I am taking power from my 200amp entrance panel to a separate building with woodworking tools. I'll start with 100amp breaker at the panel then travel 75' inside my basement before going out through 1 1/2" conduit another 75' to the building where i'll have another 100 amp breaker at the subpanel. My question is cable type. I know i'll need URD for outside and am looking at either 2-2-2-4 or 2-2-4-6. Which will suffice? Also what should i use for wire inside the house between service panel and the exit to conduit? Thanks for your help
Wiring – Running power to outside workshop
wiring
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National Electrical Code 2014
Article 250 Grounding and Bonding
II. System Grounding
250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).(1) General. The grounding electrode conductor connection shall be made at any accessible point from the load end of the overhead service conductors, service drop, underground service conductors, or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.
This means that the grounded (neutral) from the service must be connected to ground, and that the connection can be made by bonding the neutral bus bar to the grounding electrode.
(5) Load-Side Grounding Connections. A grounded conductor shall not be connected to normally non–current carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article.
This means that the grounded (neutral) conductors should only be grounded at the main service disconnnect.
If the main service panel happens to be the same place that the grounded (neutral) conductor is bonded to the grounding electrode, then there is no problem mixing grounds and neutrals on the same bus bar (as long as there is an appropriate number of conductors terminated under each lug). If the two bus bars are not connected; as would be the case anywhere other than the main disconnect (exceptions exist), then you cannot mix them.
Notice how the grounded, and grounding bus bars are connected in the main service panel. This means that; electrically speaking, they can be considered a single bus bar. Which means that both grounded (neutral), and equipment grounding conductors can be terminated on either bus bar.
In the subpanel, the bus bars are kept separate. So grounded (neutral), and equipment grounding conductors cannot be mixed.
reasons going to the service entrance will be difficult:
- you'll have to open the meter box, and most meter boxes are tamper sealed by the electric company so you will have to involve them.
- the meter box lugs are probably not sized for multiple connections.
- this creates a potentially dangerous wiring setup. someone may assume that the main panel in the house covers the entire house.
i would go with connecting a subpanel to your main panel. i assume that the existing circuits in the shop go to the main panel? take them out and use the space you just freed up to wire in your subpanel.
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Best Answer
First off, you likely want type UF cable for this application -- URD isn't even a recognized NEC type to begin with, and even URD/USE cable can't be run indoors as per 338.12(B)(1) as it lacks the fire-resistive insulation of NM, UF, and SE cables.
Second -- while 2AWG copper USE has an ampacity of 115A as per the NEC, since you cannot use USE in this application, you must use 1AWG wire here, as UF has a lower temperature rating than USE (60deg vs USE's 75deg) and thus a lower ampacity (2AWG copper UF can only carry 95A, while 1AWG copper UF is limited to 110A). This may also require you to upsize to 2" conduit in order to keep the conduit fill acceptable. Of course, since you are doing the run in conduit, 2AWG copper THWN is also an acceptable choice of wire for this task, provided the indoor run is done in conduit as well, and would allow you to keep the existing 1.5" conduit.
As to the neutral and ground wires, a 2-2-2-4 configuration would be used -- this is as per 215.2(A)(1) Exception 2, which would prevent an undersized neutral in your situation when the feeder is fully loaded. Of course, 2-2-2-2 is also acceptable, and may be easier to manage when working with individual THWN wires in conduit.
If you do decide to use URD/USE for the outdoor run -- you will need to use a different wire type for the run indoors. You'll either want copper THHN/THWN in conduit (2AWG works), or a 2-2-2-4 copper SE (note, NOT USE) cable, either in conduit or run as if it were NM.