Class-T Fuse Block and Fuses. T-105’s and 2/0 cable.

[JDCAVE]

The overcurrent protection on the house bank cable is located at the terminus in the electrical locker in the engine room. I want to rethink this and move it much closer to the battery bank.

I have two battery boxes with 4 T-105’s each with 2/0 cable. My intent is to have 2 short lengths of cable (~18”) from the the battery boxes to a Class-T Fuse Block. I realize this is 18” instead of ABYC 7” but, I think this is the best I can do and I don’t think mounting a fuse on a stud on the “last battery” will work in this instance.

I have been using the Blue Seas circuit wizard to do the calculation for the size of fuse and cannot get it to work. It asks for cold cranking amps and it’s not specified in the T-105 spec sheet. I think that’s where it’s tripping up.

I’m thinking of landing at 250 amp fuse, Class-T.

Thoughts?

Jim

 

[Comodave]

The fuse should be sized for the cables it is there to protect. Not sure what 2/0 is rated for but check that.

 

[Eli27]

The fuse should be sized for the cables it is there to protect. Not sure what 2/0 is rated for but check that.

I dont think length is an issue with 7 or 18" like the OP but dont ever forget length of run.

Eli

 

[JDCAVE]

It’s not clear to me from this Fuse selection chart either:

https://www.bluesea.com/support/art..._Fuse_Holder_For_Your_DC_Product_Installation

It looks like it could be greater than 250 amps. Not sure. I think the length of the cable run has more to do with voltage drop than fuse selection.

Jim

 

[Comodave]

Length is related to voltage drop. I am referring to ampacity or how many amps can the cable conduit without melting down. That is what you should fuse it for. Something less to where it will melt down. The fuse is there to protect the cable.

 

[JDCAVE]

Length is related to voltage drop. I am referring to ampacity or how many amps can the cable conduit without melting down. That is what you should fuse it for. Something less to where it will melt down. The fuse is there to protect the cable.

Agreed. And that is why I asked the question.

Jim.

 

[O C Diver]

I may loose points for posting this, but this was my solution.



The connection to the battery terminal is copper bus bar. My cable is 4/0 and the fuse is 400 amps. In reality, I could down size it to 300 amps as the highest potential draw is the inverter, no longer the 12 VDC bow thruster (300+ amps).

This setup was originally used on my T-105s with the standard threaded studs.

Ted

 

[JDCAVE]

Very good Ted! I’ve found my answer in the Western Marine Catalogue. Looks like single wire, engine room, 300 amps Class T. This chart isn’t referenced, but it looks like it could be from Blue Sea. The original Blue Sea one I looked at had empty fields on the table.

Jim

 

[JDCAVE]

Further to this, CMS, Marine-how-to on the subject. As always, he is very compelling in his review.

https://marinehowto.com/battery-banks-over-current-protection/

Jim

 

[Comodave]

Gotta love marinehowto.com.

 

[mike66]

You could size the fuse down as low as your max current supplied to your house system. You are probably never going to draw 250 or 300 amps. Don't want nuisance trips, so go a bit higher.

 

[Bacchus]

Further to this, CMS, Marine-how-to on the subject. As always, he is very compelling in his review.

https://marinehowto.com/battery-banks-over-current-protection/

Jim

Very good article and worth a read as it includes a point many overlook when considering fuses for batty banks. It is not only the ampacity of the cable that beeds to be considered (normal ampacity protection).
The short circuit ampacity of the batty bank needs to be considered and many folks have never heard or considered this. AIC...ampere interrupt capacity... needs to be considered and fuses capable of handling it should be used on batty banks.

 

[Antares]

This is another great thread. The Marine How To article is excellent, well written and it clearly communicates the information. I made a donation as this site has so much value and it would be a shame to loose it.

 

[CharlieJ]

@ O_C_Diver #7: While seemingly elegant, this is the incorrect way to mount a Class T fuse. Fuse ratings are valid only when the fuses are installed in their designed fuse holders.

To the concerns about abiding by the 7" Rule; here are the exceptions in E-11.1.1.1:

2. If the conductor is connected directly to the battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box, or enclosed panel, the overcurrent protection shall be placed as close as practicable to the battery, but not to exceed 72 in (183 cm).
3. If the conductor is connected to a source of power other than a battery terminal and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box, or enclosed panel, the overcurrent protection shall be placed as close as practicable to the point of connection to the source of power, but not to exceed 40 in (102 cm).

 

[CharlieJ]

OP: AWG 2/0 Boat Cable has an ampacity of 330A outside of machinery spaces and 280.5A inside machinery spaces. These ampacities are for a single cable not bundled and in free air. To be conservative, I would use a 300A fuse outside the machinery space and 250A inside a machinery space.

If the cables are bundled, further derating is required.

 

[O C Diver]

I may loose points for posting this, but this was my solution.

@ O_C_Diver #7: While seemingly elegant, this is the incorrect way to mount a Class T fuse. Fuse ratings are valid only when the fuses are installed in their designed fuse holders.

Oh the indignity of an "Invalid fuse"!

Ted

 

[CharlieJ]

Invalid fuses are bad but not worse than no fuse! And, yes, they suffer from a total lack of dignity.

 

[JDCAVE]

OP: AWG 2/0 Boat Cable has an ampacity of 330A outside of machinery spaces and 280.5A inside machinery spaces. These ampacities are for a single cable not bundled and in free air. To be conservative, I would use a 300A fuse outside the machinery space and 250A inside a machinery space.

If the cables are bundled, further derating is required.

Thanks Charlie. Obviously, this is inside the engine space. The Magnum inverter charger at 2800 watts, would draw 258 amps at 11 volts, the voltage sag not being unreasonable to 11 volts. Is a nuisance trip a likely outcome with a 250 amp Class T? Would there be any serious danger going to the next size up at 300 amps?

Jim

 

[Comodave]

I would not go to 300 amps. That is exceeding the ampacity of the cable in the engine room. It could, not likely, cause a fire. DC is the most common reason for fires aboard boats.

 

[kchace]

Thanks Charlie. Obviously, this is inside the engine space. The Magnum inverter charger at 2800 watts, would draw 258 amps at 11 volts, the voltage sag not being unreasonable to 11 volts. Is a nuisance trip a likely outcome with a 250 amp Class T? Would there be any serious danger going to the next size up at 300 amps?

Jim

I have that same inverter/charger. I think your calculations do not include the adjustment necessary for the inverter's 90% efficiency. According to the manual, the maximum *continuous* DC input current is 373A. If you want to wire for maximum, you really should go to a larger supply cable.


Ken

 

[CMS]

Thanks Charlie. Obviously, this is inside the engine space. The Magnum inverter charger at 2800 watts, would draw 258 amps at 11 volts, the voltage sag not being unreasonable to 11 volts. Is a nuisance trip a likely outcome with a 250 amp Class T? Would there be any serious danger going to the next size up at 300 amps?

Jim

A2800W I/C needs 4/0 not 2/0...

 

[JDCAVE]

Thanks JChase, CMS. My electrician and I discussed cable size when we installed the Magnum 2812 and reviews cable requirements. The current situation with the 2/0 cable isn’t ideal but I never exceed 1200 watts on the inverter right now and I always leave it off unless it’s required. The boat is substantially left as a DC boat. Upgrading to 4/0 is not an option at this time.

I will go to 250 amps on the Class T, and relocate it closer to the battery bank terminal. That will be an big improvement over the existing cabling.

Jim

 

[diver dave]

I may loose points for posting this, but this was my solution.



View attachment 132970



The connection to the battery terminal is copper bus bar. My cable is 4/0 and the fuse is 400 amps. In reality, I could down size it to 300 amps as the highest potential draw is the inverter, no longer the 12 VDC bow thruster (300+ amps).



This setup was originally used on my T-105s with the standard threaded studs.



Ted

Can you makes those nuts under the lugs go away? Or convert to Si-bronze? Or, just put me on ignore ?