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I am in the throes of learning enough to do a DIY install of a small heat pump package unit.

The installation manual calls for (and suggests that NEC requires) a "fused disconnect" inline with the power from the panel to the main unit.

I have two points of confusion on this. If my HVAC unit is right next to the main breaker box (like 3 feet away), what's the point of this (to my mind) "extra" disconnect box?

  1. why does it need a "fuse" (the breaker will be dedicated to the hvac unit)
  2. why does the HVAC needs its own dedicated disconnect rather than just shutting off the aforementioned breaker in the breaker box (right next to the HVAC unit)?

While we're at it, this is 2015... does ANYTHING in residential wiring still use an actual "fuse"? Why would it be a fuse rather than a breaker?

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    So is your breaker panel outdoors? Usually a heat pump implies there is some sort of condenser unit that would be outside and would require a local disconnect. – mjohns Jul 7 '15 at 19:12
  • Yes, the house has two breaker panels, one outside next to the meter, and a second one inside the house. The HVAC package unit is all outside, and conveniently, the best spot for it is right next to the outdoor breaker box. – Jeff Pritchard Jul 8 '15 at 11:22
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Disconnects are required when the equipment is too far from, or not within sight of the breaker (or branch circuit disconnect) (commonly known as "Serviceman disconnects" or "Serviceman switches"). This is required so that while you're working on the equipment, it's less likely somebody will accidentally energize the equipment (flip the breaker on).

National Electrical Code 2014

Chapter 4 Equipment for General Use

Article 440 Air-conditioning and Refrigerating Equipment

440.14 Location. Disconnecting means shall be located within sight from and readily accessible from the air-conditioning or refrigerating equipment. The disconnecting means shall be permitted to be installed on or within the air-conditioning or refrigerating equipment.

The disconnecting means shall not be located on panels that are designed to allow access to the air-conditioning or refrigeration equipment or to obscure the equipment nameplate(s).

Fuses are used for overload protection, mostly for motors. The fuse is typically sized to the motor load it's protecting, and may or may not be a different size than the circuit breaker. If a motor can't spin, it will quickly overload, overheat, and burn up. A fuse is used to prevent that.

For more information on protecting the equipment, and sizing the protection. See the rest of article 440. As Mjohns answer points out, the manufacturer's documentation rules, so make sure you read and understand it.

  • If a unit calls for a "fused disconnect", it doesn't have to be actual fuses, it just has to have either fuses or CBs, correct? Although, I think you're going to want (slow-blow) fuses on motors. – Mazura Jul 8 '15 at 1:12
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    @Mazura with A/C and refrigerating equipment, fuses are often used in addition to circuit breakers. When the installation instructions say "fused disconnect", they're likely referring to a device with a pull out handle with fuses in it. Something like this. – Tester101 Jul 8 '15 at 18:57
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This is the simplest answer based on the NEC. The NEC also outlines when and where a disconnect is needed, but I believe this is the overriding rule in your case.

110.3 Examination, Identification, Installation, and Use of Equipment.

(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

As far as fuse vs breaker, the unit was probably UL tested with a fuse, so therefore it got UL listed for use with a fuse. You can check the unit's data plate to be sure-- you might luck out and see something like "Max fuse or HACR breaker" instead of just "Max fuse".

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