True or Not??


Old 11-05-04, 11:14 PM
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Need help

To All:

My previous post have been confusing ,apparently, regarding the install of a simple single conduit 40 ft long, so I will try this as simple as possible.

IN my garage I am installing a single"one" 3/4" sch 40 pvc conduit.

In this run "CONDUIT" I would like to install:

(1) 50 amp, 220 volt, welder recepticle 25 ft from panel, which requires 8 gauge per maufacturer, also 8 gauge ground.

(1) 220volt 20 amp recepticle 30 ft from panel, for various 20 amp or less tools.

(1) 220volt, 23 amp at the vewry end of the run ,40ft, for the 5 hp air compressor.

All single phase.

Suggestions on how to do this is one single conduit, regardless of size/junction boxes etc; also if possible using only one ground for all.


Last edited by John1; 11-06-04 at 08:24 PM.
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Old 11-06-04, 08:35 PM
P Michael
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For one thing, you only need one ground wire. It might be #10.
Most houses are single phase so that isn't an issue.
40' is short so voltage drop isn't a consideration.
Is the compressor 240 volt?
Why not EMT instead of PVC?
#12 for 20 amps.
Old 11-06-04, 10:30 PM
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I only have my Canadian Code to go by so I can't give you the specifics as they would apply to you.

If you have all three circuits in a single conduit, you'll likely need to jump up one wire size for each because of the 80% de-rating. (i.e. your #8 wire is only good for 80% of its normal rating).

That said, I'm guessing you need #6 for the welder, #8 for the compressor, and #10 for the plugs. In the last 10' segment of conduit you should be okay to drop down to #10 for the compressor as there's only two 'hot' wires in the conduit now.

It appears unlikely this number of wires of these sizes will fit in the 3/4" conduit. You can either upsize the conduit, or maybe supply the welder (shortest run, largest conductor) with a separate cable or conduit with #8 conductors.

I would be on the verge of considering a garage sub-panel at this point. You'd save some breaker spaces in your main panel and give you some expansion room in the garage should you need it in the future.
Old 11-07-04, 02:26 AM
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The panel to the garage has it's own meter therefore a large main panel.

The air compressor is 240v.
was thinking using 5 total 8, 4 hots and 1 for the groung for all, therefore staying within the 40 rule>

Old 11-07-04, 07:46 AM
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I couldn’t determine if you were making your 40' run from the panel attached to the garage or a panel attached to your house out to the gargage.
So, If from the panel attached to your house. I would use 1” conduit for that run. It will easily accommodate the 5 wires you intend to run and will allow you to make pull with less effort. I would also suggest you go with U2Slow’s post on wire sizes. Use copper THW or THWN wire which is approved for outside use (not TW). Although, the manufacture suggests# 8 for that 50amp breaker, I would use# 6. It's more in the range to support that breaker.
Old 11-07-04, 08:56 AM
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The run is "inside" along the wall.
Old 11-07-04, 09:29 AM
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True or Not??

When wiring for any electrical item, welder/motor, lights etc; you can never use too large of condutor (wire)(overkill) as long as you use the proper breaker to protect the item and wire from excessive amp draw??

Old 11-08-04, 05:22 AM
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To answer your last question first:
Essentially true, but there are a couple of additional caveats. Basically, as long as you follow all of the rules for installing the larger wire, you can legally and safely 'oversize' the wire for a load. The additional rules are:
1) The terminals have to be rated for the wire; no trimming extra strands or otherwise modifying the wire to make it fit. Note: if you need to use oversize wire, you _can_ splice on a short length of smaller wire to fit the terminals.
2) You have to be careful about conduit size/box fill/bending radius, and make sure that the wire properly fits in whatever enclosures you are using.
3) Increasing wire sizes can do strange things to required ground size (as I expressed in previous threads that you found confusing.)

But if you watch for all of these rules, there is no problem at all 'oversizing' wires for branch circuits.

Now to your original question, how to fit the circuits into one conduit:
1) All of your circuits can share a ground; simply use the largest ground required.

For the welder you need: #6 red, #6 black, #6 green.
For the 20A general purpose circuit you need: #12 red, #12 black (use green from above)
For the compressor you need: #8 red, #8 black, (use green from above).

3) Use a 1" conduit to fit the above.

4) I would recommend that for the 20A circuit, you also pull a #12 _white_ wire. This will give you the option of adding a 120V receptacle later. There is more than enough room for this in the 1" conduit, and it is very cheap.


A) By putting _3_ 240V circuits into one conduit, you introduce 'derating' requirements. You will need to use larger than normal conductors. It _may_ be cheaper and easier to run _two_ conduits side by side.

B) Question: does the welder manufacturer normally require the #8 ground, or do they specify #8 for the hots and #10 for the ground? If they only require a #10 normally, then you can replace the #6 green with a #8 green in the list above.

C) Conductor size calculations:
For the welder wire sizes, I simply took the manufacturer recommended wire size, looked up its ampacity, multiplied by 1.25 (inverse of derating) and looked up the new wire size.
For the 20A circuit, derating says that we need to upsize the wire, but #12 is _already_ upsized for 20A circuits, because of 240.4(D). The derated ampacity of #12 exceeds 20A, so it is still good here.
For the 5Hp compressor, 430.148 says that we have to treat this motor as having a full load current of 28A. Conductors for motors must _always_ be 125% of full load current, so we go with #8. Derating would add another 125%, but we get to use the 90C column for this, so #8 is still good.

D) Conduit size calculations:
#6 THHN 32.71mm^2
#8 THHN 23.61mm^2
#12 THHN 8.581mm^2

3*32.71 + 2*23.61 + 2*8.581 = 162.5mm^2 requiring a 1" sch40 PVC conduit

Old 11-08-04, 02:06 PM
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Why Make Life So Hard?

Maybe I have missed something, but it would appear to be a whole lot easier to install a 100 amp sub-panel in the garage? This would allow for any future requirements, and would save slots in the main panel. For example he needs 50 amp for a particular piece of equipment now, and so in installing #8 cable. What happens if he buys a new and upgraded equipment? Does he really want to be doing this again in 5 years? A 100 amp panel is “bog standard”, and cheap, so no need to worry about all the details of de-rating, conduit fill, over sizing cables, voltage drop, etc. Or maybe I have missed something?

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