Thermal Breaker vs fuse & switch

scottreb
scottreb Registered Users Posts: 7 ✭✭
Hi all

Building a small (200 aH) RV 12v system.  The question is regarding wiring/circuit protection going from the battery positive to the busbar.  I most often see some type of fuse, ANL/Mega, and a battery switch.  Is there any reason not to use a like-sized thermal breaker such as the Blue Sea 187 series?   For further info, the system will have a 500-watt inverter with 8G/80A and was going to use 6G/125A for the battery to the busbar.

Comments

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    You need to size the overcurrent protection according to the rated current capacity of the conductors downstream of the energy source, the battery in this case. Using NEC charts as a guide #8 AWG is rated at 50A for 75°C, #6 AWG  65A,  these are very conservative but better to adhere to than other charts, marine for example.

    A 500W inverter at maximum capacity at minimum voltage, let's use 11V, for worst case scenario's sake, would draw 45.5A, therefore the minimum conductor size should be able to carry at least that. The maximum of 50A for #8 should be derated to 80% or 40A continuous, so for the purpose of this exercise it would be better to use #6 minimum.

    Naturally it is better to use the largest gauge that can physically fit into the lugs used, especially with 12V nominal and keep the distance as short as possible. A smaller capacity breaker can be used on a larger capacity conductor as long as it can support the loads plus a 120% margin, skimping on copper for short runs is false economy, keep all conductors downstream of the battery the same, this will result in a single breaker being required to protect all conductors.

    Being that this is not a marine application, look elsewhere for protection as marine rated generally increases the costs below is a recent purchase I made for a 125A DC breaker, it is a DIN rail mount type perhaps  not suitable for your particular application but look for options which may be.




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  • scottreb
    scottreb Registered Users Posts: 7 ✭✭
    Thanks Mcgivor.