CB or Fused Switch as Main Svc. Disconnect?

[Designer69]

How do you know whether to use a circuit breaker or fused disconnect switch at your main service disconnect means?

Are there instances where one vs. the other is better?

Thank You

 

[kwired]

Available fault current can factor into which one may be selected.

Disconnect with RK-5 fuses will be rated for 100kAic.

Loadcenters with factory main breaker often are 22-25 kAic rated as well as most in stock individual breakers that could be mounted in an enclosure intended for housing a single breaker. Cost goes up to get higher rated AIC, yet they typically only go up to about 65 kAIC, so if you need more then that you probably looking at fuses as the only option.

 

[Jraef]

It’s also sometimes easier to use fuses to coordinate systems, such as using series rated devices and panels downstream of current limiting fuses, especially if your down stream sub panels are not the same brand as the main breaker.

 

[petersonra]

I am not a huge fan of large fuses. It always seems like when they blow there are no spares and it is not like you can come up with a 1000A fuse from Home Depot.

These days you can readily get current limiting MCCB upto 100 kAIC and 1600-2400A so unless you are above that level I'd be inclined to stick to circuit breakers. Cost wise the MCCB is often less at larger sizes than the comparable fuses and fuse holders, especially at lower AIC ratings, although a lot of that depends on your pricing structure and that can be all over the place.

If I was the owner I would not allow fused mains unless a spare set of fuses was in a box next to the main.

I like lower rated fuses. I don't have a problem with 30 and 60 A class CC and Class J fuses because they are cheap and one can afford to have spares for them, and even if you don't have spares they can be gotten pretty easily. But you start to get above 60A ratings and they can be tough to come by at zero dark thirty on a Sunday morning.

 

[zbang]

If I was the owner I would not allow fused mains unless a spare set of fuses was in a box next to the main.

As long as there's enough maintenance supervision so they're replaced as they're used; finding two empty boxes and no fuses in the spares cabinet on a Sunday is Not Fun (it also leads to creative engineering to get the lights back on).

 

[retirede]

As long as there's enough maintenance supervision so they're replaced as they're used; finding two empty boxes and no fuses in the spares cabinet on a Sunday is Not Fun (it also leads to creative engineering to get the lights back on).

No empty boxes - the blown fuses are placed in them.

 

[petersonra]

No empty boxes - the blown fuses are placed in them.

put a seal on the spare fuse box so it is obvious someone opened it.

we once got several boxes of fuses from the supply house that many were open.

at first the supply house and the fuse manufacturer thought we had just gotten a bad batch but the manufacturer checked them and said they were actually blown.

turned out one of their customers had returned a bunch of fuses apparently including a bunch that were blown.

probably 30 or 40 blown fuses in the bunch.

 

[Jraef]

No empty boxes - the blown fuses are placed in them.

:lol:
So true... and sad
:slaphead:

 

[electrofelon]

Im not sure but I think a CB+ enclosure is a little smaller than a safety switch, in case space is tight. Here is a 1000A switch, as you can see its not small.

Maybe I'm just lucky but I cant remember the last time I needed a standalone service disconnect, and if you dont need one, usually a factory PB with MB or using the 2-6 rule works out the best, unless you get into real high fault currents as others have mentioned.

 

[JFletcher]

I suppose a couple of those bottles of Guinness will make the conductors all look the same length LOL.

As for spare fuses, I have seen when they were on site, and the electrician pop them in without finding the fault and promptly blew one or more of the new fuses. Last time was at a dollar store type place that lost a phase due to a tree root growing up into the high voltage side of the Transformer

 

[kwired]

I suppose a couple of those bottles of Guinness will make the conductors all look the same length LOL.

As for spare fuses, I have seen when they were on site, and the electrician pop them in without finding the fault and promptly blew one or more of the new fuses. Last time was at a dollar store type place that lost a phase due to a tree root growing up into the high voltage side of the Transformer

I was going to mention since the bottle was sitting on the line side terminals - that must be what is powering it.

 

[electrofelon]

I was going to mention since the bottle was sitting on the line side terminals - that must be what is powering it.

Well it was, but we couldnt get quite enough power, here is the second attempt, which almost worked but we couldnt get anything on the C phase.......

P.S. the conductors we laid out on the ground and each phase is indeed the exact same length.

 

[kwired]

Well it was, but we couldnt get quite enough power, here is the second attempt, which almost worked but we couldnt get anything on the C phase.......

P.S. the conductors we laid out on the ground and each phase is indeed the exact same length.

You are missing C phase and neutral. Better get something on that neutral or you will have unpredictable voltages on line to neutral loads.

 

[JFletcher]

I see the problem... You don't have Corona beer... LOL... Oh wait, this is a low voltage installation.


I was sorta kidding about the conductor lengths anyway, without seeing the other end, or knowing how many bends and which way they go, judging the lengths by looking at 1 panel can give obviously errant results.

 

[petersonra]

What's that green wire? Is that a grounding electrode conductor? Must be because it's connected to the neutral. I don't think grounding electrode conductors are allowed to be green only equipment grounding conductors can be green.

 

[kwired]

What's that green wire? Is that a grounding electrode conductor? Must be because it's connected to the neutral. I don't think grounding electrode conductors are allowed to be green only equipment grounding conductors can be green.

There is no requirement to identify the GEC by color.


from 250.119:

Conductors with insulation or individual covering that is green, green with one or more yellow stripes, or otherwise identified as permitted by this section shall not be used for ungrounded or grounded circuit conductors.

This prohibits using green for most other uses other then EGC but a GEC is one thing that does not fit into that description.

I say it can be green or pretty much any color you want.

 

[petersonra]

There is no requirement to identify the GEC by color.


from 250.119:

This prohibits using green for most other uses other then EGC but a GEC is one thing that does not fit into that description.

I say it can be green or pretty much any color you want.

I think you may be right.

 

[MBLES]

It’s also sometimes easier to use fuses to coordinate systems, such as using series rated devices and panels downstream of current limiting fuses, especially if your down stream sub panels are not the same brand as the main breaker.

What if the Main breaker/panel is 65k and you add a disconnect with rk5 fuses. Would this be series comb rated? Does the down stream equipment need to be the same rating? higher or lower? I struggle with this requirement. If you can kinda elaborate a little please

 

[electrofelon]

What if the Main breaker/panel is 65k and you add a disconnect with rk5 fuses. Would this be series comb rated? Does the down stream equipment need to be the same rating? higher or lower? I struggle with this requirement. If you can kinda elaborate a little please

Probably. That situation is "opposite" of the way it would usually go, but still valid. Here is one list of series ratings

http://www.cooperindustries.com/con...ducation/solution-center/series_ratings0.html

 

[kwired]

What if the Main breaker/panel is 65k and you add a disconnect with rk5 fuses. Would this be series comb rated? Does the down stream equipment need to be the same rating? higher or lower? I struggle with this requirement. If you can kinda elaborate a little please

I agree kind of the opposite of the way one usually looks at series ratings. If you had RK-5 fuses upstream, you probably wouldn't shell out extra $$$ for 65 kA breakers to be located downstream. I think most "standard interrupt rating" breakers will be series rated for use with RK-5 fuses. Most all that I have encountered in Square D certainly are.