Are there instances you use a non-fused disconnect?

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starbright28

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Minnesota
As many of you know I am an electrical designer/drafter for an engineering company. I have shown in the past the disconnects are a plain 'ol disconnect symbol. But then I got to thinking - some instances are going to require the disconnect fused or not fused.

The more I thought about it this question popped into my mind. Will I ever show a disconnect on my drawings that is not fused?

We do mostly commerical and industrial work. Of course in the state of MN, drawings are not necessarly required to do the electrical in buildings.

Hopefully I'm not off topic here, but just trying to get clear in my mind when would you use a non-fused disconnect.

I can give examples as to the stuff I show disconnects for. I feel the more and more that I field out these questions, the better off our drawings are going to be and less questions coming in from the field.

Thanks a bunch.
 
Nfds

Nfds

We do alot of commercial work in Florida and to me the issue of whether to use either fused of NFDS is a matter of choice. Take a 60a RTU for example. If the OCPD id rated at 60 amps the RTU disconect is so the HVAC contractor can service their unit safely so a NFDS would be fine. FDS are more expensive and also pose additional maintenance issues as far as you have an additional fuse that will go bad eventually and need to be replaced. However all that being said the majority of the jobs we do require the use of FDS. Hope this helps.
 
If the overcurrent protection is already being taken care of by the OCPD in the panel, and a disconnect is needed only for service, than an unfused disco is ok.
A lot of motor applications require an oversized circuit breaker at the panel to allow for start up, in this case the overcurrent protection must be provided at the motor disconnect, usually with time delay fuses.
 
monkey said:
If the overcurrent protection is already being taken care of by the OCPD in the panel, and a disconnect is needed only for service, than an unfused disco is ok.
A lot of motor applications require an oversized circuit breaker at the panel to allow for start up, in this case the
overcurrent protection
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must be provided at the motor disconnect, usually with time delay fuses.
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Wouldn't the OC protection be either built in to the motor or part of the starter?
 
In the past, most HVAC equipment was not rated for protection by circuit-breakers; so the easy fix was to provide a fused switch at the equipment.

Sometimes a batch of welding receptacles are supplied from a common feeder in an industrial plant, so a fusedd switch may be appropiate. However, where only a disconnecting means is required, an unfused switch is adquate.:)
 
petersonra said:
Wouldn't the OC protection be either built in to the motor or part of the starter?
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That would be OverLoad protection, and perhaps this is what monkey meant...
 
Okay I beleive I am understanding when and when we don't need fused disconnects.

For electrical drawings here I show disconnect switches for Roof Top Units (even those Mechanical details show and specifiy one), Exhaust Fans, Motos, Ait Handling units, Make up Air Units, door openers.

All jobs we do are commerical and industrial based.

As I mentioned with my initial post - I was starting to show fused disconnects.

Shouls I always show fused disconnects, or what is the preference on drawings?
 
Unless what you're supplying requires fuses or overcurrent protection I would never spec a fused disconnect. Sounds like a waste of someone's money.
 
starbright28 said:
Shouls I always show fused disconnects, or what is the preference on drawings?
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It's really quite simple. A fused disconnect switch serves two purposes:
  1. as a disconnecting means, and
  2. as an overcurrent protection device.
If both are not required, show only a disconnect switch.
 
infinity said:
Unless what you're supplying requires fuses or overcurrent protection I would never spec a fused disconnect. Sounds like a waste of someone's money.
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It's more annoying than that; someway down the line a protection device will open the circuit, and if theres a protection device at each end of the cable then you have to determine which device it that has opened; you can design that piece of pain out of the system whilst saving the customer money - a definite win-win.
 
use nfdc where possible .
Who wants to have to go onto the roof to replace a fuse.
As long a ocd in the panel is correctly sized and acceptabel for the equipment protected.
nfd for me.
 
In many cases an AC unit will specify on the nameplate maximum "fuse" size instead of maximum "OCPD." Are we supposed to interpret this as having to use fused disconnects ? Many times you do not get to know what to provide until the unit actually gets to the jobsite. Just curious.
 
starbright28 said:
Okay I beleive I am understanding when and when we don't need fused disconnects.

For electrical drawings here I show disconnect switches for Roof Top Units (even those Mechanical details show and specifiy one), Exhaust Fans, Motos, Ait Handling units, Make up Air Units, door openers.

All jobs we do are commerical and industrial based.

As I mentioned with my initial post - I was starting to show fused disconnects.

Shouls I always show fused disconnects, or what is the preference on drawings?
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IMO, it is better to avoid adding these fuses if you can. I know a lot of times they provide very cost effective protection, but it seems like when the miserable things blow at 2 AM, there are no spares to be found. Much easier to just reset the CB and move on.

Many of our customers that need high equipment uptime just do not allow fuses.
 
Shouls I always show fused disconnects, or what is the preference on drawings?[/QUOTE]


I would say non fused disco and if the EC wants to install a fused disco that is his/her chioce but as other said First OCPD in Panel board should protect equipment.
 
Non-fused have very low withstand ratings.
Fused discos have much higher AIC/withstand ratings.
You need to determine the available fault current at the disco and design accordingly.
 
goldstar said:
In many cases an AC unit will specify on the nameplate maximum "fuse" size instead of maximum "OCPD." Are we supposed to interpret this as having to use fused disconnects ?
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Yes if you want to comply with the UL listing.

Many times you do not get to know what to provide until the unit actually gets to the jobsite. Just curious.
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Info would be on the submittals if there are any.
 
HACR rated breaker

HACR rated breaker

using a fused disconnect is not a matter of choice. whether you use one or not is dependent on the device. The plate on the unit will state whether you need a fuse or an HACR rated circuit breaker. A Heating AirCondition Refrigerant rated circuit breaker is always required when the disconnect for any A/C unit is not fused! 2005nechb 440.21:cool:
 
Joseph Alexander said:
using a fused disconnect is not a matter of choice. whether you use one or not is dependent on the device. The plate on the unit will state whether you need a fuse or an HACR rated circuit breaker. A Heating AirCondition Refrigerant rated circuit breaker is always required when the disconnect for any A/C unit is not fused! 2005nechb 440.21:cool:
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The units I've been seeing state "fuse or HACR breaker. Are modern circuit breakers in general all rated HACR?
 
Joseph Alexander said:
using a fused disconnect is not a matter of choice. whether you use one or not is dependent on the device. The plate on the unit will state whether you need a fuse or an HACR rated circuit breaker.
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Not quite true Joseph. Most modern unit nameplate list MOCP or list Max Fuse or CB. [edit] If so, then you have a choice.

A Heating AirCondition Refrigerant rated circuit breaker is always required when the disconnect for any A/C unit is not fused! 2005nechb 440.21
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No, it could be fed from a fused disconnect in an MCC or switchboard etc.
 
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