Finding Breakers and fuses

micahv

I'm having a hard time finding all the correct over current protection to adhere to manufacture recommendation...

So, I'm using Morningstar MPPT CC and Suresine 300 Inverter, along with a 205W kyocera.

Inverter Manual says 100A fuse between battery and Inverter
and 3A fuse on the AC out.

Charge Controller manual 25A fuse between battery.

I was hoping to use breakers throughout to avoid having to deal with fuses if and when one blows.

I called Morningstar a few times. First guy said ok to use 125A breaker second guy said use 100A cause internals only protect up to 120A. I got a 63A Midnight solar breaker which they said would be fine since that is more then 600W. This is obvious, going to low only risks nuisance trips? I have the midnight solar 125A mini DC disconnect so it was nice to be able to use the 125A breaker.

I currently have a 15A breaker for PV in to CC, 30A(should be 25A) from CC to Bat. and 63A from Bat to Inverter for a total of 3 Breakers in one box. could just get by with a small breaker box instead of this Midnight solar DC disconnect? Will a standard DIN rail home depot box work? Also, can't find a 25A DC breaker so I use a 30A one for the charge controller.

My last call to Morningstar the guy said use 6A on the AC out since 3A is lower then the surge power, 3*120=360W. Ok fine, but where do I find a 6A AC breaker? Is the best solution here to use an AC disconnect switch and a fuse then? Where can I find the right combination of disconnect and 6A fuse? Looked at HD and found a 30A fused disconnect, doubt I can find a 6A fuse for it though.

I guess I can get a couple of these midnight solar baby box, http://store.solar-electric.com/baby-box.html one for AC and one for DC. But I'm still stuck with what to use for AC over-current protection? Everything I find always seems to be for DC...calling Morningstar, they where even at a loss as to what to use?!?!? I was like, but you guys make this stuff, they say, "yea, but we don't install it".

Anyhow thanks again for input...

westbranch

Re: Finding Breakers and fuses

a fuse is a fuse is a fuse... it disconnects supply from load to protect the wiring and reduce the risk of fire/shock.

I would look to an automotive supply place for fuses in 1 amp increments. there are many styles but all work the same. price is the only difference... should be less than 5 bucks for holder and fuses.

Yes, anything less than 10 amps is difficult to be 'on the money' CB wise... then they go up in 5 - 10 amp increments.

Small loads lead you to improvise...

HTH
Eric

crewzer

Re: Finding Breakers and fuses

Micahv,

A 25 A circuit breaker rated for DC applications and 100% continuous duty would be appropriate for the output of the charge controller. IIRC, 25 A breakers are not commonly available, so using a 30 A model would be OK.

See: http://store.solar-electric.com/oudcbr.html (panel mount)
and: http://store.solar-electric.com/mnepv.html (DIN rail)
and: http://store.solar-electric.com/pspv.html (DIN rail)

The Morningstar SureSine inverter contains internal fuses. Although its nominal rating is for 300 W, it’s also rated for 600 W for 10 minutes. I’d calculate the external DC circuit breaker (100% duty rating) this way:

(Max power / min input voltage) / Efficiency at max power = CB rating, or

(600 W / 10 VDC) / 70% = 86 A. The next standard size up is 100 A.

See: http://www.morningstarcorp.com/en/support/library/SureSineENG_R2_1_08.pdf
and: http://store.solar-electric.com/obdc-100.html

A 5A or 10 A AC/DC continuous duty breaker is OK for the main output from the inverter.

See: http://store.solar-electric.com/obac-10e.html
and: http://store.solar-electric.com/obdc-5.html
or check for a 10 A regular-duty AC breaker from Home Depot.

HTH,
Jim / crewzer

john p

Re: Finding Breakers and fuses

The Morningstar SureSine inverter contains internal fuses. Although its nominal rating is for 300 W, it’s also rated for 600 W for 10 minutes. I’d calculate the external DC circuit breaker (100% duty rating) this way:
(Max power / min input voltage) / Efficiency at max power = CB rating, or

(600 W / 10 VDC) / 70% = 86 A. The next standard size up is 100 A.

A 40 a circuit breaker is more than adequate. 100a is useless the whole inverter would be completely melted long before that circuit breaker triped.

what size wire you using with the 100a breaker#2? as anything less is also going to melt before the 100a breaker trips

inverters have a shutdown at 10.5 v but should never be used at that low a voltage as it damages the batteries.

crewzer

Re: Finding Breakers and fuses

A 40 a circuit breaker is more than adequate. 100a is useless the whole inverter would be completely melted long before that circuit breaker triped.

what size wire you using with the 100a breaker#2? as anything less is also going to melt before the 100a breaker trips

inverters have a shutdown at 10.5 v but should never be used at that low a voltage as it damages the batteries.

The SureSine already contains three 40 A automotive-style fuses. An external 40 A fuse or circuit breaker should sustain full rated power operation {(300 W / 10 V) / 85% = 35 A}, but would likely blow during a sustained surge demand.

I know several of the sales engineers at Morningstar, and they're comfortable with their external fuse rating of 100A. And the SureSIne is factory-rated to sustain 600 W for ten minutes at 25C. Depending on the size-, state of charge-, and overall health of the battery bank, the inverter could indeed pull 80 A or more.

You're on the right track about the cabling: Its "conditions of use" rating must be adequate for the application.

However, Morningstar recommends UL-listed #6 or larger, which is rated for 105 A per the US National Electrical Code for wiring rated at 90C and used in "free air" at an ambient temperature of 30C (ref Table 310-17). More conservative "conditions of use" such as lower temp wire, wire run in a conduit, and/or higher ambient temps would require larger wire (i.e., #4 or even #2).

I agree that running a 12 V battery at 10.5 V under load (the definition of empty, BTW), is not a good thing. However, battery voltage dips briefly when a load is applied (the "coup de fouet") before recovering to normal operating level. Depending on the load, it's not uncommon for a small, old, low SOC, and/or poorly maintained battery bank to dip to 10.5 V or so.

HTH,
Jim / crewzer

john p

Re: Finding Breakers and fuses

HOW?And the SureSIne is factory-rated to sustain 600 W for ten minutes at 25C. Depending on the size-, state of charge-, and overall health of the battery bank, the inverter could indeed pull 80 A or more.

600W/10.5 V THE SHUTDOWN FIGURE =57A
And really using a 300w inverter to supply 600w is not good as is really is going to run very hot.
if you really need 600w you should be using a 600 or 800w inverter.
If the inverter already has internally 40 amp fuses what the point of a 100a fuse externally??? if #6 wire is used it is going to go close to meltdown before the 100a blows.

I have a 400w inverter that powers all our lighting permanently and 3 cheap ceiling fans and use a 40a circuit breaker and never had a problem with it tripping and at short times the inverter output is close to the 400w

but the person with the inverter is able to use whatever he thinks is best for him
I always prefer to use as close a rated circuit breaker as possible just under max load.. as I have always found circuit breakers have a very slow response time so can withstand short term small overloads

crewzer

Re: Finding Breakers and fuses

HOW?And the SureSIne is factory-rated to sustain 600 W for ten minutes at 25C. Depending on the size-, state of charge-, and overall health of the battery bank, the inverter could indeed pull 80 A or more.

600W/10.5 V THE SHUTDOWN FIGURE =57A

As I indicated in my first post above, you also need to include the inverter’s efficiency (see linked spec sheet) in the calculation:

(600 W / 10.5 V) / 70% efficiency (output/input) = 81.6 A. (I used 10 V in my earlier calculation because that's Morningstar's lowest input voltage spec.)

And really using a 300w inverter to supply 600w is not good as is really is going to run very hot.

I agree. However, the 600 W rating, as I’ve mentioned twice above, is for just 10 minutes. The inverter’s most efficient operating range (>90%) is between 50 W and 150 W output. However, some users may have occasional short-term power loads (i.e., starting a motor), so the surge capacity may come in useful.

If the inverter already has internally 40 amp fuses what the point of a 100a fuse externally???

The inverter has three internal 40 A fuses. I don’t know for sure, but I suspect that some (all?) are in parallel or protect separate internal circuits. The 100 A external fuse / circuit breaker provides for easy-to-replace protection and/or disconnect for servicing.

Don’t misunderstand: My suggestions above are based on this particular inverter’s specs and architecture. Another "300 W" inverter with different specs might result in a different analysis and recommendation.

HTH,
Jim / crewzer

john p

Re: Finding Breakers and fuses

Please accept im not trying to argue for the sake of arguing.
but this statement is hopeless:-(600 W / 10.5 V) / 70% efficiency (output/input) = 81.6 A. (I used 10 V in my earlier calculation because that's Morningstar's lowest input voltage spec.)

Think about it ,that inverter can NEVER draw 81.6 amps as its internal fuses are rated at 40a. . it not matter what its effeciency is . that is only going to effect the output as the input is curent limited by the internal 40a fuses. and if morningstar say it can go down to 10v that crazy also as the batteries are going to have very short unhappy lives.. that is why most good inverters make noises to complain when voltage drops to 11 v then cut out at 10.5 v to save the battery.

If this is true its a poorly designed inverter,The inverter’s most efficient operating range (>90%) is between 50 W and 150 W output
I be very supprised if it really is above 90% at 50w . But the cut off at only 150w is poor. it should be above 90% up to about 250w.
My first 300w inverters I designed in the early 80,s were about 70%effecient from about 100w to 250w,, but by todays standards that is very poor and now good ones are above 90% from about 100 to over 250w

BB.

Re: Finding Breakers and fuses

But there are 3x 40 amp fuses in the SureSine... We don't know how they are wired (three fuses in parallel or 1 fuse each to 1/3 of the current drawing devices internally)--but it would indicate that the inverter is sized for >80 amps of draw.

Remember that external breakers and fuses are there to protect wiring. Internal breakers/fuses (if any) are to help prevent the device from catching fire/hazardous failures.

Another reason to design the inverter to operate down to 10 volts--you not only have the "dead battery" issue (<10.5 volts)--you have the voltage drop of the wiring too... Just gives a little more headroom for operation in less than optimal conditions (weak battery, less than optimum wiring, heavy surges, etc.).

-Bill

john p

Re: Finding Breakers and fuses

mabe my maths not what it used to be but isnt 3x40 closer to 120 than 80???
if in parallel or to 3 seperate output device strings its the same thing? yes?:cool:

120a x 10v=1200w or 120a x14v=1680w ..not bad for a 300w inverter?????????

crewzer

Re: Finding Breakers and fuses

mabe my maths not what it used to be but isnt 3x40 closer to 120 than 80???
if in parallel or to 3 seperate output device strings its the same thing? yes?:cool:

120a x 10v=1200w or 120a x14v=1680w ..not bad for a 300w inverter?????????

I suspect the fuse value was rounded up, and they also probably include a 125% safety factor. For example, 27 A per circuit x 125% = 33.75 A; rounded up to next standard size = 40 A. And, 27 A x 3 = 81 A x 10.5 V = 850 W x 70% = ~600 W.

The math looks OK to me.

If this is true its a poorly designed inverter,The inverter’s most efficient operating range (>90%) is between 50 W and 150 W output
I be very supprised if it really is above 90% at 50w . But the cut off at only 150w is poor. it should be above 90% up to about 250w.

Just quoting mfr specs; see attached.

HTH,
Jim / crewzer.

P.S. Thx, Bill.

Cariboocoot

Re: Finding Breakers and fuses

Trust in the Instruction Manual. Really. If it says "use a 100 Amp fuse" then do so. There's no point in trying to second-guess the engineers who designed the thing and the technicians who tested it, and there's no sense in short-changing yourself on functionality and safety.

Side question: why are so many people reluctant to use fuses and always looking for breakers? In a properly designed and installed circuit, the fuses don't blow unless something goes wrong. It's not like you'll be having to change them every week or so. I've known houses to run the same 15 Amp fuse on a circuit for decades without trouble. How often do you have to change a fuse in your car? It's a "once in a blue moon" occurrence that a fuse blows when there's no trouble. They're very inexpensive safety, and the only real advantage a breaker offers over a fuse is the built-in disconnect.

Don't be so quick to discount fuses; they're very practical safety devices.

BB.

Re: Finding Breakers and fuses

john p wrote: »

mabe my maths not what it used to be but isnt 3x40 closer to 120 than 80???

600 watts * 1/10 volts * 1/0.70 eff at 600 watts = 85.7 amps

85.7 amps * 1.25 derating (never run fuses/wiring at 100% rated current) = 107 amps minimum circuit/breaker/fuse rating

Round up to next "standard" wiring/fusing... 3x40=120 amps (automotive fuses) or 125 amps for typically available external breaker/fuse.

if in parallel or to 3 separate output device strings its the same thing? yes?:cool:

Actually no... It is very difficult to get equal current sharing among multiple low impedance (low resistance) devices/circuits/connectors.... When I was designing computer systems--I used a 1/N derating for sharing current (10 amp + 1/2*10a + 1/3*10a + 1/4*10a ... etc.). So, for me, I found that reliable current sharing became a diminishing return -- in terms of reliability.

Interestingly, the failure mode for paralleled devices is the low resistance device passes more current and eventually overheats (2x the normal current is 4x the heating effect). The first low resistance connection fails (overheats, cooks contacts, blows fuse, etc.), then the next lowest resistance path begins the failure mode.

Also, internally, that is the difference between protecting a "failed shorted" device/circuit behind a 40 amp fuse or a 120 amp fuse(s). Since heat is I^2*R --- the difference into a failed shorted device is a factor of 9x difference between 40 amp vs 120 amp protection.

120a x 10v=1200w or 120a x14v=1680w ..not bad for a 300w inverter?????????

Or, if you back out the deratings and allow for a 1/2 volt wiring drop:

(120 amps * 10 volts * 1/1.25 derating * 0.70 inv eff) - (0.5 volt wiring drop * 120 amps) = 612 watts rating for "typical" wiring

So, that all works out just about perfectly...

-Bill

BB.

Re: Finding Breakers and fuses

PreScript: Your welcome Jim.

By the way, breakers fail too... I take care of a small 50 year apartment building with electric hot water and stoves... About 1/2 of the 20 amp double pole breakers have been replaced so far because they where tripping under normal loads (and I have another one that is probably just about to need replacement).

Fuses/breakers are "unreliable" by design. They have to break/open at current levels not to much higher than normal operation. Heat, usage, starting surges, even flipping on and off (like a light switch) will eventually result in failure.

-Bill