Circuit for Outbuilding

[-dirt-]

Hi all,

My power needs have reached the point to where I need 220v at my small detached outbuilding, and I want to be sure that I install a new circuit that is up to code (we are under NEC 2011) and be able to handle the loads I plan to have. I have read up on the various applicable codes for this, but still have some questions.

I have a subpanel on the side of my house that was used for a 220v pool pump. There is no disconnect here -- just room for a few breakers. I would like to install a 50 amp breaker here to feed the outbuilding. I would run PVC conduit down the house, 18' below grade, and back up into a subpanel at the shed. The wire from the 50 amp breaker to the shed would be a 8/3 THHN/THWN (I know its OK for direct bury, might pull a coax out to the shed too.) It would be ~80' from 50 amp break to shed panel. The panel in the building does not have a disconnect.

Ok, here are some questions:

1) Am I required to use a ground rod at the outbuilding? I would like to avoid that if possible, just for the hassle. My "feeder" wire has 3 conductors and 1 bare wire, so it has enough to connect separate neutral and ground bars. Please elaborate on the ground/neutral setup...

2) Must I run the PVC conduit all the way inside the building to the subpanel, or can it just clear the floor ?

3) What is the largest amperage breaker I can install, by code, in the shed?

4) Is there an issue with the subpanel on the house being feed from another subpanel near the main panel?

Thanks all!

 

[pattenp]

8/3 CU is not good for 50A, it’s only good for 40A

What size breaker is the pool pump sub panel on in the main panel? All neutrals and grounds in sub panels down stream from the main panel need to be isolated (not bonded to each other).

1) Am I required to use a ground rod at the outbuilding? I would like to avoid that if possible, just for the hassle. My "feeder" wire has 3 conductors and 1 bare wire, so it has enough to connect separate neutral and ground bars. Please elaborate on the ground/neutral setup...

You need to place 2 ground rods at least 6’ apart at the out building.

2) Must I run the PVC conduit all the way inside the building to the subpanel, or can it just clear the floor ?

Depends. If you use individual wires then it has to be in conduit from panel to panel.

3) What is the largest amperage breaker I can install, by code, in the shed?

All depends on wire size.

4) Is there an issue with the subpanel on the house being feed from another subpanel near the main panel?

Short answer is no.

 

[ishiboo]

Hi all,

My power needs have reached the point to where I need 220v at my small detached outbuilding, and I want to be sure that I install a new circuit that is up to code (we are under NEC 2011) and be able to handle the loads I plan to have. I have read up on the various applicable codes for this, but still have some questions.

I have a subpanel on the side of my house that was used for a 220v pool pump. There is no disconnect here -- just room for a few breakers. I would like to install a 50 amp breaker here to feed the outbuilding. I would run PVC conduit down the house, 18' below grade, and back up into a subpanel at the shed. The wire from the 50 amp breaker to the shed would be a 8/3 THHN/THWN (I know its OK for direct bury, might pull a coax out to the shed too.) It would be ~80' from 50 amp break to shed panel. The panel in the building does not have a disconnect.

Ok, here are some questions:

1) Am I required to use a ground rod at the outbuilding? I would like to avoid that if possible, just for the hassle. My "feeder" wire has 3 conductors and 1 bare wire, so it has enough to connect separate neutral and ground bars. Please elaborate on the ground/neutral setup...

2) Must I run the PVC conduit all the way inside the building to the subpanel, or can it just clear the floor ?

3) What is the largest amperage breaker I can install, by code, in the shed?

4) Is there an issue with the subpanel on the house being feed from another subpanel near the main panel?

Thanks all!

1) Yes, probably two.

2) The wire must be protected from physical damage. That does not indicate conduit is required, but I'd highly recommend it. PVC is cheap and easy to do.

3) There is no "maximum breaker size" since you are protected 50A at the house, but logic says there is no need to go above 50A since it would trip at the house instead if you went over it.

4) No, other than being messy

THHN/THWN is not allowed for direct burial. You could use conduit underground the entire length, it is rated for wet locations.

 

[-dirt-]

8/3 CU is not good for 50A, it’s only good for 40A

310.15(b)(16) says 8 gauge THHW is OK for up to 55 amps at 90 degrees...

Am I missing something?

THHN/THWN is not allowed for direct burial. You could use conduit underground the entire length, it is rated for wet locations.

Can you point me to the code section for that? I can't find the burial code info and was just going on what was printed on the wire insulation.

thanks guys!

 

[pattenp]

If your sub panel doesn't have a main breaker/disconnect, then you are limited to having 6 breakers in the sub panel by code.

 

[pattenp]

310.15(b)(16) says 8 gauge THHW is OK for up to 55 amps at 90 degrees...

Am I missing something?

Being used as a feed under ground it's limited to the 60C column, which is 40A.

 

[-dirt-]

Being used as a feed under ground it's limited to the 60C column, which is 40A.

Ah ha! Trying to put together all the different, applicable sections of the code. Thank you!

So I *could* put a 50 amp breaker on there and be out of compliance, and worse case a later inspection would just necessitate a change of breaker...

 

[-dirt-]

8/3 CU is not good for 50A, it’s only good for 40A

What size breaker is the pool pump sub panel on in the main panel? All neutrals and grounds in sub panels down stream from the main panel need to be isolated (not bonded to each other).

Just saw this question. Will check and report back. Regardless, this is more a function of the size wire from that is feeding the pool subpanel, correct?

Another question: does GFCI breaker or outlets factor into this setup?

 

[pattenp]

Yes. Example: You don't want to feed a 50A sub panel off of a sub panel that's only a 30A.

You need GFCI outlets if the floor is concrete on or below grade. Or if you're going to hook up cords and use them outside on the ground.

Just saw this question. Will check and report back. Regardless, this is more a function of the size wire from that is feeding the pool subpanel, correct?

Another question: does GFCI breaker or outlets factor into this setup?

 

[pattenp]

Cable must be marked as suitable for direct bury. THHN/THWN is not mark as such. I don't know of a code section that goes into what wire is direct bury and what's not.

310.15(b)(16) says 8 gauge THHW is OK for up to 55 amps at 90 degrees...

Am I missing something?



Can you point me to the code section for that? I can't find the burial code info and was just going on what was printed on the wire insulation.

thanks guys!

 

[-dirt-]

Cable must be marked as suitable for direct bury. THHN/THWN is not mark as such. I don't know of a code section that goes into what wire is direct bury and what's not.

I meant what section goes into the temperature variance for burial -- sorry that wasn't clear the first time.

But I am interested in what wire I have though... Here is what it has marked on it:

Type TC
3/C
8 AWG CU
THHN or THWN CONDRS
SUN. RES.
DIRECT BURY
600 Volts (UL)

 

[ishiboo]

I meant what section goes into the temperature variance for burial -- sorry that wasn't clear the first time.

But I am interested in what wire I have though... Here is what it has marked on it:

Type TC
3/C
8 AWG CU
THHN or THWN CONDRS
SUN. RES.
DIRECT BURY
600 Volts (UL)

That's Type TC cable - tray cable. The conductors inside are THHN, but they're protected by the cable.

An interesting choice, I'm not very familiar with TC specs, though it is approved for direct burial.

 

[-dirt-]

It was given to me, so there's that...

 

[-dirt-]

You need GFCI outlets if the floor is concrete on or below grade. Or if you're going to hook up cords and use them outside on the ground.

Ok, so no GFCI breakers? I thought I saw a reference to those for detached buildings...

 

[ishiboo]

Ok, so no GFCI breakers? I thought I saw a reference to those for detached buildings...

Code specifies the outlets need to be protected by GFCI under those conditions, you can do it with GFCI outlets or breakers. Both accomplish the same requirement when used properly

 

[pattenp]

This why I stated about the floor being on or below grade. If your outbuilding is on a raised foundation with a wood floor I would say GFCI are not required. But I do believe it’s best to install the GFCI outlets.

Here’s the NEC wording:
.. locations specified in 210.8(A)(1) through (8) shall have ground-fault circuit interrupter protection for personnel.
210.8 (A) (2) Garages, and also accessory buildings that have a floor located at or below grade level not intended as habitable rooms and limited to storage area, work areas, and areas of similar use.

Ok, so no GFCI breakers? I thought I saw a reference to those for detached buildings...

 

[pattenp]

NEC Article 338.10(B)(4)(b) and Article 340.80

I meant what section goes into the temperature variance for burial -- sorry that wasn't clear the first time.

But I am interested in what wire I have though... Here is what it has marked on it:

Type TC
3/C
8 AWG CU
THHN or THWN CONDRS
SUN. RES.
DIRECT BURY
600 Volts (UL)

 

[-dirt-]

Hey guys -- bringing this back from the dead. I *still* have not completed this project, but its on my spring to-do list.

I picked up some 2-2-2-4 last night to use for this instead of the TC that I have now. (The TC is currently being used as a HD extension cord, so I'll be able to retain it for that purpose.)

Can someone tell me a few things:

1) What is the maximum amperage I can use on this 80' circuit if buried directly?

2) Does this amperage change if ran in an underground conduit?

3) What is the min. required conduit size for these conductors?

4) Is there a prohibition against using "non-approved" conduit to run these conductors? I'm considering using reg. PVC to provide some protection from landscaping, etc. but if this would cause hassles I'll just throw it in the trench...

Thanks!

 

[theoldwizard1]

I picked up some 2-2-2-4 last night ...

Can someone tell me a few things:

1) What is the maximum amperage I can use on this 80' circuit if buried directly?

What is the cable type and what kind of conductor ?

 

[pattenp]

If the 2-2-2-4 is aluminum then you can over current protect it to a max of 90 amps, direct bury or in conduit. And 80' isn't a long enough run to be an issue. 1 1/2" conduit would be the smallest, but I suggest 2". To be NEC compliant you should use electrical PVC conduit.

Also in rereading your previous post I see that I gave you some wrong info on the 8/3 wire amps. I was thinking 8/3 UF which is 40A, but when you said THWN I told you 40A. The #8 THHN is 55A and can be over protected with the next size standard breaker whick is a 60A. I hope this did not cause you to buy something incorrectly.

Edit: Damn I'm having bad brain farts over this. #8 THHN is 50A not 55A. Have to look at 75 degree not 90 degree column.

 

[-dirt-]

Yes, it is AL MHF 2-2-2-4 like this:

pattenp: I got the stuff at Lowe's on the discount rack for 50% off ($0.74/ft or something like that), and I would rather keep my TC/THWN cord, so no harm, no foul.

Will using conduit with this MHF degrade the amp rating (like the 60C column mentioned in post 6 above)?

Thanks as always for everyone's help.

(EDITED to remove talk of me using reg. PVC vs. conduit. The cost difference is minimal, so I should just use conduit, if I use anything.)

 

[pattenp]

It's okay to put the MHF in conduit and it will not reduce the amp rating.

I also had to edit my last post because #8 THHN is 50A not 55A. It shows 55A in the 90 degree column but you need to use the 75 degree column which is 50A. My brain is just not working for some reason.

Edit: When you say PVC vs conduit, the plastic electrical conduit is PVC. Just don't use the white PVC plumbing pipe.

 

[-dirt-]

Yes, I mean the grey sch 80 PVC vs. water pipe.

 

[pattenp]

Since you mentioned sch 80. The PVC conduit under ground only needs to be sch 40. Any of the exposed conduit above ground only needs to be sch 80 if it's in an area that it may be subject to damage.

Yes, I mean the grey sch 80 PVC vs. water pipe.

 

[-dirt-]

That is good to know. I will "stub up" the above the ground portion with Sch. 80 since my weedeater could attack it at where it comes out of the earth at either end. But I will be sure to use Sch. 40 for the run in the trench to save a little $$.

 

[pattenp]

Depending on the length of stub out of the ground if it's subject to wide temperature swings such as freezing in the winter and being in direct hot sun in the summer you may need to install a slip joint to abate any expansion and contraction movement.

 

[-dirt-]

Like this?

Is this a code requirement as well? I have not noticed slip joints on similar connections in my area. Not opposed to this -- just learning.

Also, is 6/2 NM what I should run from the subpanel in my shed for a 50 amp circuit? This circuit will terminate at a 6-50R outlet primarily to be used for my welders. (I know dedicated welding circuits can be downgraded, but I want this circuit to to be a fully rated 50 amp circuit JIC future needs arise.)

 

[pattenp]

Slip joint is required by code only if needed. For only a few feet it's not a problem. I really shouldn't have mentioned it because it's not a problem until you get up to long lengths. With a 80 degree temp swing a 100' will expand and contract about 1 1/2".

#6 NM is good for 55A. That's good for the 6-50 outlet


*

 

[-dirt-]

Thanks for the info, as always!

 

[wyliesdiesels]

310.15(b)(16) says 8 gauge THHW is OK for up to 55 amps at 90 degrees...

Am I missing something?



Can you point me to the code section for that? I can't find the burial code info and was just going on what was printed on the wire insulation.

thanks guys!

Breakers and wire are NEVER sized based on the 90c deg column because there aren't any terminals, lugs, etc that are rated for 90c deg terminations! So what is the 90c deg column for then, u ask? Its for derating purposes such as adjusting for multiple conductors in a raceway or bundle, higher than normal(86f deg) ambient temperatures(such as in an attic) and wire ran through insulation!

So, never use the 90c deg column for sizing breakers!

 

[theoldwizard1]

HIJACK !

Looking at some wire tables online (admittedly not "official" NEC). They show amperage capacity versus wire size (at different temperatures) but have nothing about distance. Obviously the total resistance of a wire changes with distance and so does the voltage drop.

So how is distance figured into these charts ?

 

[-dirt-]

Breakers and wire are NEVER sized based on the 90c deg column because there aren't any terminals, lugs, etc that are rated for 90c deg terminations! So what is the 90c deg column for then, u ask? Its for derating purposes such as adjusting for multiple conductors in a raceway or bundle, higher than normal(86f deg) ambient temperatures(such as in an attic) and wire ran through insulation!

So, never use the 90c deg column for sizing breakers!

That makes sense. Thanks!

 

[tylernt]

Obviously the total resistance of a wire changes with distance and so does the voltage drop.

So how is distance figured into these charts ?

You do a separate calculation. I believe the limit is 5%. There's a calculator here:

http://www.electrician2.com/calculators/wireocpd_ver_1.html

 

[wyliesdiesels]

I do all my VD calc by hand(cause i love #s!). If u want the formula i can post it!

 

[theoldwizard1]

I do all my VD calc by hand(cause i love #s!). If u want the formula i can post it!

Post away !

I'll just put it into a spread sheet !

 

[wyliesdiesels]

These formulas are for single phase circuits:

Vd= 2K * L * I / Cm <--- this will tell u what the voltage drop will be(in volts NOT percents) using a particular size(in circular mils) and type of wire

CM= 2K * L * I / Vd(in volts NOT percent) <---- this will tell what size wire u need for a particular voltage drop; for 120v branch circuits(5% max Vd recommended by NEC) the # to use would be 6, for 240v use 12; for 240 feeders(3% max Vd recommended by NEC) use 7.2...If u use this formula, remember to make sure that the wire size gotten with this formula has an ampacity equal to or greater than the load/breaker protecting it!

Key:

Vd= voltage drop
I = amps
L = one way length of circuit
Cm = circular mills of conductor
K = resistance in ohms of 1 circular mil foot of conductor
K = 12.9 for Cu wire @ 75c deg
21.2 for Al wire " " "

For cir mils referencing(when i dont have my books in front of me) i go here:

http://www.engineeringtoolbox.com/awg-wire-gauge-circular-mils-d_819.html

 

[-dirt-]

Still working on this project. Been working on the trench a little along and hope to finish by this weekend. 75' or so hand dug to ~20".

Picked up 12' of 6/3 w/ ground off the discount rack at Lowe's today for $1/ft. (!!) I plan to use this for my 6-50R welding outlets.

Going to pick up the conduit once the trench is finished. I plan to pull the wire a section at a time as it's laid unless someone has a better idea. I do not have a fish tape.

Regular PVC glue (like for water pipe) is what I should use for connections, right?

 

[pattenp]

Do you own a shop vacuum? Glue up all the pipe with regular PVC glue and tie a small piece of cloth on to a light string and **** it through with shop vac. Then tie on a piece of rope and pull back through and use the rope for pulling the wire. Your way will work too.

 

[-dirt-]

That sounds like it should work.

Is there a maximum size breaker box/panel I can use in the shop off this circuit? I'm thinking I may want to replace my existing one with a bit larger one.

My current one has maybe 6 breakers in a horizontal row w/ no main disconnect.

 

[tylernt]

Is there a maximum size breaker box/panel I can use in the shop off this circuit? I'm thinking I may want to replace my existing one with a bit larger one.

Physical size, no, you can go as big as you want. But if it has more than 6 handles, it must include a Main breaker or disconnect so that all power to the structure can be killed with one throw.