I have a 400 amp Siemens main panel it has a 200 Amp breaker to the house and a secondary 150 amp breaker added to run power to a metal building approximately 150 feet from the main. what size wire should i use to compete this? Also grounding the panel to the building and to the ground rod what size wire would be adequate? And can i use a bolt to run thru the base rail of the building to complete the ground for this?

3 Answers 3


For your 150' run depends on whether or not you use Copper or Aluminum (aluminum being cheaper). You would need

  • Copper 1/0 @75 degrees C or better 90 degree c
  • Aluminum 3/0 @75 degrees C or better 90 degree c


Grounding Panel to the building - the following article gives a more complete answer than I should write here.


NEC code section 250 should be of some help to you.

  • 2
    Aluminum is the better choice. Aside from being far cheaper, the lugs you'll be attaching to are themselves aluminum. so you have Al-Al contact instead of Al-Cu contact, which has "had issues"... Commented Jan 24, 2018 at 23:07
  • 1
    If you use aluminum wire you need to use anti-oxidation paste on the terminations no matter how it terminates.
    – ArchonOSX
    Commented Jan 25, 2018 at 8:09

Ideally you would want to use 90 deg rated wire which would help carry a higher load without needing a bigger wire, but realistically you're probably better off just sizing everything at 75 deg rated wire for simplicity sake.

Thus, a typical job like this would be:

  1. From the Main Panel SER 4/0-4/0-4/0-2/0 aluminum service entrance cable fed 1st to a outdoor rated box where it would...
  2. Splice into direct burial rated USE 4/0-4/0-4/0-2/0
  3. At the metal building the USE would again terminate but this time into a outdoor rated 150A main disconnect.
  4. From the disconnect you would run a grounding electrode conductor to fed two ground rods for transient voltage protection. If a cold water pipe comes into the metal building it too would need a grounding electrode wire and clamp and bonding jumper over the PRV.
  5. Also from the disconnect you would run SER cable just like in step 1, to a main lug sub panel.

4/0 Al @ 75 deg is good up to 180 Amps [See NEC Table 310.15(B)(16)]. Also 4/0 AL @ 75 deg maxed out at 195A @ 150 ft will only have 5.8% voltage drop. This is very acceptable for a 240volt feed where it is recommend to stay below 7.2%

3/0 AL @ 75 deg is good for 155A so that would work too and is a smaller wire. It also has very little voltage drop. 6.2% @ 165Amps

For the grounding electrode conductor size, using NEC table 250.102(C)(1) 4/0 Al requires #4 copper. 3/0 Al requires #6

To attach the grounding electrode conductor to the building foundation the building needs to be inspected prior to the foundation being pored to answer correctly. You could read up here for more info on that.

  • 1
    What are the prerequisites for using the 75/90 columns in residential? Commented Jan 25, 2018 at 2:27
  • Indoor wire needs to be Listed. E.g., THHN is listed at 90 deg in conduit. The lugs need to be listed for 90 deg too, if not then the THHN magically reduces to whatever the lugs are rated at...usually lugs are rated at 75 deg.
    – Kris
    Commented Jan 25, 2018 at 2:34

The deration of 90° wire is sound engineering since the terminations are not rated for 90°C you cannot use the wire at that rating. However, you can use that rating to start the deration process for ambient temperature and number of conductors in a raceway. It is the weakest link concept. Unless all of the equipment in the circuit is rated 90°C you cannot use that rating. Manufacturers do not make 90° rated terminations yet, hence the deration.

This article from the National Electrical Code is why you have to use the 75°C rating or lower depending on the terminations and ampacity of the equipment.

110.14 (C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conduc‐ tors with temperature ratings higher than specified for termi‐ nations shall be permitted to be used for ampacity adjustment, correction, or both.

(1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7).

(a) Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following:

(1) Conductors rated 60°C (140°F).

(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used.

(3) Conductors with higher temperature ratings if the equip‐ ment is listed and identified for use with such conductors.

(4) For motors marked with design letters B, C, or D, conduc‐ tors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.

(b) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:

(1) Conductors rated 75°C (167°F) (2) Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors

(2) Separate Connector Provisions. Separately installed pressure connectors shall be used with conductors at the ampacities not exceeding the ampacity at the listed and identified temper‐ ature rating of the connector.

Informational Note: With respect to 110.14(C)(1) and (C)(2), equipment markings or listing information may additionally restrict the sizing and temperature ratings of connected conductors.

  • 1
    Thanks for the clarifications - that should help the op to make a good decision.
    – Ken
    Commented Jan 26, 2018 at 9:10

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