Adding an Inverter need fuse info

Sumner · Nov 27, 2009

Look on the inverter. There should be a label there what will tell you the input amps at 12 volts. From that you can determine the wire size and fuse you need, but the instructions should also tell you that. If it has a 1800 watt continuous it might have a lot higher temporary rating and that might also need to be taken into account on the fuse (maybe a slow blow one) and/or the wiring size.

The fuse has to be rated though for the wire size you end up using. You want the fuse or breaker to blow before the wire gets hot and catches something on fire in case it is over loaded.

Personally I hate inverters as they waste a lot of electricity converting from the DC to the AC. It is much more efficient if you can find a 12 volt appliance for what you are doing or if it is something like a computer get the smallest inverter that will handle the job. My feelings about it anyway,

Sum

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Ron20324 · Nov 27, 2009

My Xantrex manual says to use 2/0 size cable of no longer than 5 feet and a 250 fuse.That Xantrex says cable rated to 90 degrees C should be used.

That "open ckt amps" threw me for a while, too- until I got to the page with the above numbers. That fuse would be a "mega" or master fuse capable of withstanding anything short of a screwdriver across the battery terminals.

Bill Roosa · Nov 28, 2009

Inverter math

Figure twice the power rating for momentary "in rush" current or in your case 1800x2=3600 watts. Now it is a little known fact that Power= volts x amps. Since you know power and the system is "fixed" at 12 volts you can easily calculate the amps needed. P/V=I (amps). You would want to use the lowest volts that the inverter is capable of operating at as that would give the greatest amps. Most shut down at 11.5 volts so 3600/11.5=313 amps. you have to account for the efficiency of the inverter also and I'd guess it is 85% at max load. 313/.85=368 amps. That would be an inverter operating at max power and you would only see that momentarily when a large inductive load (motor) turned on. Most of us don't have those kinds of motors though (AC comes to mind and that would be a really dumb way to get AC BTW). I'd settle for a cable/fuse that can safely handle 156 amps continusaly unless you have a big inductive load. The surge of current when you turn on resistive loads is pretty low so you don't need to worry about the X2 factor the inverter "may" have to deal with.

Sumner · Nov 28, 2009

som1335 said:
...........I asked for help because the primary objective is to NOT burn down the boat. Looks like 150 amp breaker should do it and again, I appreciate all the advice.
Mike

I've probably missed it, but what size wire are you running? The breaker has to protect you and trip before that wire overheats. If you were to put a 150 amp breaker on say a #8 wire you could have the fire you are trying to avoid.

Sorry if you already have this part figured out,

Sum

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Ron20324 · Nov 29, 2009

som1335 said:
Thanks for all the help. I asked for help because the primary objective is to NOT burn down the boat. Looks like 150 amp breaker should do it and again, I appreciate all the advice.
Mike

I'm not sure what you are thanking us for- you seem to have ignored what we have told you. The next time you post a question to this forum, it will probably ask why your inverter is not workling properly and/or blowing fuses.

Don't come back and ask us what size wire you should use. If you can't figure that out, you shouldn't be doing this installation job.

Maine Sail · Nov 29, 2009

som1335 said:
The instructions for the inverter I am installing state you need a fuse or circuit breaker in the batter positive line. The size is determined by the "short circuit current rating" It doesn't say how to calculate and the internet has been no help for a 12 volt systems and newbies.

I believe the OCPD they are referring to in the battery positive line is to protect the wire itself from a "short circuit" and this data is in ABYC E-11. It is an ABYC standard suggestion to protect any power supply/source within 7" of the battery terminal..

11.12.1.4. Non-motor Loads - The rating of overcurrent protection devices used to protect a load other than a DC motor shall not exceed 150 percent of the ampacity of its supply conductor. (See TABLE IV .)

If you use 2/0 UL 1426 105C wire it has a 330 amp rating outside an engine compartment and a 280 amp rating inside an engine compartment. While you can go to 330 X 150% with an OCPD I would fuse it at the warm rating for the wire of 280 amps. In your situation a 250 amp ANL type fuse used on 2/0 wire would be fine. Remember 2/0 is not the same as 2 gauge and 2/0 is significantly larger than 2 GA. A smaller fuse than the wire can support is always better than a fuse to large for the ampacity of the wire..

P.S Keep in mind that the total ampacity the conductor can safely pass drops as the wire gets longer..

Stu Jackson · Nov 29, 2009

West Marine Catalog

It has a number of different inverters for sale. Many of them show the recommended fuse size. Those are based on you being able to size the wire correctly between the bank and the inverter. The wire sizing charts in the catalog and on their website are pretty easy to use.

Maine Sail · Nov 30, 2009

som1335 said:
I just bought the inverter from West and the directions were to "calculate" the fuse size but didn't say how. Maine sail's answer is the most logically put and understandable. When I do household wiring I base it on the wattage (amps X Volts). The calculation for the battery fuse was new to me. This was not 1 where the fuse size was given adn a call to West Marine resulted in "no information available"
Mike

If you use 2 GA UL 1426 105-C wire it has a 210 amp rating outside an engine compartment and a 178 amp rating inside an engine compartment but this is really to small for an inverter with an 1800 continuous rating as surge or intermittent use can be much higher.

At 1800 watts you are on the border of 2 GA and 1 or 1/0 wire however if this is a Xantrex XM 1800 it requires 2/0 wire and a run of less than 5'.

If you're total run is 8' or less, round trip, 2 GA will be marginal for loads nearing 1800 watts. If the pos & neg combined lengths are more than 11 feet you'll want to use 1/0 wire. This is for 1800 watts ONLY. Most inverters can surge to nearly double the constant rating thus manufacturers suggest larger wire such as 2/0 as Xantrex does on many of their 1800 +/- units.

1800 Watts at 10.5 volts = 171 amps of potential draw and the wire, and it's length, round trip, should ideally be sized to handle the full load (perhaps 3600 watts) even if you never plan to do that. What you are trying to do is sizing the inverter to the bare minimum, not the max rated, as ideally should be done.

I used 10.5 volts based on your battery, a Xantrex XM 1800 cut out of 10.5 volts, and how fast a single group 31 would sink to about 10.5 volts if supplying anywhere near 1800 watts.

Just six minutes of run time with a 1000 watt microwave (your may be smaller) will burn 8.3 amps. If you have a single group 31 deep cycle battery this is a fairly large portion of your usable Ah's. Most group 31's are about 110 Ah's. For good battery life you don't want to discharge a battery below 50% of capacity. So your usable capacity is now just 55 amp hours not 110 amp hours.

To further compound the issue, when out cruising, you will rarely if ever be able to recharge your bank beyond 80-85% of capacity without some very, very long engine run times. So now you have 85% of 110 Ah's which is 93.5 usable Ah's. 93.5 minus 55 (to a 50% depth of discharge) leaves you with 38.5 actual usable Ah's from a 110 Ah battery.

Six minutes of a 1000 watt microwave @8.3 Ah's is nearly 22% of your real usable battery capacity when you take in (SOC) state of charge and (DOD) depth of discharge.

Perhaps you have two group 31's as a house bank? If so just double the numbers above. Coffee makers run significantly longer and I have not calculated inverter inefficiencies into the equation..

Just food for fodder...

As I have mentioned before the best money you can spend is $200.00 on a good battery monitor like the Victron BMV-600. It will really help you understand your system and how it is working..

The bottom line is that you want to protect the wire, as close as possible of within 7" of the battery. This fuse should not exceed the ampacity of the wire while at the same time not blowing fuses.

Joe A · Nov 30, 2009

Portable generator

I don't mean to derail an informative discussion on 12V electrical practices, but people who intend to use coffee makers and microwaves should probably think about using a portable 120V generator. They bring a lot of advantages even if used only sparingly.
Joe

Ron20324 · Nov 30, 2009

To those who took exception...

som1335 said:
Whoa!! some attitude here and for what reason? I am using 2 gauge. I chose the other precisely to prevent fire. You recommended 250 amp fuse and the other reply recommended 156 calculated, not a guess. Lower amp fuse greatly reduces the risk of fire. get a life

...To my answer, or maybe the way I put it, if you would read the repiles, you would find that Sum's first reply and Bill's answer # 3 pointed out that max watts can be double the rated 1800 watts, and to figure on that amount, but then says he would "settle" for a 156 fuse.

MY reply #2 spells out what the installation manual calls for for an 1800 watt inverter, though you never do tell us the brand, only that you bought it at West. Mainsail's nicely put #7 reply spells out what I said in #2, but you chose to ignore one (so i assume you'll ignore both), so in effect, MS was wasting his time saying the same thing. As Stu's #8 reply ponts out, the VERY BASIC reference manual is the West catalog. Alot of people could answer their own questions if they would consult it or Calder's book. I'm surprised Stu didn't ask what brand the inverter was, as that is important to know.

"Whoa!! some attitude here and for what reason?" said OP. Well, I gave you an authoritative answer staright from the factory book, so it's factual and not an opinion and not an ego trip.

MS presented a nice dissertation that ended up with the same numbers:"At 1800 watts you are on the border of 2 GA and 1 or 1/0 wire however if this is a Xantrex XM 1800 it requires 2/0 wire and a run of less than 5'."" Most inverters can surge to nearly double the constant rating thus manufacturers suggest larger wire such as 2/0 as Xantrex does on many of their 1800 +/- units." from MS again.

So where the "some attitude here and for what reason?" comes from is your "Lower amp fuse greatly reduces the risk of fire." I can see somewhere down the road someone will say, "Hey, this is an 1800-watt unit and should have a 250 fuse". They'll put in a 250 to replace the 150, and a fire WILL start that, maybe because the wire was sized for the origional 150, but will NOT be sufficient for the 250. It's like putting a skimpy wire on a table lamp because you'll only use a 60-watt bulb. Somewhere down the line someone will put a bigger bulb in, and you'll have a fire. Same with the boat. While your boat is your concern, my concern is the innocent boats that may be consumed by YOUR boat going up in flames. Do it right the first time with the properly rated supply line. I think even MS can agree with that?

Do it right, or do it wrong- your boat, your choice.

But to cut to the chase, I bought a B&D coffee maker with a thermal pot. There is no hot plate to continually draw AC. At 30 bucks, it was better to buy two to have the extra pot- which can be $25.

Maine Sail · Nov 30, 2009

Ron20324 said:
To those who took exception...To my answer, or maybe the way I put it,

Ron,

I don't think it was the answer so much, but rather, as you said, the way you put it.

The OP plans on sizing this for his use, which you and I may not feel totally comfortable with but as you said it's his boat. If he at least protects the 2 GA wire for its ampacity rating with a fuse within 7" from the battery the worst he can do is pop fuses at $12-$15 per pop. In the event that somone did put in a 250 he is covered to 276 amps, in an engine compartment, with 105C wire if you use the ABYC E-11 as a guide.

What really matters is that the wire is protected to prevent melt down and a fire.