No Overload Protection on DB wired to Busbar

[Ian1981]

It is a dist board and should have been given circuit protection from day one unless its switch and supply conductors were rated at 200A.

It is a dist board and should have been given circuit protection from day one unless its switch and supply conductors were rated at 200A.

its the same as when you tap of a large busbar with a smaller CSA of cable overload is located on the load side of not more than 3meters.
If originally the load profile wasn't going to exceed the rating of the main switch then it would have been acceptable.

In this case then yes it now may not be

 

[davesparks]

Regulation 433.2.2 would have allowed the downstream mcbs to act as the overload protection so when it was first installed the tails need not have required overload protection due to the original loading requirements

that regulation allows the device protecting a conductor to be placed downstream of the point in reduction of csa, this can be applied to a switchfuse fed from a busbar as the switchfuse contains a device which protects the conductor.
You can apply this to a DB as it does not contain a device which protects the cable, it contains many devices. You can't just add up the ratings of all of the protective devices in the DB because they will not operate together as a single device to protect the reduced case cable. The DB would need to have a protective device as its income, for example an MCB or MCCB in place of the main switch.

 

[davesparks]

View attachment 36713

That table is unlikely to be of any use as it is for a conductor operating temperature of 90C, the DB incoming terminals will almost certainly be rated for 70C operation and so the conductor operating temperature must be limited to 70C

 

[westward10]

Plus grouping factors are required for tri-rated.

 

[westward10]

that regulation allows the device protecting a conductor to be placed downstream of the point in reduction of csa, this can be applied to a switchfuse fed from a busbar as the switchfuse contains a device which protects the conductor.
You can apply this to a DB as it does not contain a device which protects the cable, it contains many devices. You can't just add up the ratings of all of the protective devices in the DB because they will not operate together as a single device to protect the reduced case cable. The DB would need to have a protective device as its income, for example an MCB or MCCB in place of the main switch.

Mccbs although they would perform the task are rarely used it is nearly always a switch fuse disconnector.

 

[Ian1981]

That table is unlikely to be of any use as it is for a conductor operating temperature of 90C, the DB incoming terminals will almost certainly be rated for 70C operation and so the conductor operating temperature must be limited to 70C

I know it also states the correction factors for different temperature which is why I posted both

 

[davesparks]

I know it also states the correction factors for different temperature which is why I posted both

I have that same PDF and it doesn't give conductor operating temperature correction anywhere. It only gives correction for ambient temperature.

 

[Ian1981]

I have that same PDF and it doesn't give conductor operating temperature correction anywhere. It only gives correction for ambient temperature.

You are correct Dave my mistake I've misread it.
It does state that the ccc ratings given in the tables are based on 90 degrees tho.
Even the biggest panel board I've fitted with a 200amp mccb the terminals were only rated at 75 degrees max.
I've never come across anything that requires 90 degrees operating temperature.
Industrial perhaps?

 

[Flanders]

I have come across this in old factory installs a few time and in one case main fuse protecting the bus bar have been a 400amp and the same set up as the op I have always flaged these up as code 2 observations, and agree with op that fitting a switch fuse is the best option , unless there is enough room to replace the main switch with an mccb as daves says, the problem with that is you normaly nead an expansion box on the bottom of the DB to fit the mccb in , so may not be as straight forward

 

[westward10]

I know it also states the correction factors for different temperature which is why I posted both

Tri-rated cables require grouping factors the table you show is for a single conductor in free air. Four conductors together into the DB require a factor of 0.65 applied to this table.