parallel a circuit breaker

[skyking2]

I need a class T fuse or circuit breaker equivilent between the battery and a Prosine 2.0 I want to buy. Blue Sea has the fuse but I would like a 300 amp breaker like the surface mounted model 7148. That style only goes to 150 amps. Could you buy two and parallel them together for a combined circuit protection of 300 amps?

[never monday]

Not a good idea. ABYC wants the fuse at the battery to prtoect the wiring to the inverter.

[GordMay]

No - you cannot parallel CPD’s / OCD’s (fuses & breakers) for a cumulative combined rating.
I am unaware of any 300A* rated surface mounted circuit breaker.

Blue Sea Systems offers several Fuse Selections
< http://www.bluesea.com/dept.asp?d_id=7463&l1=7463 >
“SEA” Fuse Block #5001 c/w 300A #5108 Fuse
“ANL” Fuse Block #5005 c/w 300A #5133 Fuse
“Class T” Fuse Block #5002 c/w 300A #5119 Fuse

* 300A DC Input seems a bit high for a 2000W Inverter. The DC cables may be sized to imply 300A fusing, but may be oversized to mitigate voltage drop.
Ie:
2000W / 12.5V = 160A
160A / .80 Efficiency = 200A
Fuses can be Lower Ampacity than the Wire they protect (but NOT larger); but must be high enough ampacity to accommodate the Load.

[skyking2]

Originally Posted by GordMay

* 300A DC Input seems a bit high for a 2000W Inverter. The DC cables may be sized to imply 300A fusing, but may be oversized to mitigate voltage drop.
Ie:
2000W / 12.5V = 160A
160A / .80 Efficiency = 200A

The class T 300amp is what the owner's manual calls for. I downloaded a copy of the manual from xantrex so I can start searching for parts to get it hooked up. I wonder if the 150 amp would be enough. I won't be using all 2000 watts at one time. It'll be mostly for powering a microwave oven on occasion and a few other small loads like a drill. What got me thinking about it is that Blue Sea model 7266 is a 300 amp breaker that looks like three breakers ganged together. I haven't found a picture yet of the back of the breaker so I'm not sure how it's connected.

[Lynx]

Try you local comerical electrical supply house. They should be able to get one. I am sure that the big boats use them.

[never monday]

The triple gang breakers are just that. THey are designed for 240V shore power where you have two hots and a neutral. If you look at the back there are 6 conections.

[GordMay]

Approximate Amps Load = (Watts Load / DC Volts Input) / Efficiency
Amps @ 2000Watt Load = (2000W / 12.7V) / 0.80 Eff (???) = 158 / 0.80 = 197 A
Amps @ 1500W Load = 148 A

The #7266 is a Three-Pole DC-Rated Panel Mount Breaker. Though pricy, you could use it in this application, connecting only 1 (or 2*) cables to the both line & load side of the breaker.

*You could also protect & disconnect the DC Negative, as well as the required Positive.

[GordMay]

Please note:
OCDs (Breakers/Fuses) are generally required to be located within 7" of the DC Source (Battery or DC Distribution Bus), as per ABYC Section E-11.12.1 (formerly E-9.11.1), and A

[skyking2]

eh, I just bought the class T 300 amp off eBay along with the fuse holder. It's what the book calls for so I might as well not dick with it, for now anyway.

[hellosailor]

Skyking-
There are some less obvious issues about fuses versus breakers. Among them, breakers aren't as "sharp" about blowing, they may in fact carry a load higher than their rating for a long time without tripping, which could let wiring overheat, etc. (Although fuses come with different "blow" time ratings, too.)

And, with breakers, if you put enough overload on them they can weld in the "on" position, and never open at all. Breakers that are rated to work in high loads are damned expensive. Primary breakers, i.e. at a battery, even a Group31 single 100AH battery, can be subject to loads approaching 4000A during a dead short, which would weld the typical (3500A rated) breaker "on" making it useless. 5000A rated breakers, the next step up, are damned expensive and make fuses into a very reasonable proposition. I'm afraid to guess what a breaker suitable for your service might cost.<G>

[skyking2]

Originally Posted by hellosailor

Breakers that are rated to work in high loads are damned expensive. Primary breakers, i.e. at a battery, even a Group31 single 100AH battery, can be subject to loads approaching 4000A during a dead short, which would weld the typical (3500A rated) breaker "on" making it useless. 5000A rated breakers, the next step up, are damned expensive and make fuses into a very reasonable proposition. I'm afraid to guess what a breaker suitable for your service might cost.<G>

I beginning to find out just how expensive they are. I found some high amperage breakers being auctioned and they had buy it now prices in excess of $2000. So it looks like $70 total for the fuse and its holder isn't so bad after all.

[skyking2]

Originally Posted by hellosailor

Breakers that are rated to work in high loads are damned expensive. Primary breakers, i.e. at a battery, even a Group31 single 100AH battery, can be subject to loads approaching 4000A during a dead short, which would weld the typical (3500A rated) breaker "on" making it useless. 5000A rated breakers, the next step up, are damned expensive and make fuses into a very reasonable proposition. I'm afraid to guess what a breaker suitable for your service might cost.<G>

I'm beginning to find out just how expensive they are. I found some high amperage breakers being auctioned and they had buy it now prices in excess of $2000. So it looks like $70 total for the fuse and its holder isn't so bad after all.

[GordMay]

Originally Posted by hellosailor

...
1. breakers aren't as "sharp" about blowing, they may in fact carry a load higher than their rating for a long time without tripping, which could let wiring overheat, etc.
...
2. Primary breakers, i.e. at a battery, even a Group31 single 100AH battery, can be subject to loads approaching 4000A during a dead short, which would weld the typical (3500A rated) breaker "on" making it useless.
...

1. Circuit Breakers are rated according to their Instantaneous Trip characteristics, and Time/Current (delayed action) characteristic curves. Most 12-50VDC Breakers will trip in less than ½ second, at about 400% rated current.

2. A Trojan “1-AGM” (Group 31, 120 AH) Battery is rated at 720 CCA, requiring (see table) a minimum 3,000 AIC Main Breaker, such as a Blue Sea Systems #7115 (3000AIC) , or the #7147 (5000AIC) (both 135A frame size).
Alternatively an “AL”, or “T” fuse could be selected (6000AIC & 20000AIC respectively).
An “MDO” “Maxi”, or “Sea” Fuse would NOT suffice (rated: 1000, 1000, & 2000AIC respectively).
Downstrean “Branch” breakers would only require 1500A IC’s.

ABYC Section 11.12.1.7.2.3 (Formerly Section 9.12.5.2.3, Table III) requires (in part) that Circuit Breakers shall:
“be capable of an Interrupting Capacity (AIC) according to Table V, and remain operable after the fault:”

Table V specifies IC’s according to Total Connected Battery Capacity in CCA’s*, as follows.
Main Circuit Breakers at 12 & 24 Volts DC (Series Branch Breakers**):
Up to 650 CCA Battery Capacity = 1500 Amps Ic (750 A)
651 - 1100 CCA = 3000 A Ic (1500 A)
Over 1100 CCA = 5000 A Ic (2500 A)

* Cold Cranking Amps ~ This is somewhat problematic, as most Deep Cycle Batteries don’t have published CCA’s (ie: Trojan T105).

** Subsequent, series connected, downstream Branch Circuit Breakers are considered to be current limited, resulting in reduced available fault currents.

[hellosailor]

Gord-
"2. A Trojan “1-AGM” (Group 31, 120 AH) Battery is rated at 720 CCA, requiring (see table) a minimum 3,000 AIC Main Breaker, "
Dunno about the Trojan, haven't read their specs. The specs I have seen indicate that even an 88AH rated deep cycle battery can throw 3300A across a dead short, for a period long enough to weld breaker points.

Which probably just brings up the more important issue, to be conservative and use safety margins in safety systems. IIRC the folks at Blue Sea were among the ones who told me a 3000A rated breaker specifically would be dangerous in this type of application, because it was simply cutting things too close regardless of the real-world issues (i.e. cables limiting current, placement, state of charge, etc.)

Apparently they don't like hearing that their equipment has caught fire or exploded. My kind of people.<G>