Fused disconnects
- Thread starter kfenn
- Start date
- Status
- Location
- Massachusetts
Re: Fused disconnects
You will have to be a little more specific.
In a general sense the NEC does not require fused disconnects.
The NEC requires overcurrent protection and the NEC many times requires a disconnect switch.
The choice to combine these requirements is more of a design issue.
You will have to be a little more specific.
In a general sense the NEC does not require fused disconnects.
The NEC requires overcurrent protection and the NEC many times requires a disconnect switch.
The choice to combine these requirements is more of a design issue.
renosteinke
Senior Member
- Location
- NE Arkansas
Re: Fused disconnects
The NEC requires that equipment be used/installed according to the makers' instructions.
Often equipment- especially air conditioning equipment- will say on theirnameplate "maximum fuse..." When only the word 'fuse' is used, rather than a more general 'overcurrent protection,' than there must be a fuse somewher in the circuit.
This fuse can be anywhere- in the disconnect, at the starter, at the panel, or standing alone. Since a disconnect is almost always required, this is a very convenient place for the fuses to be placed.
Since fuses are not always required at the disconnect, the disconnect and fuse makers often sell solid metal blanks to use in place of fuses.
Fuses have two advantages over circuit breakers. They resopnd to a short a lot quicker (1/4 cycle, rather than 6 cycles), and can often be more closely sized to the load.
The NEC requires that equipment be used/installed according to the makers' instructions.
Often equipment- especially air conditioning equipment- will say on theirnameplate "maximum fuse..." When only the word 'fuse' is used, rather than a more general 'overcurrent protection,' than there must be a fuse somewher in the circuit.
This fuse can be anywhere- in the disconnect, at the starter, at the panel, or standing alone. Since a disconnect is almost always required, this is a very convenient place for the fuses to be placed.
Since fuses are not always required at the disconnect, the disconnect and fuse makers often sell solid metal blanks to use in place of fuses.
Fuses have two advantages over circuit breakers. They resopnd to a short a lot quicker (1/4 cycle, rather than 6 cycles), and can often be more closely sized to the load.
Fusing and conductor size
Fusing and conductor size
OK, Say you have an air conditioning unit with a dataplate showing Min. circuit amps of 22 and Max. HACR CB or Fuse of 35 Amps. It is a dedicated circuit, presently with a 40 amp breaker and #10 wire. If I install a fused disconnect at the unit with 35amp fuses, can I leave the 40amp breaker and #10 wire? or does this become a feeder cicuit and require #8 wire?
Fusing and conductor size
OK, Say you have an air conditioning unit with a dataplate showing Min. circuit amps of 22 and Max. HACR CB or Fuse of 35 Amps. It is a dedicated circuit, presently with a 40 amp breaker and #10 wire. If I install a fused disconnect at the unit with 35amp fuses, can I leave the 40amp breaker and #10 wire? or does this become a feeder cicuit and require #8 wire?
1793
Senior Member
- Location
- Louisville, Kentucky
- Occupation
- Inspector
klrogers said:
I've seen discussion about this before. I tend to lean toward the feeder at #8, because of the fuses. The fuse panel, disconnect, is being fed from the 40 amp.
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
Often it may be part of the specs even thow its not needed.Often you will not know what the equipment will say till it arrives.In residential i have never seen it needed but commercial yes.
DaveBowden
Senior Member
- Location
- St Petersburg FL
Wouldn't it be less expensive to install a 35 amp breaker and a non-fused disconnect and leave the # 10 wire?
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
40 amp breaker and #10 wire is fine. You don't have to follow 240.4(D) per 240.4(G) for specific conductor applications.
Many electricians will use a 40 amp CB and 8AWG as it just doesn't seem right to use a 10 AWG on a 40Amp CB, but its OK
Many electricians will use a 40 amp CB and 8AWG as it just doesn't seem right to use a 10 AWG on a 40Amp CB, but its OK
- Location
- Massachusetts
tom baker said:
Tom I am not sure we can this time, the 40 amp breaker is larger than the max allowed by the units label.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
renosteinke said:
This is False information when used as a blanket statement.
For at least the past 30yrs, molded case circuit breakers very rarely exceed 1/2-1 cycle opening times when operating in their magnetic region. Fuses rarely clear in 1/4 cycle unless they are in or approaching their current limiting range.
6 cycle operating times are only associated with large >800A power circuit breakers. These breakers go by the names of iron frame, air frame, insulated case, and drawout.
- Location
- Windsor, CO NEC: 2023
- Occupation
- Hospital Master Electrician
I agree with Bob's read on this situation.iwire said:
Kfenn, one other circumstance I can think of that a fused disconnect would be required: Some residential furnaces state "Max OCPD 12.5 A" or something along those lines. A fuse would have to be used to meet this, as no circuit breaker would be available at that setting, IMO.
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