LESS THEN NO 2 COPPER WIRE, assuming the Inverter is located near the battery as I suspect and recommend (to reduce voltage drop and save on copper). If its way back in the rear I would bump the wire gauge up in size. (There are all sorts of voltage drop calculators on line where you type in the length and amps and wire size and it calculates voltage drop) It dosent hurt to use bigger wire which reduces the voltage drop of course.

Good cable terminations are a MUST and I would use those insulated type of cable clamps and use stranded wire and wire loops in any critical places to allow for flexing and vibration. You do NOT want any No 2 hot wire shorting to frame ground OUCH...

Of course the battery would be where the inverter cables attach (disconnect and fuzing sure wouldnt hurt) and a 160 amp alternator should handle the load of the inverter and other typical truck loads such as lights and heater fan etc


Best wishes and God Bless

John T

 

[John T]

JD, Youre my kinda guy, I thought I was the "King" of conservative design and overkill to avoid voltage drop, but for 125 amps in 10/20 feet of cable to go all the way up to 4/0 Copper now thats PLENTY OF AMPACITY Woooooooooo Hooooooooooo The voltage drop should be miniscule and I cant argue with that (Im not paying for the cable lol)

Wayyyyyyy back when I designed Secondary Power Distribution if there was any doubt or things got close, I usually, to be on the safe side, would up the wire size at least one size higher, but if I went two sizes over the boss and "bean counters" would raise old Ned and rightfully so actually as copper gets expensive especially when not needed.......

Fun discussion, take care now

Ol John T Too long retired from this stuff.........

 

[JDemaris]

I was citing the usual mininum standards. I am assuming this guy has a cheaper modified wave inverter, and not a full-wave inverter.

That is fine and the cheaper inverters are usually more efficient anyway. Often 95% efficient instead of 90 %.

But, modified wave inverters advertised as "1500 watts" can have 2500 to 4000 watts surge for 10 seconds, 1500 watts for up to 30 minutes steady, and 1200 watts for full-time use. So, if you want the inverter to work properly and handle momentary surges - e.g. for starting motors, you need cables that can carry 2500 watts and sometimes more.

I have a 1500 watt inverter here that has a surge rating of 4000 watts. It calls for 4/0 cables to the battery bank that is 10 feet away.

Advertising can be very deceptive. Reputable sellers with advertise by the mid-watt rating. Rip-off dealers will advertise by the max. surge rating.

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[buickanddeere]

The alternator maybe "rated" for 160amp but....... that's for about 2 minutes. Continuous loads of over 80amp and it's going to run hot.

 

[JDemaris]

Since the guy is asking here at a non-electrical based forum, I assuming his knowledge of inverters is limited.

There have been some very dumb posts and arguments on these forums about inverters in the past. Several posters have complained that " you can't get good 120 VAC from an inverter."

I'm not saying this post is "dumb" but am trying to put some myths to rest. When someone claims it can't be done, and I'm sitting here in a house running on 240 VAC from twin 12K watt inverters - it gets a bit frustrating.

With household electricity and also with household appliances, there's a pretty good match of terminolgy. A wall socket with a 15 amp rating will work with an applicance with a 15 amp rating (or 1800 watts), even if it takes 25-30 amps when first started. This was not always true with old fuse boxes that had quick-blow fuses in them. Slow fuses handle it better as to modern circuit breakers.

The problem is, most inverters rated at 1500 watts will not run many 1500 watt apliances or tools. So . . . sombebody buys a 1500 watt inverter. Then plugs in their 15 amp hand-held saw and the inverter breaker trips - and nothing works. He then comes to this forum and says "inverters don't work."

Or, somebody buys a good and genuine 1500 watt inverter (that's rated at 3000 watts surge or more), hooks it to ten feet of #4, #8, or even smaller wire and again - plugs in a saw and it won't work. Note that someone already suggested 10 gauge wire which would not even run a true 500 watt inverter.

The problem is mis-information and bogus advertising. A 15 amp wall socket in a house can deliver 30 amps for short periods of time. So, you plug a 15 amp saw in, hit the button, and the outlet handles the surge at starting.

If proper convention was followed, most inverters sold as "1500 watts" should really be sold as 800 or 1000 watts.

The other problem is, even when a person gets an inverter that is truly rated for the job, it often gets mis-wired.

 

[John T]

WELL SAID, as an engineer myself I see some good answers here others maybe not so good. (Its even worse when legal questions are asked and EVERYONE jumps out of the woodwork with an opinion) Often theres insufficient information provided such as the ratings and/or nameplate data from which we can give a more accurate response. Thats why I said I take him at his word that he required 125 amps HOWEVER as you so well noted it depends on the peak or continuous or all other sorts of ratings or hyperbole as to what the inverter may actually draw .....

Im still like you it appears, I prefer oversize AS OPPOSED TO UNDERSIZE when it comes to wire..

Best wishes, great info and discussion

John T too long retired EE

 

[Anonymous-0]

If that chart is for 120 volt and higher circuits, it neglects voltage drop that is crucial at 12 volts. At 120 volts a couple volts drop is no problem to the load. At 12 volts a couple volts drop reduces the available power nearly 20% and that's not acceptable.

Gerald J.

 

[MrFixit]

Thank you John. I was asking about how to determine the proper wire gauge because the inverter came with 6' 4 AWG cables. The inverter will operate 1200W continuous and 1500W for 30 minutes. The problem was the cable would warm up when the inverter supplied 1200W and become hot to the touch providing 1500W, which was a concern.

I calculated the inverters current draw using Iin = (Vo x Iload) / (Vin x Efficiency). The formula is used to determine the switching power supply's input current at a given input voltage providing output voltage and ampere. The power inverter is a switching power supply. The inverter operates at 87% efficiency. Since power(watts) is E x I, I already know it's 1500W max. I have nothing in the truck to produce a power surge similar to that of an electric motor.

Iin = (Vo x ILoad) / (Vin x Efficiency)
Iin = (120 x 12.5) / (13.8 x 87%)
Iin = 1500 / 12.006
Iin = 124.9 amperes

For 12V input Iin = 143.7 Amperes
For 14V input Iin = 123.2 Amperes

As you can see by running truck engine, the input current drops by 20.5 Amperes. The other problem I had, as you mentioned, was the voltage drop. The voltage drop was 1.5V. I did manage to reduce the voltage drop to .80V by shortening the ground cable and tie it to the chassis. The cables still become warm to the touch which I don't like.

Thanks for the tip John. I'll see what I can find online. I figured there had to be a way to determine a wire size for set current draw and to minimize voltage drop. Thanks again.

 

[MrFixit]

You need to get off your pedestal jdemaris! I'm not as dumb as u want others to believe. I have stripped down inverters piece by piece, rebuild them piece by piece, and built my own from what I've learned from them.... Modern power inverters create what's called a modified square-wave which simulates that of a sine-wave in house hold current. The power inverter I have does provide 118VAC at 1500W that works well for my 1000W electric stove, 1200W electric skillet, laptop, microwave, shop vac, breathing machine, slow cooker, radio, etc. in my truck! The inverter came with 4AWG cable which I believed would be too small that's why I asked if there was a way to determine what wire size to use for 125 Amperes.

Now if you like jdemaris, I'll be happy to show you ALL the calculations concerning Power inverters????

 

[MrFixit]

For those of you who need Full details about my inverter:

DC/AC power inverter

Input: 12-15VDC
Output: 120VAC modified square-wave
Power output: 1200W continuous, 1500W for 30 minutes, 2500W peak
Efficiency: 87%
supplied cable: (2) 6' 4AWG which is too small

input current requirement:
Iin = (Vo x ILoad) / (Vin x Efficiency)
Iin = (120 x 12.5) / (13.8 x 87%)
Iin = 1500 / 12.006
Iin = 124.9 amperes

For 12V input Iin = 143.7 Amperes
For 14V input Iin = 123.2 Amperes

Any Questions???

 

[MrFixit]

Thanks dusty. that will be helpful.

 

[MrFixit]

For those of you who need Full details about my inverter:

DC/AC power inverter

Input: 12-15VDC
Output: 120VAC modified square-wave
Power output: 1200W continuous, 1500W for 30 minutes, 2500W peak
Efficiency: 87%
supplied cable: (2) 6' 4AWG which is too small

input current requirement:
Iin = (Vo x ILoad) / (Vin x Efficiency)
Iin = (120 x 12.5) / (13.8 x 87%)
Iin = 1500 / 12.006
Iin = 124.9 amperes

For 12V input Iin = 143.7 Amperes
For 14V input Iin = 123.2 Amperes

Any Questions???

 

[JDemaris]

Hmmm. About your commentary . . .

"You need to get off your pedestal jdemaris! I'm not as dumb as u want others to believe."

What the heck are you talking about? You asked a technical question and I answered with technical facts. At least, as best I could since you provided incomplete information.
I'm not trying to convince anyone that you are "dumb" . . . but you seem to be doing a pretty good job of it yourself.

Also, about your futher comments:

"Modern power inverters create what's called a modified square-wave which simulates that of a sine-wave in house hold current"

That is a silly statement, if meant to prove some point. Simulated square wave does not work well with all electronics. That is why it's called "simulated." Modern inverters come as modified square wave and full wave. That's simplistic, but good enough for now. The full wave inverters cost more and are required when feeding power back to power companies or when running certain types of electronics or battery chargers. Anybody that has a grid-tie electric system where they sell their excess electricity back to the power company has to use a "modern" full wave inverter. Not a "modern" modified square wave inverter.

I'm not sitting on any "high horse" as you put it. Seems though you are way down in the valley when it comes to knowledge about inverters, amp draw, motor staring needs, etc.

Also note that modified wave inverters do not work well with most conventional battery chargers. A full wave inverter is needed in those cases.

Since you now claim to be an expert, why the heck did you ask the questions on these forums?

 

[buickanddeere]

You may not have known jdemaris and weighed his input for as long as most of us here.

As for myself, I'll believe 99% of what jdemaris says without question. He has a well rounded background of theoretical and practical real world experience.

 

[JDemaris]

You need to ask yourself if you need that peak current at 2500 watts and maybe more. If using something with a motor, you probably will and maybe more. Most hand-held circular saws rated at 13 amps won't work with an inverter rated at 2500 watts -even if wired correctly. If the input DC wire is too small, chances of working are worse. General rule of thumb is you have to double the rated wattage if a motor is involved. So a typical 1500 watts saw (13 amp) needs at least 3000 watts to get started.

I've done a lot of off-grid work with AC power tools running off inverters, and have a pretty god idea of how much one make can vary from another. The ratings they sell them by are sometimes accurate, sometimes conservative, and sometimes greatly exagerated. Also note that 87% efficiency is pretty low for a modj-wave inverter. 93% is more the norm.

Typical cheaper modified wave inverters rated at 1500 watts and wire sizes called for by the companies that sell them:

Xantrex/Trace 1500 watt, 12 volt inverter (surges to 4000 watts):
- 2/0 AWG for runs up to 5 feet
- 4/0 AWG for runs 5 to 10 feet

AIMs 1500 watt, 12 volt inverter (surges to 2500 watts):
- 2 AWG for runs up to 10 feet
- 1 AWG for runs 10-15 feet

Power Bright 1500 watt, 12 volt inverter (surges to 3000 watts)
- 4 AWG for runs up to 4 feet
- 2 AWG for runs 4-6 feet
- 1 AWG for runs 6- 10 feet

 

[JDemaris]

Thanks for that comment. His (Mr.Fixit's) irate reply has me a little baffled. I tried to answer a techincal question with a techincal reply and this guy goes wacko? Maybe it's my ex-wife using a fake ID?

Did he think when I mentioned some previous "dumb" posts I meant him? I did not, and thought I made that quite clear.

There was an inverter discussion here months ago, and some guy got upset and told me that 120 VAC cannot be made from 12 VDC. That's the post I called dumb - after the guy tried to make an idiot out me (and hopefully failed). I'm sitting here in a house running on 120 and 240 VAC being made by two Outback inverters and this guys tells me I'm not? How else does one reply?

 

[whizkidkyus]

jdemaris,

I'm not trying to get into the middle of this and make him or you mad at me. I'm just trying to diffuse a " tense " situation so that there will be no hard feelings and to prevent this from getting worse. You'll have to over look my friend , MrFixit , he gets a little ill sometimes if he thinks that someone is insulting his intelligence. He sometimes takes things the wrong way. Actually, he does know a lot about power supplies , inverters , computers and electronics. I'm not saying that he's perfect because we're not, everyone makes mistakes .To make a long story short , he's touchy when it comes to being insulted about his intelligence and he's a little high tempered . He more or less thought that you were calling him dumb and insulting his intelligence. Yes, he'll admit that he didn't give all the complete details. He's the type that will tell you what he thinks and he'll do it in a paragraph or less and get right to the point. Me , on the other hand , I'll politely tell you what I think by poltely writting an eassy or two and I'll keep it decent and social. We discussed it on the phone and I tried to argue your side and see his side. What i'm trying to say is that you just need to overlook it . He's really a nice guy when you get to know him .

Have a nice day ,
Whizkidkyus

 

[JDemaris]

There is a difference between him "thinking someone insulted him" and someone actually doing it. The reality is he screwed up and flew off the handle when all I did was attempt to respond to his question accurately.

I did not write anything directed towards him that was insulting before he lost emotional control of himself.

That is easy to verify, since these conversations are written, for all to see.

I wrote, and I quote myself. . .

" there have been some very dumb posts and arguments on these forums about inverters in the past. Several posters have complained that ' you can't get good 120 VAC from an inverter.'
I'm not saying this post is "dumb" but am trying to put some myths to rest."

Note that I made specific mention of previous posts and not this one. It was in reference to a discussion that took place months ago.

If your friend is so well versed in electronics, as he now claims, why ask these questions on any forums . . . especially an antique tractor forum?

I gave him correct information, based on common information that is common knowledge to someone that works often with inverters and battery banks. I've been doing it for over 30 years. And note, I don't say this to put myself on some "high horse" as your friend states. It's just a fact. Some things I know a lot of about, and some things I don't. I guarantee you I know more about inverter theory, history, design and real world application then your friend does. And note, that is not an insult unless he choses to make it one. I suspect there are things he may know more about than I do.

Also note that I sent him a direct email on this subject, rather then attempt to hash it here, where it does not belong. He chose not to respond.

As I see it, he lost control, made a stupid emotional mistake, and won't take responsiblity for it.

I spend a lot of time trying to help others, when I can, if I think I'm qualified. And, others have done the same for me. I won't waste time with those that are just jerking around and looking for trouble, with a huge chip on their shoulder.

 

[charles todd]

I have a 1500W/3000W Inverter, Modified Sine Wave. I use it to provide backup power to my house when storms (hurricanes) knock out the power. I am thinking of getting a 2nd one to provide 240V. I am running it off of six 127Ah 12V deep cycle batteries wired in paralell. These were former telephone company backup batteries. I maintain them on a float charge like the phone company did.

It DOSEN'T run ALL electronics perfectly. The TV, satellite reciever, CFL's, and ceiling fans work fine. Though the CFL's and fans do run noisier. My microwave sounds like SH!T and I do not run it off my inverter. I have had mixed results with MSW inverters, but they are cheap power. I gave less than $100 for my unit at SAMS.

CT

 

[MrFixit]

I owe you an Apology jdemaris. I did jump the gun and I am sorry for doing so. Please accept my apology. The reason I asked on this forum was due to other electrical questions I found. I figured someone could help me with what I was looking for. ALL I wanted to know was how to calculate a wire size for 125 amperes 12VDC for so many feet with minimum voltage drop. The cable that came with the power inverter was 6' 4AWG and gets hot pushing 1500W and had a 1.5V drop on the input supply. I have nothing in my truck that would push it past 1500W, well, unless I turn everything on at once.

I used the same formula as I do in Switching power supplies since Power inverters are switching power supplies. The formula was posted since people wanted to know how I came up with 125 amperes. The mistake I made was mentioning the inverter. Everything I know of electronics, computers, electrical is from hands-on experience. It was the only way I could learn what I wanted by taking things apart and been doing that for the last 23 years. I tried to go to school for it but kept getting turned down for various reasons. Right now, I'm fighting my company about this damn inverter. Since 2 drivers burned their trucks down after installing an inverter, guess what, I must be just as dumb since I'm a driver too. That's my companies philosophy along with a lot of other people's.

I've been on here before to help other people if I think I could be of some help, only to be criticized each and every time. You assumed my knowlege was limited about Power inverters because I made a post on this forum. I do not claim to be an expert nor pretend to be one. I only asked a simple question that needed a simple answer. I didnt ask what others have done in other posts. I do understand how inverters work. It does create a 'modified square-wave' also known as a 'stepped approximated sine wave'. The earlier versions of power inverters, like the one whizkid has on top of his metal cabinet, used a regualar square wave. They worked fine with most equipment. The square-wave did have problems. For instance, the square-wave was substantially lower than the peak voltage of a sine-wave. The square-wave produced harmonics in the waveform causing buzzing sounds and problems with some equipment. Modified square waves or stepped approximated sine wave is a combination of the sine wave and square wave. The voltage peak and the waveform closely resembles that of a true sine wave allowing more devices to be used.

All I wanted to know was how to calculate the wire size to handle the input current with minimum voltage drop....