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Thread: Cable size for 100 amp subpanel

  1. #1

    Default Cable size for 100 amp subpanel

    Greetings;
    I have a question in regards to installing a 100 amp / 220 volt subpanel (with a 100 breaker in the subpanel). What size aluminum and what size copper wire do I need to run? Is it safe to use 3 conductors using 220 off the subpanel, as well as, 110?

  2. #2

    Default

    A 100 amp main breaker is not required in a nonservice rated [sub panel]. The main breaker controlling that sub panel is found inside your main service rated panel that protects the feeders serving that sub panel getting its power from a 100 amp breaker in that main service rated panel.

    YOu must install 2 hots, one insulated neutral, and one equipment grounding conductor in that feeder serving that sub panel. You must also separate the neutral bar from the grounding bar and metal case of that sub panel inside that sub panel electrically.

    4 awg copper = 100 amp in a dwelling as a feeder serving a panel.

    2 awg aluminum = 100 amp in a dwelling as a feeder serving a panel.

    Good Luck

    Wg

  3. #3

    Default

    Thank you very much, Kevin Welch.

  4. #4
    Join Date
    Sep 2002
    Posts
    1,999

    Default New 100 amp service panel

    I've got 3 questions that are related to the thread below:

    Currently I have a 125 amp service panel that serves 2 apartments.

    I'd like to install a separate meter and a 100 amp service panel upstairs. That way, there's a separate meter for each of the apartments.

    The plan would be to run 4 AWG copper wire from where the new meter base will be to the new 100 amp service panel (this would be the new feeder). I would also run 12 and 14 AWG wire from the new 100 amp service panel to the current 125 amp service panel and join it to the wires for the branch circuits in the separate apartment. The remaining branch circuits would stay connected to the 125 amp service panel.

    The real question is the sequence of events that needs to take place. I need to have uninterrupted electrical service.
    I could:

    a) run all the wiring, get the meter installed, then, once the new feeder wire is connected, splice in the branch circuits.

    or,

    b) would it make sense to run all the wiring, connect the new feeder wire to where the current feeder wire runs into the 125 amp panel, splice in the branch circuits, then, when the meter is installed, I would connect the new feeder wire to the meter. The benefit of this is that I could activate and test the circuits before the meter is installed. I just don't know whether there is an issue with connecting the new feeder wires to the same connectors where the current feeder wires are attached.

    Question 1: Is b) possible to do? Is it safe? Is a) logical?

    Question 2: How would the new 100 amp service panel be grounded? The equipment grounding conductor would be connected to the panel box on one end, but where would the other end (the one at the new meter) be grounded?

    Question 3: Finally, the new 100 amp service panel will be recessed into the wall of a closet. The wall I'm planning to install it on is not a perimeter wall. Do electrical panels have to be installed on perimeter walls, or can it be an interior, non-perimeter wall?

  5. #5

    Default

    The NEC requires that each dwelling be served with a minimum of 100 amps. Also I am taking that this is an older building that does not have a separation wall of 1 hour rated or more fire wall. This means that you must install main service disconnects outside by your meter base. Install a ground rod and a PVC conduit up to the meter base connecting the ground rod by grounding electrode conductor to the meter base. Then a nipple out both sides of that double gang meter base to each of the two main serivice rated weatherproof disconnets. Then install either a 4 wire 2 awg SER aluminum nonmetallic sheathed cable or a 4 wire 4 awg copper nonmetallic sheathed cable or conduit containing 4 - 4 awg copper or 4 - 2 awg aluminum conductor inside each conduit to each nonservice rated panel [aka sub panel] inside each apartment.

    Remember main service rated panels or disconnects must be placed either outside or nearest point of entrance inside. These mains must be grouped or a fire wall must separate to two or more dwellings as two separate buildings totally separated by a minimum of a one hour fire wall totally separating each apartment by that fire wall.

    Remember the neutral bar of that sub panel inside each apartment must be isolated from teh metal frame of the sub panel and also isolated from the equipment grounding bar. After the main service rated panels or disconnects in your condition located outside everywhere on the load side of those main service rated disconnects must have the neutral and grounidng separated.

    Remember no breakers are allowed installed in a panel of disconnect inside a closet or bathroom. YOu also must maintain 30" dedicated space on the wall for that or those panels or disconnects and you must maintain 30" dedicated clear approach from ceiling to floor in front of that or those panels in existing buildings.

    You are not allowed to pass through one panel to get to a second panel.

    Remember when you have separate service for each apartment you are not allowed to serve other areas other than that certain apartment dwelling unit with those branch circuits. You need to separated everything in each apartment fed by its own service.

    You need to take the existing branch circuits serving that second dwelling out of that existing panel and make your junctions in a junction box not inside that existing panel to reroute you branch circuits for that second dwelling apartment's sub panel.

    Remember smoke detectors are required in each apartment bedrooms and vicinity of bedrooms and on each floor of each apartment.

    I would build the entire new meter and panel system and install branch circuit wiring to the point planned to make the junctions. Including installing a conduit and feeder to the location of the existing panel from the meter READY TO CONNECT. Then cut off the power and make the final connections. Then get your inspection approvals and re-energize your new service.

    The two sub panels must be supplied equipment grounding by use of an equipment grounding conductor ran with the feeders serving those sub panels from the main service rated disconnects located outside. The grounding source of the sub panels located inside each apartment must come through the main service rated disconnects supplying those sub panel power sources.

    Non service rated panels [aka sub panels] may be installed anywhere inside each apartment except it is forbidden to install panels or disconnects inside closets and bathrooms. Otherwise you may install the sub panels serving each apartment anywhere inside that apartment. Just be aware that panel must be always available to the tenant of that certain apartment.

    Hope this helps

    Wg

  6. #6
    Join Date
    Sep 2002
    Posts
    1,999

    Default 100 amp sub panels and feeder wire conduits

    The NEC requires that each dwelling be served with a minimum of 100 amps. Also I am taking that this is an older building that does not have a separation wall of 1 hour rated or more fire wall. This means that you must install main service disconnects outside by your meter base. Install a ground rod and a PVC conduit up to the meter base connecting the ground rod by grounding electrode conductor to the meter base. Then a nipple out both sides of that double gang meter base to each of the two main serivice rated weatherproof disconnets. Then install either a 4 wire 2 awg SER aluminum nonmetallic sheathed cable or a 4 wire 4 awg copper nonmetallic sheathed cable or conduit containing 4 - 4 awg copper or 4 - 2 awg aluminum conductor inside each conduit to each nonservice rated panel [aka sub panel] inside each apartment.
    Ok, I understand this to mean that outside the house there's a two gang meter base, which is a meter base that can accommodate 2 meters, and from each meter there's a conduit containing 4 - 4 AWG copper conductor. One of these feeds one panel, the other feeds the other panel.


    Remember the neutral bar of that sub panel inside each apartment must be isolated from teh metal frame of the sub panel and also isolated from the equipment grounding bar. After the main service rated panels or disconnects in your condition located outside everywhere on the load side of those main service rated disconnects must have the neutral and grounidng separated.

    Ok, the sub panel appears to do this: the neutral bar is mounted on a plastic strip, so it is not in contact with the metal frame. The grounding bar is in contact with the frame.

    Remember no breakers are allowed installed in a panel of disconnect inside a closet or bathroom. YOu also must maintain 30" dedicated space on the wall for that or those panels or disconnects and you must maintain 30" dedicated clear approach from ceiling to floor in front of that or those panels in existing buildings.

    Ok, so it seems I have to scrap the plan to install the sub panel in the closet. The new sub panel will be on the wall almost right behind the meter base. The existing sub panel is in the basement in the same location, but one floor below. This will make it easier to splice the branch circuits to the new sub panel. The new conduit will connect to the new sub panel, which is on the ground floor. The existing conduit will connect to the existing sub panel. Also, what do you mean by 30" of dedicated space? Is it 30" from every edge of the panel? A 30" radius from the center of the panel?



    You need to take the existing branch circuits serving that second dwelling out of that existing panel and make your junctions in a junction box not inside that existing panel to reroute you branch circuits for that second dwelling apartment's sub panel.

    There are a total of 18 spaces (4 double pole plus 10 single pole). Do each of these circuits need a separate junction box? Can it be done in one large junction box?


    I would build the entire new meter and panel system and install branch circuit wiring to the point planned to make the junctions. Including installing a conduit and feeder to the location of the existing panel from the meter READY TO CONNECT. Then cut off the power and make the final connections. Then get your inspection approvals and re-energize your new service.

    How can you install a new 2 gang meter base and connect the conduit and feeder without taking out the existing meter base? My understanding is that the electric company will only install the additional meter and base once all the connections are made indoors and approved by an electrician.



    Thank you very much for your advice.

  7. #7

    Default

    The NEC forbids main disconnect form to be in different places. The switches [main braeker or disconnects] that controls all the power to that building must be in one location. You must be able to stand at one spot and be able to shut down the entire building. This means the panels inside must be slave panels fed by a main disconnect or two breakers in one panel outside beside the meter base. There must be a main disconnect form one for each meter located in one location. These two main disconnect forms control the power to the two panels inside the apartments that are slave panels of hte two outside disconnect forms located at the meter base.

    Before you take for granted that the bar on plastic is not conneted to the grounding bar or metal case of the panel look for a metal bar that connects the neutral bar and grounding bar. Also look for a jumper strip that connects that neutral bar to the metal case of the panel. This neutral bar in each subpanel must be electrically isolated from both the grounidng bar and the metal case of the panel.

    YES you are forbidden to install a panel in a closet.

    The 30" dedicated wall space is an area 30" inches wide on the wall dedicated for that panel only with nothing other than a junction box alowed in that 30" wide dedicated wall space. That means from ceiling to floor on that wall. Teh panel may be installed anywhere in that dedicated 30" wall surface but nothing else in allowed in that 30" by ceiling to floor wall surface.

    One junction box may contain all branch circuits if that junction box is large enough to meet box fill limitations for that number of current carrying conductors.

    You may install the meter base in a different location or install the conduits and wire ready to connect to the meter base before you have your power disconnected. Then all you have left to do is remove the old meter base then mount the new meter and connect the conduits and feeder conductors already installed waiting to be connected before you ordered the power off.

    Hope this helps

    Wg

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