can you use a heavier gauge than required safely

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  #1  
Old 06-05-07, 09:47 AM
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can you use a heavier gauge than required safely

I am running an 80 amp 240 line to my garage...will have 4 - 220 circuits running off this feed for stationary power equipment (main service can handle this additional load).

The length of run to the garage is about 35 feet...online wire calculators call for 10 gauge wire to feed this circuit at 240 volts....I have some THHN #2 copper left over from a previous job...my questions are:

is it ok to use this heavier gauge for this 80 amp- 240 feed ?

The garage is separate from the house...but there are original construction grounded conduit circuits running to the garage now...do I need grounding rods for this new garage panel if it is mounted inside the garage ?

Thanks, Tim
 
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  #2  
Old 06-05-07, 10:16 AM
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The problem with using an on-line calculator for wire size is that you have to know how to use it.

When discussing a run to a sub panel, 10 gage wire is at most good for 30 amps.

Forget what the online calculator tells you. Go by the NEC tables. These will tell you the proper size wire for an 80 amp sub panel feed.
 
  #3  
Old 06-05-07, 10:21 AM
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> I am running an 80 amp 240 line to my garage...online wire calculators
> call for 10 gauge wire to feed this circuit at 240 volts

Whatever calculator you used is very wrong; #10 is only good for up to 30A. For 80A, you need a minimum wire size of #4 copper. The #2 copper you have now is good for up to 125A as a subpanel feeder. You'll need to install hot, hot, neutral with the #2 and a green or bare #6 ground.

> is it ok to use this heavier gauge for this 80 amp- 240 feed ?

Yes, code provides a minimum size.

> there are original construction grounded conduit circuits running to the
> garage now

Code allows for only one circuit per outbuilding. If you have existing circuits, they need to be abandoned and fed with the proposed subpanel. What trade size is the existing conduit? It needs to be at least 1-1/4" to accommodate the #2 conductors.

> do I need grounding rods for this new garage panel if it is mounted
> inside the garage ?

Yes. All outbuilding subpanels require a 5/8" x 8' copper clad ground rod bonded to the subpanel ground bar using #6 copper wire and an acorn clamp on the rod.
 
  #4  
Old 06-05-07, 11:34 AM
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Thanks...in using the online calculators...I selected 240 volts since this was originally going to be only for 220 circuits...when applying the 120 volt parameter in the calculator...it indeed says minimum # 4 wires..

The existing garage circuits are feed via an underground/patio 1/2" conduit. If I go with the panel...I will run the line along my connecting block wall using a minimum 1 1/4" EMT.

A couple electricians have said for these types of circuits...you can 'step down' here in Phoenix using #4 neutral with #2 hots....or #6 neutral using #4 hots...etc...?..is this a common practice and do you guys concur on this ?
 
  #5  
Old 06-05-07, 12:31 PM
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Most calculators are not telling you what you need for safety. They are only computing voltage drop for you. Do not ever rely on a wire-size calculator to design a circuit for safety or code. That's just not something they do, or even claim to do.

You can usually step down the grounding wire, but rarely the neutral.
 
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Old 06-05-07, 12:49 PM
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> using a minimum 1 1/4" EMT

Typically, you would want to use schedule 80 PVC conduit outdoors. I don't know if outdoor EMT is common in Arizona, but up here in Michigan it starts to rust away in a couple years. Definitely paint it if you use EMT.

> you can 'step down' here in Phoenix using #4 neutral with #2 hots
>....or #6 neutral using #4 hots...etc...?..is this a common practice and
> do you guys concur on this ?

Yes it is a common practice, but how much you can step down the neutral is figured by a neutral demand load calculation. It is often okay to go down one size if the feeder supplies mostly 240V loads like power tools and welders. You would not want to step down the neutral if you have mostly 120V loads instead of 240V loads or if you're using the feeder for a lot of computer or communication equipment.
 
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Old 06-05-07, 01:25 PM
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Delete - Nevermind.
 
  #8  
Old 06-07-07, 08:18 AM
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Thanks guys....since this is considered an out building...can I use a main lug panel or do I need to install a second main panel with a disconnect breaker as part of the panel itself....rather than or in addition to just the feeder breaker in the main service panel ?

Also...I live more or less on a mountain here in Phoenix...usually a few inches below the surface is solid rock....?...what type of grounding rod is usually used in that case ?

Thanks, Tim R
 
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