Hello, new member here - I'm putting together a small off-grid system in a remote cabin. I just picked up an MNDC125-Plus disconnect box and plan to use the 125A breaker to connect a high current (90A) charger to the system. I understand that this box was intended for tying in an inverter, but it seems flexible enough to suit my needs. My question is regarding the polarity and hookup of the main breaker. I think I know the answer, but was hoping one of the experts here could confirm it.
The instructions that came with the box state:
"DC positive from the battery bank connects to the lower section of the large DC inverter breaker"
I assume this is referring to the lower stud on the breaker when the box is oriented as shown in the instructions. However, the label on the breaker identifies the lower stud as "Load" and the upper one as "Line". This suggests to me that, in DC polarity terms, the lower stud is the "-" terminal and the upper one is "+". If it makes difference, the breaker that came with it is a Carling type EA1-X0-17-708-22A-ZB.
I read a post in the breaker FAQs section where Halfcrazy explained that DC breakers should always have their "+" terminal connected to the battery, even for charging sources. At first this sounded counter-intuitive, but after reading the rationale he presented it made sense to me. Assuming that's correct, it seems to me that I should be connecting the battery positive to the upper terminal of the breaker and the positive output of the charger to the lower terminal. Is this right? Are the instructions wrong? Am I confusing what "Line" and "Load" mean? I'd appreciate any insight anyone can provide.
Randy
Polarity is not an issue on the battery breakers mainly do to the high IAC and the low operating voltage.
Ryan
Ah, got it. Thank you so much for the reply.
Halfcrazy is correct. I have designed most of the DC electrical boxes installed in North America. I have always hooked the battery plus to the bottom stud. This is done for ease of wiring. When you are charging, the inverter is the source and the battery is the load, so technically it would be correct when charging. I have found over the years that with a 50,000 amp plus AIC and the relatively battery voltage as compared to the 125 volt rating, their just isn't a problem. Thousands of invereters over the years have blown and the breakers always do their job. When an inverter blows up, the fets short. This makes a direct short across the battery. The battery voltage is drawn down a bit due to the huge amount of current going though the inverter circuit and the AIC rating is overly sufficient to trip the breaker. We chose this style breaker many years ago for a good reason.
Thanks for the detailed reply. You folks seem to know what you're doing. I'm sure I'll have more questions as my design/install progresses.