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Thread: Max CB for SubPanel

  1. #1
    DIY Junior Member Ted M's Avatar
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    Default Max CB for SubPanel

    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. #2
    Electrical Contractor/Instructor jwelectric's Avatar
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    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

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    Jack of all trades DonL's Avatar
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    The amount of circuits already in the panel would have to be considered also.
    Last edited by DonL; 01-04-2012 at 02:40 PM.
    Theory only works perfect in a vacuum.

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    DIY Junior Member Ted M's Avatar
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    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. #5
    DIY Senior Member Chad Schloss's Avatar
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    Quote Originally Posted by Ted M View Post
    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
    my fluke true RMS meter is fused. I blew one, that's how I knew it was fused.

  6. #6
    Jack of all trades DonL's Avatar
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    Most meters are fused for current, but not voltage.

    That is the DA fuse...
    Theory only works perfect in a vacuum.

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  7. #7
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by Ted M View Post
    Does this mean that using fused leads is still a good idea?

    Ted M
    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. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    DIY Junior Member Ted M's Avatar
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    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. #10
    Jack of all trades DonL's Avatar
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    Quote Originally Posted by Ted M View Post
    I just wondered if the fuses put into leads were fasting acting than the ones inside the meters.
    Ted M
    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.
    Theory only works perfect in a vacuum.

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  11. #11
    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by DonL View Post
    Most meters are fused for current, but not voltage.

    That is the DA fuse...
    took the words right out of my mouth. what's a ducks *** fuse doing in a meter?
    Last edited by jwelectric; 01-05-2012 at 01:38 PM. Reason: remove ugly word
    I am definitely not a pro plumber, but I am a pro crastinator

  12. #12
    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by DonL View Post
    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.
    a super flink? That's what the Germans call em. or just a regular fast blow?
    I am definitely not a pro plumber, but I am a pro crastinator

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by BobL43 View Post
    a super flink? That's what the Germans call em. or just a regular fast blow?
    There is also a NB type.

    No Blow and normally made of copper.
    Theory only works perfect in a vacuum.

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    DIY Junior Member plummen's Avatar
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    Quote Originally Posted by DonL View Post
    The amount of circuits already in the panel would have to be considered also.
    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. #15
    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by DonL View Post
    There is also a NB type.

    No Blow and normally made of copper.
    you can buy those as cheap as 1 cent, unless you bid for it on E*B*A*Y*, LOL (missed this post)
    I am definitely not a pro plumber, but I am a pro crastinator

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