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dj1
Re: AC Hi Temp

JohnE3:

Replace your 30A double breaker at the main panel with a 40A double breaker (must be a matching brand). Then install 40A fuses in the disconnect (try Bussmann BP-FRN-R-40, available at the big stores) - put the main breaker back on and turn on the A/C to see if problem solved.

Can't or won't do it? get an A/C tech or an electrician to help you.

keith3267
Re: AC Hi Temp

JohnE3. The Semi-Retired Electric made a comment that got me to make some calculations based on the information you gave and a couple of assumptions. What I came up with is that your AC unit probably draws at least 30 amps when in use so your fuses and breakers are undersized. But you cannot just put in larger breakers and fuses.

The first thing you need to do is to have the wiring checked to make sure it can handle a higher current. My 3 ton unit, which draws about 15 to 18 amps has a 60 amp breaker at the main panel and a 50 amp at the disconnect outdoors, but I have 4 gauge wiring so I think that is OK.

Have an electrician check your wire size and if it is adequate, then go to larger capacity breakers and fuses. The fuses in the disconnect panel should be a lower capacity than the breaker in the main panel.

If your wiring is not sized for larger breakers and fuses, then it should be replaced as it is undersized for your unit. Your current fuses might be the only thing between you and a disastrous fire.

Re: AC Hi Temp
keith3267 wrote:

JohnE3. The Semi-Retired Electric made a comment that got me to make some calculations based on the information you gave and a couple of assumptions. What I came up with is that your AC unit probably draws at least 30 amps when in use so your fuses and breakers are undersized. But you cannot just put in larger breakers and fuses.

The first thing you need to do is to have the wiring checked to make sure it can handle a higher current. My 3 ton unit, which draws about 15 to 18 amps has a 60 amp breaker at the main panel and a 50 amp at the disconnect outdoors, but I have 4 gauge wiring so I think that is OK.

Have an electrician check your wire size and if it is adequate, then go to larger capacity breakers and fuses. The fuses in the disconnect panel should be a lower capacity than the breaker in the main panel.

If your wiring is not sized for larger breakers and fuses, then it should be replaced as it is undersized for your unit. Your current fuses might be the only thing between you and a disastrous fire.

Keith as I said in post #7 & 10 the nameplate will determine if the wire can be protected by a breaker (or fuses) larger than the values mandated in Art 310.15 (B)(16) etc.

Per 2011 NEC:
440.31 General.
The provisions of Part IV and Article 310 specify ampacities of conductors required to carry the motor current without overheating under the conditions specified, except as modified in 440.6(A), Exception No. 1

440.6(A) and 440.6(B).
(A) Hermetic Refrigerant Motor-Compressor. For a hermetic refrigerant motor-compressor, the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection. Where no rated-load current is shown on the equipment nameplate, the rated-load current shown on the compressor nameplate shall be used.

Exception No. 1: Where so marked, the branch-circuit selection current shall be used instead of the rated-load current to determine the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, and the branch-circuit short-circuit and ground-fault protection.

The Code recognizes the special needs of hard starting, listed, A/C equipment and allows the manufacturer to actually modify the allowable current rating of the wire and devices feeding their equipment. The nameplates are tough to understand and as you said it is best left to an electrician (or inspector).

John
Re: AC Hi Temp

Conclusion - the local fire marshal confirmed that fuses are not required. I swapped out the disconnect, unit has run just fine thru several more days of 100F weather.

The GE fuse finally died, slowly roasted.

Thanks for all the responses.

John

keith3267
Re: AC Hi Temp

Thanks for the feedback.

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