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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3349
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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I am currently wiring a sewing shop in 240v. It is a detached

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I am currently wiring a sewing shop in 240v. It is a detached building (20' away) from my main residence meter and service line. I want to slave a 125A service rated breaker box [as a sub panel] to my residence's meter via a 125A Main Disconnect. Is this feasible? And what is the best way to do it? In the Shop, I plan to run 3@ 20 A circuits for 240v Outlets (3-4 outlets per circuit in parallel) and 3@ 15 A circuits for the 120v lighting (2 lights per circuit in parallel as well). Thanks. John name is XXXXX XXXXX I will be happy to assist you with your electrical question. My goal is to exceed your expectations on Just Answer!


1) Hello John....... what is the amperage of the main electrical panel? Is it a 200 amp service?


2) Do you have room inside the existing main electrical panel to accommodate a double pole 125 amp breaker?


3) Will you be connecting the detached building as underground or and overhead aerial drop?


4) I assume that the sewing machines will require a dedicated 240 volt circuit?

Customer: replied 3 years ago.

Hi Kevin,


to answer you questions:


1) 200A service. 200A Panel


2)There is physical space in the panel for a 125A double pole breaker. Not 125A available of the 200A (If that matters?).


3) I was planning on underground from the meter/disconnect, but, if my panel supports it that will change to aerial as I will have to run lines through the attick to avoid boring through my brick walls.


4) Yes and no, each sewing machine will have its own receptacle, but as I only use 1 machine at a time, 2 if my wife is sewing, then 4 recaptacles in parallel on 1 20 A circuit seems doable. The machines can be run on 120v but the servo motors are far more efficient with 240v. The motors pull 3A on 240v and 6A on 120v.



1) John....... thanks for the replies


2) I would suggest installing Schedule 40 Gray electrical PVC conduit to connect the buildings. You can use Underground Feeder cable, but I always recommend using conduit if possible. You only want to trench and dig this one time. You can also use Rigid Metallic conduit. If using Sch 40 PVC and it is subject to damage above grade, you will need to transition to either Sch 80 PVC or Rigid Metallic conduit. Sch 40 Gray electrical PVC is not rated for areas subject to possible damage.


3) The conductors from the main panel to the sub-panel require a 4-wire circuit, ie, 2 hots, 1 neutral and 1 equipment grounding conductor. If using Rigid Metallic conduit, then no need for a separate equipment grounding conductor since the Rigid Metallic conduit acts as the Equipment Grounding Conductor.


4) For a 125 amp feeder circuit, you will require either 1/0 AWG Stranded Copper or 1/0 Stranded Aluminum, type THWN. The 1/0's will be used for the 2 hot conductors and the 1 neutral conductor. The voltage drop for a 20 foot run using 1/0 AWG is next to nothing and you are OK there.


5) For a quantity of (3) three 1/0's and (1) one 6 AWG equipment grounding conductor, a minimum of 1 1/2" diameter Sch 40 Gray PVC will be required for the conduit fill ratio of the 4 conductors.


6) At the main panel, you will also need an equipment grounding bar if using the 6 AWG equipment grounding conductor. The 6 AWG EGC only gets connected to the equipment ground bar and not the neutral bus bar. Tape the end of the 6 AWG using green electrician's tape at each end to identify this as a ground. Tape the 2 phase conductors at both ends with black and red electricians tape to identify these as the hots. Tape the neutral at each end with white electricians tape to identify this as the neutral. You'll need (1) one 125 amp double pole breaker installed at the main panel.


7) The 125 amp sub-panel will need to be a main breaker panel and not a main lug panel. This will allow you to disconnect the entire detached building via the sub's 125 main disconnect breaker.


Inside the sub, the neutral bus bar does NOT get bonded with the panel's metal enclosure. You will need to remove either the sub's main bonding jumper (green grounding screw or a copper jumper strap). This isolates the neutral and the equipment grounding conductor. In any sub-panel, the neutral is always floating. The sub will also require an equipment grounding bar to land the 6 AWG equipment grounding conductor. Since a detached building, you will also need a minimum of (1) one 8 foot ground rod connected to the equipment ground bar using 6 AWG Stranded Copper and an acorn type ground clamp. Depending upon your soil conditions, you may require 2 or more 8 foot ground rods all bonded together. The total resistance of the ground rod system needs to be 25 ohms or less.


If you will be using Romex as your conductors for the building branch circuits, the bare copper Romex grounds only get landed on the sub-panel equipment ground bar and NOT on the sub-panel neutral bar. This is for safety purposes and is an NEC requirement.


8) I would not recommend paralleling the 240 volt receptacles. Each 240v receptacle should be on its own dedicated circuit breaker for safety reasons. Reason being, if the 240v receptacles are wired in parallel and more than 1 machine running concurrently, there is always the risk of overloading the circuit and tripping the breaker.


Hope this helps.........If you have any additional questions, let me know and I'll be glad to answer them for you.


Otherwise, don't forget to rate me before you log Off.

The next time you have an electrical question, you can also request for me at:

1) All electric motors, big or small have an "in-rush" of electrical current upon the motor being activated. If the breaker is not sized accordingly and additional motors are allowed to all be connected to 1 circuit, then a high probability that the breaker will trip due to the "in-rush" current on the motor. The machines need to be on their own dedicated 240v circuit for this reason.


2) If you are concerned that the additional 125 amp load will not be able to be supported on the 200 amp main, you will need to activate as many electrical devices and appliances in the main panel. Simultaneously, turn ON all appliances such as the Central Air, all lighting, microwave oven, dryer, washer, etc. Then use a clamp-on amp probe at each of the 200 amp main phase conductors to determine the full load amp draw with the panel now being under numerous load conditions.

1) If using Rigid Metal conduit or EMT at either panel end, and you remove the concentric conduit knockouts, you will also require the use of bonding bushings or a grounding wedge. These are only required if removing the concentrics. If not removing the concentrics, then regular box connectors are only needed. If using bonding bushings or a grounding wedge, then 6 AWG will need to connect the bonding bushing or grounding wedge back to the respective equipment grounding bar at each panel.
Customer: replied 3 years ago.

Is the main breaker at the sub a requirement? Or does the breaker in the main suffice for disconnecting the detached building? I only ask because I have a 125A lug panel collecting dust.

1) Article 225.32 in the 2011 edition of the National Electrical Code discusses service disconnecting means for feeders or branch circuits in a remote structure or remote building. The requirement is to facilitate ready disconnection of power at the remote.


2) If a sub is installed next to or in line of sight from a main panel, then the sub can be a main lug and a main breaker is not required.

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