AWG Table - Current. AC or DC?

Discussion in 'General Electronics Chat' started by Iodem_Asakura, Apr 23, 2005.

  1. Iodem_Asakura

    Thread Starter Senior Member

    Sep 14, 2004
    140
    0
    Hi everybody.

    Someone knows if the current specified in the various AWG Wire Tables we find in the web, are AC Current or DC Current?

    I thougth it was the same 'cause is the RMS value of a Current Waveform and this is the same as a DC current value. But i have seen in the Relays Specifications, that they support more AC current than DC. I don't know if the current they say for an AC Voltaje is the pike value or the rms value.

    For example, i've seen some relays to say:


    this is the switch voltage--> Coil: 12 Vdc

    these values are for-------> 7A 120Vac
    the contacts----------------> 3A 30Vdc

    So, i want to know if the current specified in te AWG Wire Tables are rms, pike or dc values.
     
  2. David Bridgen

    Senior Member

    Feb 10, 2005
    278
    0
    Either r.m.s. or d.c., it makes no difference.
     
  3. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    David's correct, just assume RMS values. If you have a Vac value, just divide by sqrt(2) to get the RMS (only if its sinusoidal).
     
  4. Iodem_Asakura

    Thread Starter Senior Member

    Sep 14, 2004
    140
    0
    So, why a relay says the contacts support:

    7 Amp at 120 Vac
    3 Amp at 30 Vdc

    ???
     
  5. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    As Dave stated, the AC voltage/current rating is always RMS (unless specified otherwise). They are just telling you that you can use the contacts to switch AC or DC. But usually you can switch the higher voltage at a lower current & lower voltage at the higher current.
     
  6. Iodem_Asakura

    Thread Starter Senior Member

    Sep 14, 2004
    140
    0
    Yes, i know, but why can i switch 120 Vac (high voltage) at 7 Amp (high current) and only 3 Amp (low current) at 30 Vdc (low voltage)?
     
  7. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    More wear occurs on a contact's surface when you break the contact, than when you make it. The voltage tends to want to keep the contact, and arcs as the contacts open, until the contacts are open too far to maintain the arc. I would guess that DC has a tendancy to hold a better / longer lasting arc than AC. This is JUST a guess, as I've never studied this, or given it much thought. I've noticed that the volts / amps ratings on all of the contactors I've ever worked with are always rated higher for AC than DC, so there must be a valid, technical reason.

    I agree with everyone else's posts: When sizing your wires, size for the rated load amps. AC or DC amps will not make a difference on a given wire size. Follow the ratings printed on you contactors, as well, and you shouldn't have any problems.
     
  8. David Bridgen

    Senior Member

    Feb 10, 2005
    278
    0
    When the contacts open, and current is flowing, there will be an arc across them.
    If the contacts are interrupting d.c. the arc will last a relatiely long time.
    If they are interrupting a.c. the arc "self quenches" - because it goes out each time the current passes through zero - and is of a mch shorter duration.

    So ..... the contacts have to be rated for d.c. at a lower current than for a.c.
     
  9. Iodem_Asakura

    Thread Starter Senior Member

    Sep 14, 2004
    140
    0
    Those are good replies. I guess you are right. Now i understand why the current is rated bigger for AC than DC in the relays.

    Thank you to everyone.
     
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