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Switching capacity of a relay

  1. Aug 22, 2016 #1
    hi am planning to buy a electromechanical relay with following specification:
    256VAC 60HZ,

    this relay is used for controlling a heater. heater's specs are

    I have found one matching for this spec from TE.. its specs are
    switching capacity: 6000VA

    what does this switching capacity means?? does this specify power rating of relay???
  2. jcsd
  3. Aug 22, 2016 #2


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    VA = Watts, so it's saying that the absolute maximum switching capability is 6 kW

    you should really look at SSR's ( solid state relays) or other contactor relays maybe
  4. Aug 22, 2016 #3
    but in the outer cover of TE relay...it is mentioned like this
    16A 260VAC 50/60HZ
    3/4 HP 120VAC 50/60HZ
    1HP 208-600 VAC 50/60HZ

    I am confused with this 1 HP rating. whats the difference between this 1 HP and switching capacity of 6000VA...Please clarify me this rating.
  5. Aug 22, 2016 #4


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    not sure

    @jim hardy can you help please ? :smile:

  6. Aug 23, 2016 #5

    jim hardy

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    Contacts have a rating for how much continuous current they can carry,
    and another for how much current they can interrupt,
    and that ability to interrupt is different for inductive and non inductive loads
    and it differs greatly for AC and DC supply.
    So you might edit: should find several ratings for the same contact.

    Since a typical small motor can draw 9 or 10 times running current while it's starting, it would be a mistake to control it with a switch rated for just the full load amps listed on the motor's nameplate. To help us along they often tell us what size motor the switch should be able to comfortably handle .

    They also often give current ratings for both a non-inductive (resistive) load and inductive around 0.75 power factor. Inductive will usually be slightly lower.
    Your heater will be a resistive load.
    Incandescent lamps are resistive but have high inrush, like motors do, so you may find a tungsten lamp load rating specified.

    Here's an example from a GE relay catalog page
    note that 20 amps at 120 volts is 2400 Watts = 3.2 hp, about 6X the ½hp motor rating. That's because the motor is inductive and it has inrush.

    Here's part of an information brief from Carling Switch folks that's intended to familiarize users...
    I've not yet located the IEC and NEMA standards.

    But that's why you see so many different numbers for the same contact..

    Here's another brief from a switch manufacturer

    Last edited: Aug 23, 2016
  7. Aug 23, 2016 #6


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    Thanks Jim :smile:
  8. Aug 23, 2016 #7


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  9. Aug 30, 2016 #8
    thanks jim and davenn ....for your valuable support
  10. Aug 31, 2016 #9
    I have doubt here....
    my relay spec sheet for the above one says something like this....
    NEMA pilot duty rating

    i did some little research in internet and found that make current means pilot duty inrush current and break current is steady state current..
    A represents 1 HP and B represents 1/2 HP...
    600 means voltage rating...
    hence A600 means 600 V 1 HP...i got this from below table...
    but my doubt is
    1) i couldnt understand the meaning of pilot duty....i mean is it the one which denotes ON/OFF relays...
    2)what does that NEMA table implies?....i mean does it says that particular relay has passed this condition mentioned in the table?
    3)say am connecting a motor with 1 HP,460V
    LRA is 10.3 A
    FLA is 1.3 A
    so according to table my relay has A600 rating....
    here should i need to check that the LRA and FLA are within make and break currents respectively?
    4)what does the maximum Volt amps indicate....i mean i know my load is 1 HP...but according to A600 should my relay withstand 7200VA???

    @jim hardy and @davenn can u please add some comments to these questions?
  11. Sep 1, 2016 #10

    jim hardy

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    I am not versed in those industry standards.

    When starting, i go to manufacturers datasheets to learn the terminology. So, here we go.

    Here's one company's explanation of the term "Pilot Duty"
    Note they describe interposing relays, relays that drive relays not motors.

    I don't know anything about that table or where it's from.
    If i don't answer that question i might be thought ignorant , but if i do answer it'll probably remove all doubt.
    So here's best i can do for you:
    This continuation of Macromatic's tutorial
    and i think that is industry practice. Does your source say they tested to those conditions ? If so, probably that particular type of relay contact has passed such a test.

    Hmmm. Where did that table come from ?
    According to Macromatic's tutorial, " A " means 10 amp thermal current (continuous) and B means 5 amp. No mention of horsepower.
    But note these are for Pilot Duty relays, one ought really use a contactor for a motor. That's because it might be asked to break LRA .

    Yes, going strictly by the rules.

    I would prefer for a design with my name on it, that LRA be within thermal current because the motor could stall .
    FLA should certainly be within break current.
    But as i said above, i think i wouldn't use a pilot relay for powering a motor of significant size. Should the motor stall the relay might be asked to interrupt LRA.

    Answer: Briefly it should withstand it. 7200 is its maximum 'make' rating, observe that amps is always 7200 / voltage. That's because on "make" a contact bounces and arcs and might weld. At higher voltage they reduce the allowed current to discourage contact welding...
    Aha! I finally see it ... your table tells us an A600 contact should be equivalent to a switch rated under other standards for 1hp motor or 720 VA coil . But 7200 VA exceeds the thermal rating of the contact. It can take that for only a short interval. See Note 3 at Macromotion tutorial
    Pilot Duty relay should control the motor starter(contactor)'s coil.
    I'd have no fear of using one for a smaller motor, though, one whose LRA is within relay BREAK current.

    See recent thread Snubber Circuit. Control relays driving a motor were burning up.

    Well i learned something about those relay ratings today . Thank you !

    here's a snip from an Allen-Bradley catalog . I hope to find and read the NEMA standard.

    Corrections are welcome.

    old jim

    Attached Files:

    Last edited: Sep 1, 2016
  12. Sep 1, 2016 #11
    wow...that was a superb and lucid explanation....thank you @jim hardy .
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