Wire size

I have added on to my shop I have put in a 250amp breaker box witch is 50ft from my sub panel. I need to know what size wire I need, and can my ground wire be smaller. Thanks Alan.

It depends on several factors, primarily related to temperature.

My 2002 NEC rates 4/0 90°C THHN at 260 amps.

I would be inclined to use a larger gauge to keep the temperature down. The next larger one listed in the table is 250 kcmil.

This is pretty big stuff. Unless you have experience handling large wire and the tools to work with it properly, I would suggest letting a professional electrician do the work.

Three conductors of either size would require 2" conduit.

- Leigh

The real Leigh said:

It depends on several factors, primarily related to temperature.

My 2002 NEC rates 4/0 90°C THHN at 260 amps.

I would be inclined to use a larger gauge to keep the temperature down. The next larger one listed in the table is 250 kcmil.

This is pretty big stuff. Unless you have experience handling large wire and the tools to work with it properly, I would suggest letting a professional electrician do the work.

Three conductors of either size would require 2" conduit.

- Leigh

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The only thing you can use the 90° column for is derating there is no equipment rated to use the cables @ the 90° ampacity. Using the 75° column would be 250 kcmil THHN which is rated for 255A which would require a 2" conduit for 3 conductors or 2½" for up to 5. See table 310.16 2005 NEC for allowable ampacities.

What size is your subpanel,or do you have a switchboard that can handle a 250 amp. subfeed?

sub panel

Sub panel was not the right term. It is a junction box.

The distribution panel (main breaker box) is also called 'the panel'. Additional boxes placed downstream are termed sub panels. A junction box is mostly used to indicate an empty enclosure used to join wires together.

kinda large for a home shop

ND is north dakota i believe...i hate to say but i think u will require a electricain to calculate the load if u plan on using 250amps

most people's supply from the electrical company are incapable of supplying 250 amps on top of the original home. e.g my main is only 250 amps

judging from what u have written u are planning to land the wires on your home panel with no independent breaker to protect the wires going to the new panel. thus giving u 2 panels from the same supply. this is a no no.

realy u need to calculate the load required by your work shop. then the voltage u require 240 is 2 wires plus one ground 110/240 requires 3 wires plus a ground. does your home panel have the avalible ampage to supply a sub panel (might have to get the power company to run u a new wire). then there are a lot of other rules about the conduit and insterlation u will need to work out. 250mcm isnt easy to run especially if u have to install bends to go around stuff.

There are a couple of approaches you may want to look at, rather than tapping off the existing service.

One is to ask if the power company can bring a separate drop to your shop, with a separate meter. This can be a real advantage for tax purposes if you make money in the shop.

The other is to add a second panel next to the existing one. The power company may have to replace their existing drop with larger cable, and you'll have to install larger wiring from the weatherhead to the two panels.

can my ground wire be smaller

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NEC Table 250-94 does allow smaller grounding electrode conductors in AC systems. For a 250 MCM conductor, the grounding electrode must be at least #2 copper, or #0 aluminum.

The real Leigh said:

Three conductors of either size would require 2" conduit.

- Leigh

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By NEC, but as always, local control may be different. I've just been through this here last week, and for example, my utility requires 3" conduit for three 4/0 conductors.

mark thomas said:

By NEC, but as always, local control may be different. I've just been through this here last week, and for example, my utility requires 3" conduit for three 4/0 conductors.

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Those are EUSERC requirements for underground services not NEC and only apply to parts the parts the ultilty has control of, they would not apply to overhead services.

EUSERC: Electric Ultility Service Entrance Requirements Committee, mostly comprised of West Coast ultilities.