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Thread: AC Hi Temp

  1. #1
    Join Date
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    Default AC Hi Temp

    Indianapolis suffered a weeks worth of extremely high temps, 100+ thru the last weekend. I replaced about $30 worth of 30A fuses.

    I believe the failure is occurring while the system is running. Each time I replaced a fuse the system started up at the "cool" command from the thermostat. During the highest of the heat wave, 104 Saturday evening with each new fuse the system ran 45 minutes to an hour before failure.

    The system is a 12 year old 6 ton Crane residential split system. Electric is supplied thru a pair of 30A breakers to a 30A fused disconnect.

    House is a 1901 Arts & Crafts Victorian -- so insulation is minimal,system got little down time during the heat wave.

    Today the temps dropped back to normal, 80's, it has run all day, cycling, without a problem.

    Something I did notice, a difference in the fuses. One never failed, no matter which leg it was installed in, a GE FP35-89. All the failed fuses are Buss FRN-R-30. I looked for more of the GE fuses at the big box stores without success.

    I do know that on motor circuits larger amp fuses can be installed after the breakers. The 30A fuses are the largest available for the current disconnect. I have not yet located any label supporting a larger fuse.

    Any advice would be appreciated, thanks

    John

  2. #2
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    Default Re: AC Hi Temp

    I'm assuming these are "cartridge" fuses -- a round cylinder that snaps into the fuse block. You won't find anything larger than 30A that will fit; they don't make them, because that size of fuse block isn't rated for anything over 30 amps. They are all standard sizes.

    Look for a "time delay" fuse. It's designed specifically for motor circuits to accommodate starting current over the circuit rating. http://www.amazon.com/Bussmann-BP-FR.../dp/B00004WA2X

    If you still have problems blowing time-delay fuses, your A/C unit may need to be serviced. The compressor motor may be on the verge of failure.

    (Depending on local codes,) It's not really necessary to have a fused disconnect at the compressor unit. You may be able to replace the disconnect with an unfused disconnect.

    You still do need a disconnect in sight of and within a certain distance of the compressor unit. Service technicians need to be able to disconnect the power to the unit and "lock it out" to prevent inadvertent re-energizing of the circuit.
    Last edited by Fencepost; 07-09-2012 at 01:15 AM.
    The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.

  3. #3
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    Default Re: AC Hi Temp

    Ditto fencepost, the fuses aren't required and probably don't match the load curve.

    Either replace or jumper the fuse portion of the disconnect.

    The breakers are "inverse timelag" devices which are perfect for A/C applications.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  4. #4
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    Default Re: AC Hi Temp

    I don't know if this will help, but my HVAC unit has a disconnect box with a circuit breaker instead of fuses. I see this same box for sale at my local big box store. On mine though, I have a 60 amp breaker on the main panel and a 50 amp breaker on the disconnect for a 3 ton cooling system that is 3 years old.

    I do have to replace the breaker about every 8 or 10 years because it ages outdoors, had to replace it this summer in fact because it kept tripping on those hot days.

  5. #5
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    Default Re: AC Hi Temp

    Quote Originally Posted by The Semi-Retired Electric View Post
    Ditto fencepost, the fuses aren't required and probably don't match the load curve.

    Either replace or jumper the fuse portion of the disconnect.

    The breakers are "inverse timelag" devices which are perfect for A/C applications.
    I'm now a little confused by Fencepost and your answers.

    If the OP is blowing 30A fuses, don't you think that his 5 ton A/C has a breaker that is inadequate?

    My unit is a 6 ton Heil, which is on a double 40A breakers with 2 FRN-R-45A Bussmann fuses in the disconnect. It's functioning fine in temps over 100F.
    Last edited by dj1; 07-09-2012 at 04:12 PM.

  6. #6
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    Default Re: AC Hi Temp

    Quote Originally Posted by dj1 View Post
    If the OP is blowing 30A fuses, don't you think that his 5 ton A/C has a breaker that is inadequate?
    Two things: first, the breaker has an inherent time delay that allows for startup inrush current. Ordinary fuses do not. As a result, non-time-delay fuses can be blown even when a 30A breaker would be adequate. (The fuse part # you mention appears to be a time-delay fuse.)

    The second thing is the heat: if the sun is shining directly on the disconnect during 100 degree weather, the ambient temperature of the fuses may be in excess of 160 degrees F. This may be enough heat to cause the fuse to fail at a lower current than normal.

    The OP indicates that it's always the same fuse (of the pair) that blows; this could be attributed to a weak connection. The fuse block should be checked for corrosion, and the wire terminations in the disconnect should be checked for tightness and corrosion.
    The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.

  7. #7
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    Default Re: AC Hi Temp

    Good points fencepost. Also, Code requires the breaker (or fuses) be between the "min size" and the "max size", per the nameplate. Breakers AND fuses (outside) are just asking for trouble.

    Any connection or device outside, in the sun/rain is nothing but trouble.

    Most, if not all, of the load outside is 240V. So, if most of the current goes into the unit on one fuse/breaker it must return on the other fuse/breaker. Question: why does the same fuse keep blowing? Answer: as you said, corroded or loose connections on that one fuse.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  8. #8
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    Default Re: AC Hi Temp

    Fencepost & Semi,

    Could you be misreading what the OP wrote? He mentioned:

    "Something I did notice, a difference in the fuses. One never failed, no matter which leg it was installed in, a GE FP35-89. All the failed fuses are Buss FRN-R-30. I looked for more of the GE fuses at the big box stores without success".

    I understand it as: GE fuses don't fail. Only Bussmann fuses fail, no matter which slot they seat in. So maybe there's no corrosion issue here after all.

    What do you think?

  9. #9
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    Default Re: AC Hi Temp

    HI All:
    Thanks for all the interest. Sorry I been slow in getting back. As this is my first posting on this board I had problems with my password [I blame in on a fella by the name of Murphy, of which I am sure all members here are familiar].

    dji nailed the mystery; why does the GE fuse survive and the Bussman fuse fail? The GE fuse was installed in both legs and survived. The failure does not occur at start up, each time a new bussman fuse was installed the compressor ran 45 minutes to an hour before failure, so I don't believe the failure mode is start up surge related.

    As to the corrosion issue; checked that early on, all the connections in the disconnect and on the relay are clean. Only connection not checked is the breaker.

    As to changing out to a non-fused disconnect that will be determined by the local fire marshal. IRC and the install instructions make no requirements either way

    Regards;
    JohnE3

  10. #10
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    Default Re: AC Hi Temp

    Quote Originally Posted by JohnE3 View Post
    HI All:
    Thanks for all the interest. Sorry I been slow in getting back. As this is my first posting on this board I had problems with my password [I blame in on a fella by the name of Murphy, of which I am sure all members here are familiar].

    dji nailed the mystery; why does the GE fuse survive and the Bussman fuse fail? The GE fuse was installed in both legs and survived. The failure does not occur at start up, each time a new bussman fuse was installed the compressor ran 45 minutes to an hour before failure, so I don't believe the failure mode is start up surge related.

    As to the corrosion issue; checked that early on, all the connections in the disconnect and on the relay are clean. Only connection not checked is the breaker.

    As to changing out to a non-fused disconnect that will be determined by the local fire marshal. IRC and the install instructions make no requirements either way

    Regards;
    JohnE3
    The fuses are both dual element class Rk5 time delay fuses (just looked it up), if the connections are clean and tight manufacturing tolerances will determine which one will blow.

    NEC 440.4 B and IRC 3702.11 also mentions the nameplate data should dictate the fuse or breaker size. If your blowing 30A fuses and can't put a larger one in you have a fusible disconnect only rated for 30A where it should be a 60A. Having fuses and a breaker the same size is bad engineering.

    NEC 110 and IRC 3405.2 both allow a pull-out disconnect at the outside unit, as well a switch,breaker or fuses.

    But, your local AHJ (inspector) can overule both Codes, if it has passed a special rule to do so, which is rare.

    At this point, IMO you either have a condenser going bad, a loose connection, or the breaker & fuses are sized too low.
    Last edited by The Semi-Retired Electric; 07-11-2012 at 01:26 AM. Reason: typo
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

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