My Magnums MS4448PAE call for 2/0 wire with 175 amp protection. I intend to use a fuse and a cut off switch. This will be X2. Is it necessary to have a breaker also?

Unless they have changed their manual--one fuse

Glenn, in order to provide "Overcurrent Protection" you need only one device and I often prefer fuses versus breakers. An On/Off switch is a separate issue and hey I have no problem with those. Sure, a proper circuit breaker could ??? serve as BOTH if that's YOUR choice and Magnum approves of such ???????????

NO you DO NOT need a Fuse plus a Breaker to provide "Overcurrent Protection" ...………….The time current curves would determine which opens first.

NOTE "Overcurrent Protection" for huge cables FROM a battery TO an Inverter need located at the energy source IE the battery, not far downstream leaving cable unprotected !!!!!!!!!!!

John T

Edited December 6 by oldjohnt

The Magnum Panels all come with breakers. That is why I was asking. I agree with yall. one device is enough. 

  On 12/6/2019 at 7:41 AM, GlennWest said:

My Magnums MS4448PAE call for 2/0 wire with 175 amp protection. I intend to use a fuse and a cut off switch. This will be X2. Is it necessary to have a breaker also?

I suggest a 175A breaker instead of a fuse. I used to use fuses too until I had an electrical issue that blew the fuse. Then what? Keep replacing the fuse until you locate the problem? Good luck with that.

Also, you get a fuse & cut off switch in one.

Edited December 6 by Zulu

  On 12/6/2019 at 8:10 AM, GlennWest said:

The Magnum Panels all come with breakers. That is why I was asking. I agree with yall. one device is enough

Glenn built in breakers are all well and good for their intended purpose and I'm sure your Magnum is properly designed...

 HOWEVER FYI even if you are already aware for those who aren't,  don't forget in order to provide "Overcurrent Protection" for the huge cables FROM the battery TO the Inverter those overcurrent protection devices (fuses or breakers) must be located at the source of energy the BATTERY not later somewhere downstream which would leave that portion of the cable (from battery to a breaker inside the Inverter) UNPROTECTED !!!!!!!!!!!!!!!! Yikes  !!!!!!!!!!! That's a serious potential hazard with 2/0 Cables and a 21 KWH battery !!!! A breaker can only detect current that flows THROUGH IT not elsewhere like from a POS cable shorted to the RV frame BEFORE the Inveter !!!!!

Of course there's no problem in overcurrent  protection at BOTH locations (battery and Inverter) as well as On/Off switches 

NOTE in the area of Electrical Power Distribution there are applications where fuses are favored and others where circuit breakers may be

NOTE for certain applications "Switch Rated" breakers are required that may not be necessary elsewhere

John T  Longgggggggg retired n rusty power distribution engineer and codes change so NO warranty...Consult the manufacturer and local practicing electrical engineers and electricians where fire and life safety is concerned ………...

I am also using a 175 amp breaker on the positive lead between my battery and inverter, and normal 20 and 50 amp breakers on the ac outputs.

I have #4 running from each battery to a 8 post  buss bar. Leaving buss bar with 2/0 to protection then inverters. 

Edited December 6 by GlennWest

The suggestion of just the breaker is tempting. 

  On 12/6/2019 at 11:13 AM, jcussen said:

I am also using a 175 amp breaker on the positive lead between my battery and inverter, and normal 20 and 50 amp breakers on the ac outputs.

I had not intended to add breakers on the out side of inverters. It will connect to panel box in RV. It has its own 50amp breaker

I do not trust breakers on high ampere dc circuits. I do use Blue Sea battery disconnect switches. I also use these MRBF on the bus bar.    https://shop.marinehowto.com/products/blue-sea-marine-rated-battery-fuse   They save a couple of connections each. A catastrophic fuse could be required at the battery before the bus bar if the cable length is too long.

  On 12/6/2019 at 5:38 PM, GlennWest said:

I had not intended to add breakers on the out side of inverters. It will connect to panel box in RV. It has its own 50amp breaker

Glenn, I would expect the AC Output of the Inverter has its own built in limiting protection so no additional DIY after market protection would then be required. Of course, as I explained below, your RV Distribution Panel indeed has "its own 50 Amp Main Breaker" to monitor the current that passes THROUGH and AFTER it, but that doesn't protect the WIRING FROM Inverter output TO the Panels 50 Amp Main, the Magnums output protection and limiting should take care of that.

  On 12/6/2019 at 10:35 PM, Sehc said:

I do not trust breakers on high ampere dc circuits. I do use Blue Sea battery disconnect switches. I also use these MRBF on the bus bar.    https://shop.marinehowto.com/products/blue-sea-marine-rated-battery-fuse   They save a couple of connections each. A catastrophic fuse could be required at the battery before the bus bar if the cable length is too long.

SEHC    DITTO  Indeed there are as I recall numerous applications where fuses are preferred over circuit breakers.   DITTO  Also catastrophic fuse protection can be required "at or near" the battery, I forget the distances and current and voltage etc. requirements.  

Yall be safe and take care now

John T

  On 12/6/2019 at 10:35 PM, Sehc said:

I do not trust breakers on high ampere dc circuits. I do use Blue Sea battery disconnect switches. I also use these MRBF on the bus bar.    https://shop.marinehowto.com/products/blue-sea-marine-rated-battery-fuse   They save a couple of connections each. A catastrophic fuse could be required at the battery before the bus bar if the cable length is too long.

My positive cables are 18" from batteries to bussbar. Negatives are 24". Negatives at bottom so they had to be longer.

  On 12/6/2019 at 10:35 PM, Sehc said:

I do not trust breakers on high ampere dc circuits.

Midnite Solar’s F-series magnetic/hydraulic breakers seem to have some pretty impressive specs. Glenn, ask on the MidNite Solar forum.

  On 12/6/2019 at 5:38 PM, GlennWest said:

I had not intended to add breakers on the out side of inverters. It will connect to panel box in RV. It has its own 50amp breaker

Mine is in the S&B, so the 50 and 20 breakers are in a breaker box with receptacles, nothing else between inverter and breaker box.