Fused, Non Fused, and Starter Disconnects

[audioaddict]

I've been out of the loop for awhile with EE design nitty gritty, in particular with the usage of these 3 types of disconnect switches.

My understanding for several years has been to use whatever was necessary for each scenario....if a motor had a built in fuse, you would go with a regular disconnect, otherwise a fused disconnect. A starter disconnect would be used when you needed to combine a starter with a disconnect switch....I believe we were using these on circulating pumps and other plumbing equipment.

But this stuff is still confusing and I've been reading reports where some of this stuff is voluntary....choosing between fused and unfused disconnects to save money (sort of like 3 pole vs. 4 pole ATS switches...why else go 3 pole?).

And for the longest time I could have sworn you would only use thermal switches on 120 volt equipment but some guys have claimed to use it on 208 V single phase....

I can't claim to be the guru here, and despite an NEC bible...there seems to be a good variety on how things are done by EEs.

Discuss.

 

[Kuku]

It's usually a matter of coordinating with the mechanical engineer and a quick glance at the equipment cutsheets. Most of my work in the past few years the AHJ has required the mechanical contractor to provide the starter, disconnect etc. and it was on the EC to install and wire them. This is the best case scenario for electrical as it is one less thing we have to worry about.

 

[tesla314]

Re. fused vs. non-fused: I typically show fused disconns for all mech equipment and require the electrician to provide a fuse per the nameplate. I find that the equipment that actually gets installed rarely is exactly what's on the cut sheets (no sole-sourcing). So if what showed up needs a size smaller OC protection than what I provided for at the CB, the electrician can just fuse down at the disconnect.

 

[JRaef]

I've been out of the loop for awhile with EE design nitty gritty, in particular with the usage of these 3 types of disconnect switches.
My understanding for several years has been to use whatever was necessary for each scenario....if a motor had a built in fuse, you would go with a regular disconnect, otherwise a fused disconnect. A starter disconnect would be used when you needed to combine a starter with a disconnect switch....I believe we were using these on circulating pumps and other plumbing equipment.

But this stuff is still confusing and I've been reading reports where some of this stuff is voluntary....choosing between fused and unfused disconnects to save money (sort of like 3 pole vs. 4 pole ATS switches...why else go 3 pole?).

And for the longest time I could have sworn you would only use thermal switches on 120 volt equipment but some guys have claimed to use it on 208 V single phase....

I can't claim to be the guru here, and despite an NEC bible...there seems to be a good variety on how things are done by EEs.

Discuss.

There are several issues at play here, I'll only cover a couple off the top of my head.

1) In a motor circuit, if the circuit already has an OCPD upstream sized for the motor, you don't need a second one. So for example if you have a branch feeder circuit breaker in a panelboard that is feeding power to a motor, and nothing else, then at the motor starter you do not need another fuse or circuit breaker so a non-fused disconnect would suffice. But if you have a branch feeder going to a duct and it is tapped off to run to a motor starter as it goes on to feed something else, then you will need an OCPD after the tap off point.

2) If you have a fairly high Available Fault Current, you may need the fuses in the circuit anyway. Non-fused disconnects often have relatively low withstand ratings unless protected by narrowly specific fuse classes, which give them a higher rating by a process called "Series Ratings". But none of the non-fused disconnects are series rated with circuit breakers. So if, for example, you have an AFC of 30kA and you are using 35kAIC rated breakers in the panelboard, you cannot use a non-fused disconnect at the motor starter because it is only going to be rated for 18kA (or less) without the fuses.

3) You want closer motor winding short circuit protection than you will get with a circuit breaker, so you use a fuse that comes in a size designed to match a specific motor

In general it's never a problem to have a fused disconnect as opposed to a non-fused, but it is sometimes a problem to have a non-fused, especially if you are unsure about other aspects of the system.

As to "starter disconnects", that's just a matter of convenience. If you are going to have a starter and the disconnect is not already there, it makes more sense to use a "combination starter" where the disconnect is already built-in, otherwise you have higher installation costs. But if a non-combination starter comes on a piece of machinery and it doesn't come with it's own disconnect, they you need to add one ahead of it.