48v off grid system: battery cable advice needed please

[inetdog]

So anyone have any theories on why all of the Conext SW inverters call for a 250a breaker? I have a theory. The 24v SW4024 has maximum battery current of 230a so it needs a 250a breaker. (the maximum battery current of the SW4048 is 105a and 150a for the SW2524).

I suspect that an inverter short or battery cable short will always produce enough current to trip a 250a breaker in either a 24v or 48v system. So they only offer their DC switchgear with a 250a breaker so they don't have to produce different models of switchgear for each inverter model.

So maybe I will use a Blue Seas 125a on the positive lug?

I think that your reasoning is good, and the fuse is a very good idea too.

[Sunking]

Fuses or breakers demand to mounted directly to the battery term post or source of power. Failure to do so is ignorance and foolish. If installed downstream leaves all that cable unprotected.

More importantly are you operating the system Grounded or Ungrounded? If ungrounded or floating, one fuse will not work or protect you from anything. Both polarities are required to have protection which means a fuse on both positive and negative battery post in a floating system.

FWIW grounded system are dangerous if improperly implemented. All Grounded systems are prone to unnecessary outages, and all of them will electrocute you if you come in contact with the hot conductors.

[Sunking]

So anyone have any theories on why all of the Conext SW inverters call for a 250a breaker? I have a theory.

Not theory, I know why. Most all fuses and breakers are not rated for continuous power. They are rated for 80% continuous. Example every 20 amp breaker in your main panel can only supply 16 amps at 100% duty continuous power. It will handle 20 amps for a short period of time, but will operate if 20 amps remain for any extended period of time. That ought to give you a clue why PV conductors have to be derated. That is all I am going to tell you because there is a lot more to it.

Any 120 volt plug in appliance on a 20 amp circuit you buy will not be larger than about 1800 to 1900 watts. Now go figure out why and you will have a deeper understanding. You can find 120 volt gizmos that operate higher than 1900 watts, but leave it on the shelf. It will not be UL or made in the USA. If it were the manufacture and UL would be sued into the abyss never to be seen again.

[hammick]

Not theory, I know why. Most all fuses and breakers are not rated for continuous power. They are rated for 80% continuous.

Understood. However Midnight Solar claims all of their breakers are rated for 100% current if used in one of their enclosures. http://www.midnitesolar.com/productP...Order=15&act=p

My charge controller has ground fault protection built in and per the NEC the negative battery post is not grounded when ground fault is used. The AC portion of my system and EGC will be grounded to my rebar UFER. Out of an abundance of caution I think I i will take your advice and put a 125a Blue Seas fuse on both the positive and negative battery posts.

Thanks.

[Sunking]

[QUOTE=hammick;181790]Understood. However Midnight Solar claims all of their breakers are rated for 100% current if used in one of their enclosures. [URL="http://www.midnitesolar.com/productPhoto.php?[/QUOTE]

That is why I said Most fuses and breaker. I did not say all of them.

[hammick]

Upon looking at the Blue Seas fuses closer they have a maximum voltage rating of 58v. Not going to work for a 48v system. I will try to call Blue Seas and see what they think. Equalizing charges in cold weather can easily exceed 64v.

[Sunking]

Upon looking at the Blue Seas fuses closer they have a maximum voltage rating of 58v. Not going to work for a 48v system. I will try to call Blue Seas and see what they think. Equalizing charges in cold weather can easily exceed 64v.

Hogwash. A 48 volt DC system is nominal 48 volts. As long as the service equipment is rated 48 volts DC or higher you can use it. Manufactures and UL are quite aware of EQ voltages.

The other option which is better is Square D now made by Schneider. Some of it goes to 500 VDC used in commercial systems. They come in dual ganged breakers for each polarity. Be prepared for sticker shock, as that is what you asked for by not grounding your system. You will also need a very expensive GFD.

[hammick]

Hogwash. A 48 volt DC system is nominal 48 volts. As long as the service equipment is rated 48 volts DC or higher you can use it. Manufactures and UL are quite aware of EQ voltages.

Thanks. I will call Blue Seas tomorrow to make sure. Don't want a fuse to blow when I'm gone and return to find discharged batteries.

[Sunking]

Thanks. I will call Blue Seas tomorrow to make sure. Don't want a fuse to blow when I'm gone and return to find discharged batteries.

Fuses operate from current, not voltage. When a manufacture make a product like a fuse has to submit it to UL or some recognized testing agency for certification. When a manufacture submits a sample for testing, they state the intended operating voltage. UL does destructive testing. They crank the voltage up until it flashes over. So a 50 volt device is going to take at least a 1000 volts to Flash over.

The exact same fuse can be submitted for say 150 volts and will likely pass. In fact manufactures test and list at much higher voltages and relabel them for lower voltages and charge more for it.

[inetdog]

Fuses are rated according to the maximum voltage that they can safely interrupt when they blow from overcurrent.
Your batteries can deliver quite a few amps to an external short circuit, but in that process the terminal voltage will go down below the resting voltage for that SOC, and will not be anywhere near the EQ voltage by the time the fuse blows.

[hammick]

Thanks for the responses guys. So what size Blue Seas fuse do you guys recommend? The SW4048 manual calls for a 250a breaker which I have installed on the positive 4/0 battery cable at the e-panel. The SW4048 specs say max DC current is 105 amps. I don't know what to do. I called Schneider and their techs see no reason to fuse at the battery terminals so they are no help. They also can't give me a good reason why the SW4048 requires a 250a breaker. The just say all of their inverters require a 250a breaker.

I also spoke with Blue Seas and they are no help.

Blue Seas does make a 250a terminal fuse: http://www.amazon.com/Blue-Sea-Syste...blue+seas+250a

[inetdog]

There is no reason to fuse at the battery terminals in order to protect the inverter, and that is all that the Schneider folks are advising you about.
The reason to fuse at the battery terminals is to protect the batteries and the associated wiring against a short circuit anywhere between the batteries and the inverter. (or between batteries and CC or batteries and DC loads. One fuse at each battery terminal can protect all three groups of wires. (Although it may not keep the smaller size CC or load wires from getting really hot before the fuse blows.)

[Sunking]

There is no reason to fuse at the battery terminals in order to protect the inverter, and that is all that the Schneider folks are advising you about.

I agree in general all Schneider is doing is covering their equipment and no engineer, electrician, technician, or inspector would agree with them. Ni Inspector would pass it because it violates code and all known engineering practices. It is incompetent advice.

uses primary objective is to protect the wiring, equipment is secondary. The only way to protect the wiring and equipment is to have the OCPD (over current protection device) installed at the source of the power. You Battery is the SOURCE of POWER to the INVERTER. What Schnieder is telling you is extremely dangerous. If you were to have a fire, and they put that in writing, would be sued to bankruptcy by your survivors. Even a fist year engineering student could be the Subject Matter Expert in that court.

If you were to do what Schneider told you to do installing the fuse down stream, if you had a fault on the wiring between the battery inverter you wil have a huge fire on your hands because there is nothing to interrupt the circuit and turn off power. You cannot put out a electrical fire until power is removed which could not happen, The fuses installed on the battery post eliminates any of that from happening.

Thank about it. In any house with commercial AC service you take service from the meter directly into your main breaker box. At the meter is a 100, 150, 200, 250 amp Breaker depending on what size service you have feeding your main breaker panel. Then in your breaker pane is all your distribution breakers that distribute power to your home. All of that OCPD are at the SOURCE of power protecting the wiring in your home.

As to the size required we can debate about. What I will tell you is if you have 4/0 AWG from the battery Term Post to the Inverter the largest you can use is 230 AMPS. 250 is pushing it, but could be used if you use a 90 degree C cable insulation like THHW.

Having said that is Non-Sense to use a 250 amp fuse. The 4048 is a 4000 watt Inverter correct? 48 volt Input correct? Intermittent to 8000 watt correct? Pretty sure I got all that correct. So assuming the Inverter efficiency of 85%, continuous 4000 watt. Max fuse size is [(4000 watts / .85) x 1.25] / 48 volts = 122 amps. Perfect Fuse Size is 125 amps. You can safely go to 200 amps if you like which is way over kill and not reason to allow that much current. IMHO 150 amps is more than enough and plenty safe on a 4/0 AWG copper conductor. If I were your engineer on record would place my PE STAMP on it which would make me solely responsible. JPM, Inetdog, and Mike would concur with that statement and back me up with no question. Just ask them.

Think of it like this. There is no harm, no foul if you under size the fuse. All that could happen is you put too much load on it, and it operates, then the lights go out. Replace it and you are back in biz. If you over size it, you risk fire and Kaboom.

As for the 250 amp fuse at the E-Panel does nothing but adds extra profit for Schneider. It has no function unless the upstream fuse is equal to or larger than what you have at the battery. 150 amps is not larger than 250 amps. The 150 amp covers the wiring and equipment. End of story.

[hammick]

Thank you very much Sunking. Exactly what I was looking for. The SW4048 is max rated for 7000w for five seconds. Shouldn't I be using 7000 watts in the equation instead of 4000 watts to prevent blowing fuses if all the big loads hit at once? These fuses are $20 each and I can't just go to NAPA and get them.

If so, [(7000 watts / .85) x 1.25] /48 volts = 214 amps

Next size fuse is 225a

If I was crazy enough to cross the positive and negative cables would that blow a 225a fuse?

[inetdog]

Thank you very much Sunking. Exactly what I was looking for. The SW4048 is max rated for 7000w for five seconds. Shouldn't I be using 7000 watts in the equation instead of 4000 watts to prevent blowing fuses if all the big loads hit at once?

If so, [(7000 watts / .85) x 1.25] /48 volts = 214 amps

Next size fuse is 225a

The fuse itself has a time/current characteristic too, as well as being specified to hold a continuous current higher than its nominal rating to allow for production tolerances, etc.
To know what the effect of 214A for five seconds would be you need to know the time characteristic of the fuses used in the Blue Sea holders.
The only way to know for sure is to contact them directly. But it appears that MRBF fuses have a time characteristic similar to the ANL fuse standard, and you can see that curve on this page. http://www.pbase.com/mainecruising/battery_fusing
At a time of 7 seconds, a 100A fuse will hold something upward of 150A.