Max CB for SubPanel

Discussion in 'Electrical Forum discussion & Blog' started by Ted M, Jan 4, 2012.

  1. Ted M

    Ted M New Member

    Messages:
    12
    Location:
    Northern New Hampshire
    I would like to know the maximum CB size I can use in a SubPanel.

    My main panel supplies little information. Just the name, catalog number, etc., along with an area for listing what the CBs are for.
    But no schematic diagram.

    The main panel is a Square D Load Center; QOC40MW225, Series L7, Issue L-821.

    The main disconnect CB is 200 amps, a 2QMT, 10,000 amps SYM, 240 V.

    It is a 40 space panel, about 30 years old. All CBs are QO type.


    I'd like to use a 100 amp QO2100 CB, two spaces, for the SubPanel.

    Finding data online for such an old panel was difficult.

    In one place I can find that CBs from 10 to 125 amps can be used.

    In another place the higher limit is 60 amps.
    However, in that instance the data was for panels made after 1996.

    Any long-time electricians around that can help me here?

    Why would Square D make such CBs if they could not be used?

    Physically, the CB can fit. I wonder more about the stab current ratings.

    Thank you very much.

    Ted M
  2. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,529
    Location:
    North Carolina
    to feed the remote panel with a 100 amp feeder protected by a 100 amp breaker would be fine in your panel. If you are using copper conductors of a cable then use #3 or #1 aluminum
  3. DonL

    DonL Jack of all trades

    Messages:
    3,820
    Location:
    Houston, TX
    The amount of circuits already in the panel would have to be considered also.
    Last edited: Jan 4, 2012
  4. Ted M

    Ted M New Member

    Messages:
    12
    Location:
    Northern New Hampshire
    Thanks JW. I had planned on using #3 copper to the subpanel.


    BTW, JW, I found your recent article on multimeters electrifying.

    My multimeter is CAT III for 1000 volts, and CAT IV for 600 volts.

    It's a Craftsman Professional, True RMS, model 73753. 500,000 count, if I remember right.
    Sears most likely bought the meter from some OEM and put their name on it.
    It has 10A fused and 0.5A fused from the 10A / mA to Common connections, inside the meter.
    Fluke seems dead against fused leads.

    Does this mean that using fused leads is still a good idea?

    Ted M
  5. Chad Schloss

    Chad Schloss Member

    Messages:
    328
    Location:
    USA
    my fluke true RMS meter is fused. I blew one, that's how I knew it was fused.
  6. DonL

    DonL Jack of all trades

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    Location:
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    Most meters are fused for current, but not voltage.

    That is the DA fuse...
  7. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,529
    Location:
    North Carolina
    A fuse in the leads while checking voltages is useless. A fuse opens on current not voltage. Should enough current pass through to blow a half amp fuse the meter is going to explode in your hand before the fuse blows. A half amp fuse will allow ~ 3 amps to flow for two full seconds before blowing.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,834
    Location:
    New England
    Most modern DMMs have many megaohms of input resistance, so unless it is broken in the first place, it's almost impossible to get it to blow a fuse when measuring volts. Now, if you tried to measure ohms when the power was on in the circuit, it might blow. Unless using an inductive current probe, putting the meter in series with the load using the probes to measure current (not for the feint of heart!), a fuse is prudent and usually present.
  9. Ted M

    Ted M New Member

    Messages:
    12
    Location:
    Northern New Hampshire
    I know the fuse would only blow measuring current.

    I just wondered if the fuses put into leads were fasting acting than the ones inside the meters.

    Since almost all quality meters have internal fuses, why are the Canadian authorities so insistent in having fused leads then?

    Ted M
  10. DonL

    DonL Jack of all trades

    Messages:
    3,820
    Location:
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    That would depend if they are FB or SB fuses.

    Even $10 meters have a fuse.

    I use FB fuses in my meters, and they only blow when I am in DA mode and make a stupid mistake.
  11. BobL43

    BobL43 DIY Senior Member

    Messages:
    1,786
    Location:
    Long Island, NY
    took the words right out of my mouth. what's a ducks *** fuse doing in a meter?:p
    Last edited by a moderator: Jan 5, 2012
  12. BobL43

    BobL43 DIY Senior Member

    Messages:
    1,786
    Location:
    Long Island, NY
    a super flink? That's what the Germans call em. or just a regular fast blow?
  13. DonL

    DonL Jack of all trades

    Messages:
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    Location:
    Houston, TX
    There is also a NB type.

    No Blow and normally made of copper.
  14. The number of existing breakers wouldnt matter as long as he has to empty spaces in line.
    You could fill the main panel top to bottem with 30a 2p breakers as long as the actual load being placed on the system at one time doesnt exceed the main breaker rating
  15. BobL43

    BobL43 DIY Senior Member

    Messages:
    1,786
    Location:
    Long Island, NY
    you can buy those as cheap as 1 cent, unless you bid for it on E*B*A*Y*©, LOL (missed this post)
  16. DonL

    DonL Jack of all trades

    Messages:
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    Location:
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    True on the number of breakers, But %80 percent load would be a better design practice.
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