Fuse/breaker location for inverter
westyd1982
Solar Expert Posts: 85 ✭✭
I have a Morningstar Suresine 300 inverter, and in its installation instructions it recommends a 100 amp fuse no further than 12" from the battery terminal. I'm looking at upgrading the charging capabilities of my system and am thinking about building the upgraded system around a Midnite Solar Mini DC Disconnect Power Center.
My question is do I need fuses in the battery box close to the positive battery terminal for each positive wire? Or can the disconnect with appropriate breakers handle the circuit protection correctly at the end of 5 feet of 2/0 wire? Or should I have a fuse larger than the disconnect breaker in the battery box to protect the wire in case of a catastrophic failure (earthquake or fire that damages the insulation on the 2/0 wire)?
I currently have fuses on each of the three positive wires attached to the battery: charge controller, Suresine, and a 1000W MSW inverter. With the Midnite Solar Mini DC Disconnect Power Center, I'd just have the 2/0 cables.
My question is do I need fuses in the battery box close to the positive battery terminal for each positive wire? Or can the disconnect with appropriate breakers handle the circuit protection correctly at the end of 5 feet of 2/0 wire? Or should I have a fuse larger than the disconnect breaker in the battery box to protect the wire in case of a catastrophic failure (earthquake or fire that damages the insulation on the 2/0 wire)?
I currently have fuses on each of the three positive wires attached to the battery: charge controller, Suresine, and a 1000W MSW inverter. With the Midnite Solar Mini DC Disconnect Power Center, I'd just have the 2/0 cables.
Comments
I am not a NEC expert, but I think the answer depends on whether the wires from the battery to the midnite disconnect are in conduit. If in conduit, I think you do not need a fuse in the battery box.
--vtMaps
The wires run through conduit from the battery box.
What size breaker came with your MNDC, and are you using it as the breaker for the inverters or as a battery disconnect with more fuses/breakers between it and the inverters and controller?
The idea of breakers/fuses is to protect the wiring from short circuits. And placing the breaker as close as practical to the source of energy keeps the "unprotected" wiring distance short too.
Personally, I would probably want a breaker before I put a pair of cables into a conduit. Yes, the conduit does protect against falling sharp objects and wear+tear--However, there have been shorts that develop inside conduit too (why? don't know--damage during installation, failure from chemicals, stress on wiring, pinched conduit???). Having an insulation failure in heavy cable with no breaker back to the battery bank--can be scary.
If possible, I would install the breaker on the outside of the battery box. Keep the unprotected cable short and the breaker/fuses outside of the box (away from acid mist and hydrogen gas).
-Bill
BB -
Thanks. Right now, I have a 150 amp fuse on the 2/0 wire for the MSW inverter. I'll keep it in the upgraded system. Right now I have fuses close to where they attach to the battery terminal. Unfortunately, I don't have space to have them outside the battery box (see photo below). I occasionally look for signs of corrosion. So far after 3 years my battery box stays very clean inside and I haven't noticed anything.
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Consider using a Bluesea battery terminal fuse system. Comparitively cheap, and easy to customized both for charge circuits and load circuits.
Tony
rgk1 -
I haven't purchased a MNDC yet. I was looking at the 125 or 175 amp versions. I'm thinking of having it as the battery disconnect with everything else on its own breaker after the main breaker.
This is the link to the BlueSea bus/fuse terminal (very neat device) that Tony typed about (two and one fuse devices are available):
http://bluesea.com/products/2151
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-Bill
Westyd1982, as everyone says, having a fuse on or at the batteries is a goood idea. I use a 110 amp class T fuse at the batteries as the first line of protection for all the wiring. I currently use Anderson connectors to disconnect the batteries, but will be removing that, running the battery leads in conduit into the MNDC and use the (175 amp) breaker as the disconnect. Then like you said, separate breakers to the other components. The MNDC is a great box to neatly connect everything and can hold 5 din breakers or 3 panel mount.
This Class T fuse (in a fuse holder block) is on the battery positive terminal. The cables than go to a Midnite Mini DC with a 250 amp breaker. Code wise, any fuse is supposed to have a means of de-energizing it so it will not have any voltage potential if you need to replace it. In this case I'd rather have the protection at the terminal than be to the letter of the code. The inspector agreed in this case.
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Thanks Bill, not a lot of time these days, and not too much bandwidth.
T
Tony--And I have, obviously, have too much time and bandwidth.
Love,
-Bill