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fusible disconnects #84314
03/17/03 09:08 AM
03/17/03 09:08 AM
S
steve66  Offline OP
Member
Joined: Mar 2003
Posts: 25
My boss is under the impression that disconnects for air conditioners have to be fused. Does anyone know a code reference that requires this? If not, is there still a good reason to use fuses? Does commercial vs. residential make a difference?

2017 / 2014 NEC & Related Books and Study Guides
Re: fusible disconnects #84315
03/17/03 09:48 AM
03/17/03 09:48 AM
D
dana1028  Offline
Member
Joined: Jan 2002
Posts: 163
San Carlos, CA
Fuses are generally preferred over breakers but NOT required....as long as you use HACR breakers. See Art. 440 for AC requirements, Section B - Disconnecting Means.

Re: fusible disconnects #84316
03/17/03 10:12 AM
03/17/03 10:12 AM
I
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
You must follow what is on the Tag of the unit if the only type of protection listed on the tag is fuses you must use fuses.

Some tags will say "Fuse or HCAR Breaker" then you can use either

You must also use the size listed. If the tag says 25 Amp fuse that is what you must use even if your branch circuit is #10 with a 30 amp breaker.

2002 NEC 440.52(A)(3) and as always 110.3(B)

[This message has been edited by iwire (edited 03-17-2003).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: fusible disconnects #84317
03/17/03 10:15 AM
03/17/03 10:15 AM
E
ElectricAL  Offline
Member
Joined: Oct 2001
Posts: 597
Minneapolis, MN USA
Historically, there was a problem created by the wording printed on the nameplates attached to the installed equipment. The label listed maximum fuse size, which was taken to mean that a circuit breaker wasn't enough to protect the equipment, when the AHJ considered 110.3(B). Manufacturers have changed the wording on the nameplate to not describe one form of OCP (over current protection) over another.

If the branch circuit OCP at the panel is approved to protect the AC, the disconnect at the AC unit could be just a pullout (non-OCP) or unfused switch.


Al Hildenbrand
Re: fusible disconnects #84318
03/17/03 10:34 AM
03/17/03 10:34 AM
I
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Read 2002 NEC 440.52(A)(3) this requires the makers to state fuses or inverse time breaker, if the overcurrent is part of the motor current protection

Now look at table 430.52 and you will see that fuses and inverse breakers react differently.

Units still come with a specification as to minimum circuit ampacity as per 440.4(B) and overcurrent device requirement as per 440.52(A)(3)

But I agree if the unit does not state a specific type of overcurrent device it is up to you.


[This message has been edited by iwire (edited 03-17-2003).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: fusible disconnects #84319
03/17/03 11:09 AM
03/17/03 11:09 AM
E
ElectricAL  Offline
Member
Joined: Oct 2001
Posts: 597
Minneapolis, MN USA
Thanks Bob,

You are saying this clearer than I.

The AC nameplate must be in agreement with the choice of the hardware the electrician installs to supply and protect it.

It is interesting to note that even one type of fuse compared to another behaves differently, and the same is true when comparing circuit breakers. 440.52(A)(3) states: A fuse or inverse time circuit breaker responsive to motor current . . . shall have sufficient time delay to permit the motor-compressor to start and accelerate its load

For Steve66's question with respect to residential vs. commercial, the same is true in both cases.


Al Hildenbrand
Re: fusible disconnects #84320
03/17/03 06:34 PM
03/17/03 06:34 PM
S
steve66  Offline OP
Member
Joined: Mar 2003
Posts: 25
It sounds like I can't go wrong if I just follow the manufacturer's instructions and recomendations.

Thanks everyone.

Re: fusible disconnects #84321
03/18/03 07:55 AM
03/18/03 07:55 AM
B
Big A  Offline
Member
Joined: Jan 2003
Posts: 46
Lynchburg, VA
Hello All,

Am I missing something here? I have installed non-overload protected disconnects for AC's and heat pumps lot's of times.

I am talking about the disconnect that is required to be at the condensor outside the home. Usually I just install on of those pull out type disconnects that you flip over. The overload protection comes from the breaker in the panel.

Re: fusible disconnects #84322
03/18/03 11:42 AM
03/18/03 11:42 AM
E
ElectricAL  Offline
Member
Joined: Oct 2001
Posts: 597
Minneapolis, MN USA
You're probably not missing anything [Linked Image] Big A.

What Iwire and I are talking about is that the nameplate on the side of the metal enclosure of the condensor will call out whether only fuses, only HACR type circuit breakers or either can be used to provide the overcurrent protection for the machine. We always read the nameplate to get the numbers for the min. circuit ampacity and the maximum overcurrent protection (OCP). . .the wording on that label may vary from model to model when it describes the OCP types the manufacturer allows.

Much of the time, as you are describing, the condensing unit will allow protection by HACR type circuit breaker, and that breaker can be in the service center with a simple unfused pull out disconnect outside at the AC condensor. However, the manufacturers wording on the AC condensor must agree with what you do. Read the nameplate closely to comply with 110.3(B).

The 2002 NEC Handbook has a great example (Exhibit 440.1) of the possible hookups after 440.21.


Al Hildenbrand
Re: fusible disconnects #84323
03/18/03 07:18 PM
03/18/03 07:18 PM
J
John Steinke  Offline
Member
Joined: Apr 2001
Posts: 518
Reno,Nv., USA
You almost have to be psychic sometimes... If the plate on the AC says "maximum fuse..." without any mention of a circuit breaker, or other type of overcurrent protection, you are required (see UL white book) to use a fuse somewhere in the circuit.
Circuit breakers are not the same as fuses; a fuse, even a "time delay" fuse, will typically respond (blow) a lot quicker than a breaker.

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