Breaker Sizing

[SolarVet]

I have a question. I will be using 2 ga Welding cable from my battery box. But I will be using 2 pair  , 2 Postive wires and 2 Negative wires. from battery box to central place inside my house. I want to use 300 amp rating.
Is it okay to use 150 amp Bussman Breakers on each pair. Instead of one 300 amp Breaker.
12 volt system
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[Vic]

Hi Vet,

There may be several issues with what you want to do;

Parallel conductors smaller than about 1/0 are not code,  as I recall.
These cables should probably be in conduit,  and in conduit,  the maximum ampacity is around 115 Amps,  if your cable is rated to 75 degrees C.  Then if you wanted to have four current-carrying conductors in conduit,  there would be a derating of the Ampacity to 80% of the 115 amps.

AND,  then there is the possible issue that fairly often,   welding cable does not have correct ratings for this use,  including the correct fire rating for use inside habitable structures,  and so on.

The Bussman breakers may be OK  on  a 12 V system,   although these breakers are usually Thermal,  not Magnetic-Hydraulic (which is often the preferred type for this use).

In general circuit breakers are intended to protect wire/cables,  and using a breaker larger than the Ampacity of the cable could be a safety concern (fire).

This post does seem a bit negative ...   sorry...

EDIT:  You may be looking at trying to meet the Surge capacity of your inverter.  300 A is a lot of current,  especially at 12 V,  where large currents make smallish voltage drops a larger percentage of the system voltage.

Hope that the planned cable run is SHORT,  as,  at high currents the voltage drops become extreme,  and the remedy is to spend lots on copper (usually).

If you really had to have a system to supply a large current,  you would probably want to use a 250 A  Mag/Hydraulic breaker and 4/0 cable (which may be just a might too small for 250 A continuous).

Here is a 250 A high quality breaker:
http://www.solar-electric.com/mnedc250.html

It really should be in a metal box,  but you could adapt a standard NEMA box,  by cutting the correct size rectangular cut out for the front of the breaker.  This might not fly if the system needs to pass an inspection.

MidNite e-panels are great for collecting all of the breakers,   mounting for CCs and inverters etc.  You may be operating on a shoestring,  and need to save some cash by a DIY approach.

Just my opinions.  Good Luck,   Vic.

[SolarVet]

To be Honest I dont find myself using more than 200 amp continuous, I have a Vanner VLT1000 and Xantrex 2000 PSW power inverters.
The reason I say no more than 200 amp because I monitor loading and really dont care for loads of more than 150 amps.
Most of my loads are 12 VDC LED lighting and what nots. And my 12 VDC loads dont pull more than 20 amps all together.
The Power Inverters when I use them mainly for backup or I feel itchy.
I may check on some pricing on wire and go to 3 or 4/0.
But I had to the 2 ga Welding cable..Which is Carol brand. So I didnt see why waste it. The runs will only be 10 feet max if that.
So I may use 100 amp breakers instead.
The reason why I use the bussman is that they tend to handle the surging better. I had seen the 250 amp MN you talked about. Its something I have considered also.
And yes everything is in conduit.

[Vic]

Hi SolarVet,   Thanks for the added detail.

To safely use the  2 gauge cable,  agree that a 100  amp breaker would do the job for three of fewer current-carrying conductors in conduit.

I am not a real Code expert,   but believe that most common circuit breakers have a 75 degree C limit for the temperature of the conductor.   So,  even if your #2 gauge cable had a 90 C rating,  you would need to use the 75C rating from an Ampacity Table  (115 A for cables with 75 C maximum ratings on my Table).  There are some variations in these Ampacity Tables,  and if there was an Inspector looking at your installation,    that person would have a reference to just which table to use.

You should use the max ampacity value for the exact rating of the cable that you plan to use.

The MidNite breakers are rated for continuous DC current at the labeled current value on the breaker.   Thermal breakers are normally affected by the ambient temperature,  which usually can change the exact current at which they will trip.

Good luck with your project.   Just my opinions,    Vic