Question about fuses on a

Coleman Evcon electric furnace

Question sent as the result of the feedback form.
It was submitted on Tuesday, December 26, 2000 at 20:18:07
By Robert Stewart of Austin, tx.
Brand: Coleman Evcon
fuel: electricity
location: closet
problem: intermittent
    The problem that I keep having, is the buss fuses keep blowing out. We had it repaired, or we thought so. They replaced a transformer and said it was fixed. Now the heater just went out again this time the fuses caught on fire, and burned a couple of wires. I do know that it can be rewired, but my question to you is why does the fuses keep blowing out, not only do they blow but they are also really hot to the touch.

Sperry Digisnap clamp on ammeter avaliable from Lowes or The Home Depot for about $80.
    From: Scott Meenen n3sjh: It sounds like you have the wrong size fuses or a problem with dirty or crummy connections on the fuses and holders causing both to over heat and fortunatly the fuses will blow to protect the equipment from fire (you hope). Loose connections cause as much if not more problems than short circuits.
    I would suggest buying or borrowing a clamp on ammeter (shown above) and checking the current draw on the heaters. Unless the heaters are shorting to the case of the cabinet they should draw the same or less of their rated current. The rule of thumb for fuses, circuit breakers and wiring is to load within 80% of the rating of the wiring and the fuses should blow when over loaded to 125% (the reciprocal of 80) of the circuit capacity. A 5kw heat strip will draw 21 amps at 240 volts and 18 amps at 208 volts. Following the rule you need a 30 amp circuit for the 21 amp load.
    Example a 12 gauge wire normally can handle 20 amps that means you should have no more than 18 amp flowing through it and when you get to 25 amps the 20 amp fuses or circuit breakers should blow. This does not mean that you should run 20 amps even though the wire can handle it and the fuses or breakers may not blow. This margin is to allow surges for motors and other starting currents.
 Scott.
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  Definitions:
  1. Blower: usually a squirrel cage centrifugal air moving device. Will move large volumes of air relatively quiet. Will use less energy with more back pressure.
  2. Fan: a paddle type air moving device used where noise is not a major consideration. Will use more energy with more back pressure.
  3. Combustion blower: A blower used on high efficiency furnaces or oil burners to move combustion air. usually 1/20 to 1/6  horsepower.
  4. OEM: Original equipment manufacturer.
  5. Horsepower: 746 watts
  6. KW Kilowatt (1000 watts) or 3400 BTUs per hour
  7. RPM: revolutions per minute.
  8. Service factor: the extent to which a motor can be safely overloaded beyond its name plate ratting without over heating.
  9. Air over horsepower: The rating of a motor assuming air flow through the windings usually as a result of the air moving device.
  10. SAE: Society of Automotive Engineers.
  11. Ton 12,000 BTUs per hour of heating or cooling.
  12. Short circuit: an unwanted low resistance electrical path.
  13. Short: (slang) any electrical malfunction. Please avoid using this term in this manner.
    This page will be updated soon
 Good Luck Scott
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Written By:  Scott Meenen N3SJH of:
G&S MECHANICAL SERVICES.
Specializing in Mechanical, Controls and Electrical Modifications Of
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