Regular Guest
Is this a correct set-up?
1.Older electric panel in residential home with 50A A/C breaker.
2. Outside disconnect with pull switch (vs a visable set of 2 fuses)
(30A disconnect)
3. Name plate reads 30A max fuse/breaker
4. three ton unit (36)
My question? Do these disconnects with a manual pull out bar in them act as a breaker to protect unit or are they just what they are called...A disconnect?
To shut off unit?
Should the 50A breaker go ... better yet does it HAVE to go and be replaced with a 30 Amp breaker?
Thanks
Last edited by gary cox; 01-03-2009 at 11:37 AM. Reason: clarification
Regular Guest
I am not an electrician, but a) a disconnect is a disconnect, not a circuit protector, and b) a fuse or circuit breaker is supposed to be the *weakest* link in a ciruit, not the strongest.Originally Posted by gary cox
You should have a qualified electrician take a look at this before you suffer an electrical fire.
Global Moderator
From a protect the equipment standpoint, the 30 amp fuses in the disconnect should do it. You'd need to know the size of the wire between main panel and disconnect to know if the 50 amp breaker is protecting it properly.
Regular Guest
The OP seemed to be saying the disconnect was of the non-fused type. If so, then the 30A disconnect is electrically weaker than the 50A breaker, a definite no-no.
God Speed, Jim. We'll miss you - 2-26-23
Either need to have a fused disconnect installed, or the correct breaker.
Professional Member*
It sounded to me like there is a 50A circuit feeding a 30A fused disconnect that is fused at 30A feeding a 30A max unit.
If that is the case the disconnect would need to be rated for 50A, so a 60A disconnect would need to be used but fused at 30A, you would need reducers for that fuse size.
The easiest thing would be changing the breaker to 30A.
Professional Member
You need to have a licensed electrician look at your situation. This includes looking at the a/c's data plate to verify that the max brkr is in fact what you have and that everything from the panel to the outdoor unit is sized properly and meets the electrical code. Also, is this a new install? If so, a permit should have been pulled and subsequently inspected by the local inspections department.
Regular Guest
I agree with ncboston, you cant fix things over the phone (so to speak) you have to be there to see the whole picture.
Member
I was looking for something else and saw this...maybe I can help.
The equipment should list MCA (Minimum Circuit Amps) and MOP (Maximum Overcurrent Protection) - both must be met.
The equipment determines the minimum wire size, and the maximum breaker size. Often, the equipment will list a huge breaker on a small wire, and if it's listed that way - it's OK. They've done the calculations, tested the system, and had it approved (they've used all the exceptions in the electrical code.)
Often, in the past, electricians would run a big wire and a big breaker to the outside, expecting someone to provide the correct (smaller if neccesary) fuse for the outside unit (chosen later, another contractor, etc.). With today's more efficient units, we're left with those large wires and big breakers. One can lower the breaker rating to match the new unit ($), or add a fuse to match the unit ($) - the wires do not determine the breaker size - the equipment does. Big wires - properly protected - are better (they cool better, less voltage drop).
If I had to spend money on something - to met the MOP on an existing job - I would add the fusible disconnect with the correct fuses to the existing large wires with the large breaker. That existing wire size will then always be protected by an adequate breaker (in case larger equipment is installed later the breaker will not get lost) - and the equipment is protected by the proper MOP (the new fuses) - and short circuit/ground fault protection will be enhanced, as will outside access to correct the (probable)cause of the short circuit.
Breakers and fuses do 3 jobs - ground fault, short circuit, and over current protection. You generally want some fast for the first 2 (to protect people and equipment) and something slow for the last (to allow equipment to get to speed). These conflict, particularly in HVAC equipment - thus the nameplate!
Professional Member*
Be careful there! You're half right and half wrong. The purpose of a breaker is ALWAYS to protect wiring... but you have ot consier the ENTIRE circuit. The wires it's protecting might be located inside a motor, or somewhere inside the equipment, but fundamentally it's always protecting wires or current carrying circuits somewhere from exceeding the maximum current it can handle. Most equipment also has other fuses or breakers internally to protect other smaller wires... like the fuse on the low voltage control power in series with it's transformer.
But I think what you're getting at is that a small breaker can be used for agiven wire size or wiring can be oversized, but the wiring inside the equipment needs to be considered, not just the size of the wire feeding the equipment. You don't need to know that wire size... the manufacturer will tell you what that is on the nameplate or installation manual.... so yes, the equipment does determine the breaker size...but it's because of it's wire size internally.
Professional Member
The disconnect you describe is ONLY a disconnect, it doesn't provide any overcurrent protection.
You should definitely replace the 50A breaker with a 30A breaker.
With regards to prior posts; circuit breakers are for both equipment protection and wiring protection. In some applications (and this is one) the breaker size and wiring ampacity may not match.
This is true only when the circuit feeds a single permanently wired device, such as an AC unit. The breaker is sized to protect the equipment, and to accommodate the large current drawn by the compressor motor as it's starting. The wiring can be sized according to the current draw while running.
Contrary to a previous post, there is no ground fault protection whatsoever, nor any protection against shock by humans, in a typical air conditioning circuit. No overcurrent device other than a GFI breaker can provide protection against electrocution, the current required to kill is *far* less than is required to trip even the smallest breaker.
I would always recommend a properly sized breaker in the panel, (sized to the equipment nameplate maximum overcurrent protection rating) rather than relying on a fused disconnect (which the OP doesn't have BTW). Resetting an indoor breaker is a lot less hassle than trying to track down and replace an oddball outside fuse.
This is a problem that happens frequently when systems are replaced. Modern, higher-efficiency systems typically run on less current than the systems they replaced, but installers rarely bother to re-size the circuit breaker.
I had a system changed out a while back. The new system specs a max of 30A breaker, the original breaker was 60 amps, and the installer didn't change it.
-jim-
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