Oversized wire and breaker?

solarpowernovice

I recently bought a 2000 watt 24v magnum inverter and in the manual it says: Maximum DC fuse size 150 amps. Would it be okay for me to use my current 2/0 cable and a 250 amp DC breaker from my previous 12v system? I don't want to buy a new breaker and new inverter cables if possible.

mcgivor

solarpowernovice said:

I recently bought a 2000 watt 24v magnum inverter and in the manual it says: Maximum DC fuse size 150 amps. Would it be okay for me to use my current 2/0 cable and a 250 amp DC breaker from my previous 12v system? I don't want to buy a new breaker and new inverter cables if possible.

If the design calls for a 150A fuse, that is what should be used, a 150A  fuse in series with the breaker would allow the breaker to be used as a means of disconnect while still protecting the conductors, 2/0 is rated for ~200A, depending on insulation,  but the inverter may have a restriction to protect itself in the event of an internal fault, not that of the conductors, in short it's always best to follow manufacturers recommendations.

solarpowernovice

So, just to clarify... Using my old 2/0 battery and inverter cables is fine, so long as I introduce a 150 amp fuse before the breaker? This would be much better than buying a new breaker and cables. Thanks

Estragon

The 150a fuse should go as close to practical to the battery positive. It will protect the controller internal wiring. The breaker can go close to the controller - it should never trip (the fuse should open in a controller short), but can be handy as a disconnect.

solarpowernovice

Can I use a breaker instead of a fuse? the cheapest 150 amp fuse block with fuse is $55 https://www.solar-electric.com/mrcb-150-amp-dc-circuit-breaker.html
I found this breaker used on Ebay for $25. My inverter cables have 3/8 lugs on them and this breaker has 5/16 studs but thats the only problem that I see with it.

Here is another breaker I found with 3/8 studs, it says 12v but in the description it says Max 48v
https://www.ebay.com/itm/12V-DC-150-AMP-Circuit-Breaker-for-Race-Car-Trunk-Mount-Battery-Switch-3-8-Studs-/351998833465 Would either of these work? I would rather use a breaker because if it trips I can just reset it and wont have to buy a new fuse. I am open to suggestions on which breaker to buy if anyone has any.

westbranch

"solar'' breakers are usually rated to 150V ...  MidNite etc. brands.  If you expect to change your system in the future you will be money ahead with a 150V model.   Remember if you series your panels you could be well above 50 V

Dave Angelini

The breaker/fuse is for an inverter right? The key spec to match here is that it has Class T interrupt ability.
This is just another reason why I always use the exact product that the manual specifies.
 I do not want to be sued and I want my clients to be safe

solarpowernovice

Alrighty then, it looks like I will go with the 150 amp fuse block. I'd rather spend more money to do it the right way and not cut any corners. Thanks for all your help everyone.

solarpowernovice

I have one more question for you guys... In my inverters manual it reads: "In some installations, if the combined short-circuit current of all the batteries in the bank is determined to be 2,700 amps or less, then an ANL type of fuse may be used---if in doubt, use a class-t fuse." I'll be purchasing eight Costco insterstate golf cart batteries (2 strings of 4) rated at 215 AH.

My question is: How do I figure the short circuit current of all my batteries? I don't really plan on going much bigger with my system, seeing as how I just doubled it (overall system voltage/ batteries and solar modules). I may go bigger with my next battery bank in a few years, seeing I have 1600 watts of solar modules going to just the 8 golf cart batteries but I will not be adding any more PV

mcgivor

The overcurrent protection is based on the conductors maximum  current carrying  capacity , not the short circuit amperage of the batteries themselves, the purpose of the protection is to prevent the conductors from becoming the fuse, the fuse/ breaker would  protect the conductors in the event of a short circuit.

Estragon

My understanding is the ANL fuse will open in a short on the order of 100s of milliseconds to a second, whereas the class T should open on the order of 10s of ms. The risk is the inverter wiring may arc/burn before the ANL opens.

BB.

Also, the AIC rating (Amperes Interrupting Current) means that a 2,700 AIC fuse will catch fire, arc, explode if more than 2,700 amp short circuit.

For example, all house hold circuit breakers (in North America) have an AIC rating of 10,000 Amps. That is also the maximum current rating of a "typical" pole mount transformer and some resistance of the electrical drop from the pole to the house.

Lead Acid batteries output huge amounts of short circuit current. If you have a large bank and heavy copper cables, then there coudl be high short circuit current.

-Bill

Estragon

Just curious - would cranking amps be a reasonable order of magnitude guess for SC amps? I know they aren't so rated, but presumably a pair of GC battery would very roughly equal a typical auto battery CA at ~800-1000CA?

mcgivor

Estragon said:

Just curious - would cranking amps be a reasonable order of magnitude guess for SC amps? I know they aren't so rated, but presumably a pair of GC battery would very roughly equal a typical auto battery CA at ~800-1000CA?

The cold cranking amp rating is a rating of the battery’s ability to supply the rated current for a given duration/temperature  under load. The short circuit amperage would be significantly higher, in the thousands of amps,  depending on the size of the battery itself  as a short circuit is not a load designed to be applied to the voltage of the battery, unlike a starter motor.

solarpowernovice

Can I use Blue Sea Systems MRBF terminal fuses like this? https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A  --- Apparently Samlex uses these fuses in their 2000 watt inverter install kit http://www.samlexamerica.com/products/ProductDetail.aspx?pid=91... You cant get any closer to the battery than these and like you say...

Estragon said:

The 150a fuse should go as close to practical to the battery positive. It will protect the controller internal wiring. The breaker can go close to the controller - it should never trip (the fuse should open in a controller short), but can be handy as a disconnect.

The following types of fuses are included in the Samlex Kit:

Marine Rated Battery Fuse (MRBF Series) made by Cooper Bussmann

- Voltage rating of max 58 VDC

- Current ratings of 100A (MRBF-100), 200A (MRBF-200) and 300A (MRBF-300)

- AIC of 10000A at 14VDC, 5000A at 32 VDC and 2000A at 58 VDC

- Ignition protected as per SAE J1171

- Weather Proof (IP66)

Hopefully these work because I found 2 fuse blocks and 4 fuses for the price of 1 Class-T fuseblock/fuse. I'd love to save the money but I'd rather be safe.

***UPDATE*** I emailed Magnum, my inverter manufacturer and blue sea. Blue sea told me this fuse rarely blows unless a major problem has occured and I should go with the class T seeing my inverter manual recommends it. Magnum tech support told me the MRBF fuse WILL work with this inverter... Now I am totally confused. I think I will go with the Class T before the inverter and MRBF fuse on my second string of batteries.

MrM1

Looking at those MRBF terminal fuses,  they say they are rated for 58v.  So would they not work on a 48v battery as the charging and EQ voltage would be higher?   

Also,  if I have a Midnite Solar 250 amp breaker (24v system) on the inverter side,  Do I need a fuse on the battery side?  And would it be of benefit to down size the fuse to 175amps?  I am using 4/0 cable. between battery and inverter. 

And one other issue,  the MRBF terminal fuses blocks say they connect to a 3/8's battery terminal stud,   but Trojan studs are 5/16.  Do I need an adapter? Or can one bolt the 3/8 ring to a 5/16 stud with no issue?

solarpowernovice

Jerry from donrowe.com customer service answered this same question for me, so here it is from the email he sent me...
"With all the info you should be fine with 3/8’ on 5/16” posts. I would suggest using a good 5/16” ID hole washer on each side of the cable flats to provide a bit more support and to prevent slippage. Several customers have reported doing that and it works fine for them."

As for your other questions, maybe someone else with more knowledge will chime in.

Estragon

You should generally install the size of breaker or fuse recommended by the inverter manufacturer. Too small may not handle non-fault current safely, and too large turns the inverter and/or wiring into a fuse.

With properly sized breaker and wire, fusing the battery side would normally not be needed. There are exceptions though, for example if more than two parallel strings/batteries in the bank each parallel connection should be fused.

BB.

Class T fuses (NAWS sells class T, not sure if all are Little Fuse LNN"S" series or not), but here are product page and the basic data sheets:

http://www.littelfuse.com/products/fuses/industrial-power-fuses/class-t-fuses/jlln.aspx
http://www.littelfuse.com/~/media/electrical/datasheets/fuses/industrial-and-ul-fuses/littelfuse_fuse_jlln_jlls_datasheet.pdf

Specifications
JLLN
Voltage Ratings AC: 300 V
DC: 160 V (1 - 60 A)
125 V (70 - 1200 A)
Ampere Range 1 – 1200 A
Interrupting Ratings AC: 200 kA rms symmetrical
DC: 50kA (1 - 30A)
20kA (35 - 1200A)
Approvals AC: UL Standard 248-15, Class T
UL Listed (File: E81895): 1 – 1200 A
CSA Certified (File: LR29862): 1 – 600 A
DC: UL Listed (File: E81895): 1 – 1200 A
Material 1-30 A: Melamine body, Bronze caps
35-1200 A: Melamine body, Copper caps
Environmental RoHS Compliant

JLLS
Voltage Ratings AC: 600 V
DC: 300 V
Ampere Range 1 – 1200 A
Interrupting Ratings AC: 200 kA rms symmetrical
DC: 20kA
Approvals AC: UL Standard 248-15, Class T
UL Listed (File: E81895): 1 – 1200 A
CSA Certified (File: LR29862): 1 – 600 A
DC: Littelfuse self-certified
Material 1-30 A: Melamine body, Copper caps
35-60 A: Melamine body, Bronze caps
70-1200 A: Melamine body, Copper caps
Environmental RoHS Compliant

Took a bit of searching, but found MRBF fuse specifications:

http://www.cooperindustries.com/content/public/en/bussmann/transportation/products/circuit_protection/fuses/marine_rated_batteryfuses.resources.html (not much here)
http://www.peerlesselectronics.com//files/MRBF-100.pdf

Applications: Full range circuit protection for automotive and marine
applications. Break in capacity meets the requirements of conventional
vehicle batteris and 42V electrical networks
Voltage Rating: 58Vdc Maximum
Amperage Rating: 30A - 300A
Ingress Protection: IP66
Ignition Protected: Per SAEJ1117
Interrupt Rating:
10000 AMP @ 14Vdc
5000 AMP @ 32Vdc
2000 AMP @ 58Vdc
Color Coded (see next page)
Torque Rating: Maximum 12 N􀁴m (106 in-lbs)
Material:
Body - Ceramic
Housing & Cover: UL-rated 94V0 Thermoplastic
Ring Terminals - Tin Plated
Compliances: ISO 8820-6

Rating
30A - 300A
100%
> 100 h

135%
Max 900 sec
200%
max.
60s

350%
min. max.
0.1s 1 s

600%
< 0.2s

And here is a LittleFuse design document... Not a bad place to start if you have detailed questions about how to pick the "right fuse/breaker":

http://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf

With fuses and breakers--You really need to go back to the actual manufacturer's specifications to figure out if any particular choice will work for your needs.

And you may never get an "official" answer to questions like can I use an MRBF fuse (58 volt max rated) on a 48 volt battery bank (which can see 60+ volts). My unofficial answer would be (don't sue me for this) is yes because actual battery bank voltage on a shorted circuit would be less than ~58 volts.

HOWEVER, if you have a large Lead Acid (or Li Ion) battery bank and heavy cables, the 2,000 Amp interrupt current may not be high enough... And you then need to look at the Class "T" type fuse with 20,000+ Amp IC rating.

And to make the decision, you need to look at secondary questions. For example, if the DC leads are long enough (and not huge diameter copper), the resistance of the leads may limit the short circuit current (V=IR; I=V/R). For example, to get 2,000 amps at 58 volts:

• 58 volts / 2,000 Amps = 0.029 Ohms

If you have a 2 AWG cable, that is around 0.16 ohms per 1,000 feet or 0.00016 ohms per foot

http://www.interfacebus.com/AWG-table-of-different-wire-gauge-resistance.html

• 0.029 Ohms / 0.00016 Ohms per foot (2 AWG) = 181 feet round trip (or 91 feet one way)

It appears that for many of our DC off grid System Battery Bank designs, I would guess there is no enough resistance in the copper wiring to limit a short to 2,000 Amps on a fully charged 48 volt battery bank (and using 48 VDC nominal would reduce the one way wire run to ~75 feet for 2 kAmps maximum).

To get a feel of what in depth short circuit calculations look like, here is one for AC systems (search for "Cooper Bussman's Point to Point Method of Short-Circuit Calculation" if link is not permanent):

http://www.interfacebus.com/AWG-table-of-different-wire-gauge-resistance.html

Here is a DC short circuit estimate from Schneider:

http://www2.schneider-electric.com/documents/electrical-distribution/en/shared/interactive-catalogue/lvped2080006en/pdfs/page_019.pdf

Short-circuit currents
DB105060
Calculation of the short-circuit current across the terminals of
a battery
During a short-circuit, the battery discharges a current equal to
Vb = maximum discharge voltage (battery 100 % charged)
Ri = internal resistance equivalent to all cells (a function of the capacity in amperehours).

Isc=Vb/Ri

Example
consider a set of four 500 Ah batteries connected in parallel
discharge voltage of one battery: 240 V (110 cells 2.2 V each)
discharge current of one battery: 300 A with a run-time of 30 minutes
discharge current of all four batteries: 1200 A with a run-time of 30 minutes
internal resistance 0.5 mW per cell, i.e. for one battery:
Ri = 110 x 0.5 x 10-3 = 55 x 10-3 W
short-circuit current of one battery: Isc = 240 V / 55 x 10-3 W = 4.37 kA
neglecting the resistance of the connections, for all four batteries discharging the
short-circuit current in parallel, the total short-circuit current is four times that of one
battery, i.e. Isc = 4 x 4.37 kA = 17.5 kA.
Note: if the internal resistance is not known, it is possible to use the following rough
approximation: Isc = kc where c is the capacity of the battery in ampere-hours and k is a
coefficient close to 10 and always less than 20.

Other typical examples
PABXs: Isc from 5 to 25 kA at 240 V DC with L/R = 5 ms
submarine: Isc from 40 to 60 kA at 400 V DC with L/R = 5 ms.

So, looking at a "typical" worst case Lead Acid Storage Battery (large AH cell) with ~0.5 mOhms per cell, a 48 volt @ 1,000 AH or more? battery bank would have:

• 24 cells * 0.0005 Ohms per cell = 0.012 Ohms total (and could be 2-5x higher vary easily)

Some numbers measured/estimated for a 100 AH lead acid cell:

https://www.blueboxbatteries.co.uk/blog/battery-internal-resistance-short-circuit-current-47

For standard/typical Lead Acid storage batteries, they tend to have higher internal resistance. For AGM/GEL/Li Ion they tend to have lower internal resistance. And remember that a Lead Acid battery can only out peak current for a few minutes from full charge, and the current quickly drops after that.

Has anyone found/done "real life" 12/24/48 volt lead acid storage battery short circuit tests here?

-Bill

MrM1

Community Gold.   Thanks so much.   

Dave Angelini

I have done the tests and seen the results. I have used Class T fuses in my batteries since 1992.
Watch a dead short on the power bridge of an XW6848+ and you may do this also. This was a test of a good unit BTW.

solarpowernovice

Great information as always Bill and friends, thanks! As an aside here is a really good article i found on
the subject of Over Current Protection Devices...
http://www.electricityfromthesun.info/low_voltage_dc_fuse_and_circuit_breaker_applications.htm