Size wire for underground feed to shop?

[Keith Nordyke]

I have a detached shop about 75 feet from the house. It is currently fed by overhead triplex. I need to move the line underground and the path with be about 190 feet because I will need to avoid a huge concrete pad and then make a 40 foot emt run to get to the panel. Will #4 triplex work? An electrician friend indicated (offhand) that #4 could be direct buried and would handle 100 amps. Thoughts? Ideas? How deep do I need to trench? 18 inches adequate?
Thanks

[Brian Backner]

I'm no electrician, but #4 is a little light - most ampacity charts state #4's capacity to be under 90 amps - and that's for 90 degree C copper! To be safe, I would think you want a minimum of #2 (95 amps for 60 degree and 115 amps for 75 degree wires, respectively).

The other consideration is the distance you need to go - you may have to go even larger to avoid any significant voltage drop! I have seen an online calculator for this, but can't find it at the moment. Perhaps some other Creeker can provide a link to it.

Here in Taxachusetts, my local code calls for a minimum of 18" depth of trench, unless you either put down a 2" cap of concrete over direct burial wire, or you pull wire through rigid steel conduit - and then you can bury to 12" deep. When I ran power to an shed several years ago (only 20 amp, 120 v for lights and a single outlet, but over 220'), I ran 10 gauge wire pulled through PVC conduit buried at 20-24" deep. My building inspector looked at only the first five feet and signed off in a heart beat. YMMV.

Cheers,

Brian

[bruce buren]

check local codes, but mine is #4 feeding a 100 amp and i ran it in pvc buried 18" deep, subfed from my 200amp in the house.

[Chris Damm]

Here you have to bury it 24". Check with your local inspector!

[David G Baker]

I would run #2 copper and this should be good for 100 amps if you run aluminum you should use the next larger size. I would bury the wire 24" deep and would run 4 wires, two hot legs, one neutral leg and one ground leg.

[Rollie Meyers]

#4 copper is NOT suitible for 100A service to a outbuilding,if done by a so-called "professional" I have no comment.
Table 310.15(B)(6) does not apply to the OP's needs, Table 310.16 is the correct table to size the feeder, & for 100A & using copper conductors #3 AWG is the minimum size, #1 AWG is the minimum size for aluminum conductors. These were sized in the 75 degree column, one cannot use the 90 degree column to size conductors.

[Roger Frazee]

Hi Keith

I think it would be beneficial for you to gander at this website link...bookmark it...you will use it again.

http://www.jhlarson.com/ind_tables/ind_table.htm

To understand what Rollie just posted look at ' Conductor Characteristics ' in the right hand column. The first table is for conductors that are not main feeders or service entrance conductors. This is the table you will use. Whenever you are choosing ampacities that are not carrying the entire load of a dwelling or apartment, duplex etc... The table (310.15B6)that says conductor sizes 3 wire 1 phase Residential Services. This is the table that deals with dwellings etc and it is this table that allows you to use #4 copper for 100 amps. Your shop does not qualify its not a dwelling or other structure that houses and sleeps people. This table is primarily used for single family and multifamily dwellings for sizing the service entrance or main feeder(S).
So you want to use the first table 310.16 listed under the heading I mentioned. As Rollie said you would use the 75C column and be careful that you are looking at copper ampacities (left side of table) or Aluminum ampacities (right side of table). Most triplex and quadplex direct burial will have the insulation rating USE-2 and RHHW and RHW this does two things it allows direct burial and they can enter a building if placed in conduit before entering the building.

I would use aluminum for the sub-feeder.... But 190 feet at 100 amps is likely to require 2/0 or so aluminum this may pose some problems with terminations to existing equipment.

In the middle is the wire size calculator that will give you the wire size based on the distance and load. The wire size it gives will fall around 3% voltage drop. This will insure proper design getting the correct amperage and voltage to the shop. Play with it some and get a feel for it. Select 240 volts. Distance and load (amps) will change wire sizes and voltage delivered at the shop.

Now you mention you have a triplex overhead drop serving the shop presently. I am going to assume this is coming from a main breaker panel on or inside your home. If it is originating inside your meter can or an outside service panel let us know. Because this will allow a few options as to whether you need any overcurrent protection at the origin of the subfeed.

You cannot reuse the overhead in the underground feeder it will not be rated direct burial and the neutral is bare.

Next thing is 3 wire feeder vs 4 wire feeder. 4 wire is the new code standard but 3 wire is still allowed if you meet certain conditions. But if you want water using metal pipes or phone lines or data lines going from the house to the shop forget 3 wires...it is not allowed .

So if 4 wire is your option then your existing panel at the shop will need to have the neutral and ground separated. It doesnt at present because it is served with 3 wires.

To explain your wire size....You have a 100 amp panel you can feed it 100 amps or anything less than 100 amps. But wire size must fall within the main lugs at the subpanel disconnect and supply breaker wire size range. Check your exisiting sub-panel and see what the maximum wire size it will accept at the main lugs. This can be a problem if voltage drop due to distance makes the wire size too large for the lugs. Small 100 amp panels with say 12 circuits or less many times will only accept #1 awg if main lug only. Most 100 amp main breaker panels will accept 2/0. The minimum feeder for a typical woodshop is probably 60 amps IMO. Point being that the feeder only has to be capable of carrying the calculated load for the shop. If you don't need to be concerned about cost due to wire size then feed the panel it's 100 amp rating. But no matter always allow for added loads as the shop expands. One man shops generally are fine with 60 amp feeders but it does depend..... You have 100 now I believe so probably want to keep that.

24 inches for direct burial in residential applications

Your shop should have ground rods at least one anyway.

You need a building disconnect at the shop nearest the point of entrance of the feeder either inside or outside. Usually this is less than 10 feet by local code authority. You can satisfy this with a main breaker panel or backfed main breaker panel with hold down kit or a service disconnect ahead of a main lug only sub panel.

Your EMT must be sized correctly for your wires... the link above has a sizing chart depending on what you end up with in wire size. The ground wire will be smaller than the conductors so just make it the same size as the others when checking the chart. Some will say to go up one more size. I don't think you will need to.

Hope this helps......

[Rollie Meyers]

Another thing about triplex is that it cannot ever be used in conduit. 2 ground rods are required unless you can prove 25 Ohms of resistance or less & most people do not have or wish to spend the thou$ands to buy the testing equipment to verify it.

[Keith Nordyke]

Thanks, Guys. I was aware of alot of what has been posted. I agree that I should go with the larger wire. I did plan for a disconnect at the entrance to the shed where it will come out of the ground and I have ground rods at the shed panel. One thing that was mentioned that I didn't consider was the capacity of the lugs at the panel where it leaves the house for the shed. I will check that. The overhead wire was here when I moved here and I want to move it underground and disconnect the lines that have been running through my attic and then to the shed. Thanks again for the help and advice. My neighbor is an electrician at one of the major chemical plants around here and when I mentioned the project to him today, he thought it would be #2 but wanted to "check the book" before he fully opined. Sounds like the advice here is coming into consensus with what I learned today. Thanks again.

[Rich Neighbarger]

[...]unless you can prove 25 Ohms of resistance or less [...].

Is it that high now?

[Rollie Meyers]

Quote:
Originally Posted by Rollie Meyers
[...]unless you can prove 25 Ohms of resistance or less [...].

Is it that high now?

Here is a copy & paste from the 2008 NEC,this is unchanged from the 2005 edition.

250.56 Resistance of Rod, Pipe, and Plate Electrodes. A
single electrode consisting of a rod, pipe, or plate that does
not have a resistance to ground of 25 ohms or less shall be
augmented by one additional electrode of any of the types
specified by 250.52(A)(4) through (A)(8). Where multiple
rod, pipe, or plate electrodes are installed to meet the requirements
of this section, they shall not be less than 1.8 m
(6 ft) apart.

FPN: The paralleling efficiency of rods longer than 2.5 m
(8 ft) is improved by spacing greater than 1.8 m (6 ft).

[Jarrett Vibert]

PVC conduit buried at 20-24" deep.

pvc buried 18" deep

Here you have to bury it 24".

I would bury the wire 24" deep.

24 inches for direct burial in residential applications

If the varying info above hasn't directed you to the best advice, let me repeat it. Consult your local electrical safety authority.

In my neck of the woods. Anything buried, whether it be directly or within conduit must be below 1meter ~36". If your route crosses and/or runs underneath a driveway, that depth increases.

Your EMT must be sized correctly for your wires...

Never under any circumstances bury EMT.

12yrs in the electrical trade has taugh me to never trust the code book if you have the option to speak to your inspector. All codes are subject to alteration by your inspector, and all codes can be adjusted within book releases.

just my .02

[Roger Frazee]

Never under any circumstances bury EMT.

Your absolutely correct. However the person asking in this thread isn't burying the EMT....he is transitioning to EMT when he comes above grade at the shop.

It can be buried though...it is just hardly ever done because of the corrosion application that must be done to it to allow it buried by the local code authority.

In my neck of the woods. Anything buried, whether it be directly or within conduit must be below 1meter ~36". If your route crosses and/or runs underneath a driveway, that depth increases.

What woods are you from ?

In my neck of the woods there are no amendments to the burial depths listed in Art. 300....for residential.

Service laterals are 30 inches minimum ..... Generally speaking they want the other electrical on the homeowner side above that. Cutting laterals is no a lot of fun. Any thing is possible in commercial construction but residential in my experience for feeders load side of the service equipment follows Art. 300. Under driveways or streets or parking lots or roads alleys it is usually 24 inches.

Your correct the ahj can require anything he wants as long as he has some reason for it to differ from the NEC like a local amendmant but in my 25 years in the business I've never seen them want a residential feeder to a detached building 36"...

[Rollie Meyers]

18" burial depth is fine for most PVC SCH 40 conduit,unless your running it under a driveway or something similar, then 24" of cover is required.