wiring question

hot_rod

I have a 3/4 grey PVC conduit between the house and garage, a former homeowner installation with a run of #12 romex, about 60' from breaker box to shop.
.
I'd like to replace that with a 240V circuit for my welder or heat pump.

There seem to be different fill tables and current carrying capacity charts online.

3 conductors of #6 THHN, for a 50A circuit?

PC7060

@hot_rod - per the attached you can run four #6 inside of the 3/4 schedule 40 conduit. 

I believe you are fine for that distance; the #6 is rate for 65 amp for THHN which give you > 25% margin over the 50 amp breaker rating. 

mattmia2

You will need a grounding electrode system and a disconnect for the building. that single circuit is installed under an exception that allows you have a single circuit in an outbuiding without meeting the requirements for the service to a building. once you install a subpanel and multiple circuits, you need to follow the rules for a service to a building.

you should be calculating the conductor size based on the load and voltage drop, not on the overcurrent protection. you figure the load and work backward to the conductor size and overcurrent protection.

hot_rod

Thanks, would a 50A breaker in the house and a small sub panel with a ground stake be what the code requires?  Mainly a  single 50 receptacle in the shop for either welder, compressor or plasma cutter, a few lights and 120V receptacles.
Can the neutral be reduced? Or all 3 conductors need to be #6?

ChrisJ

I ran #6 wire in 3/4 pvc for my generator and it was tight.   As long as there's not many turns it'll work.   That was four #6 copper conductors. 


I'm not positive but I would expect you can run a #8 for ground but neutral probably needs to be full size.

jad3675

Another idea - have a separate meter and feed run to the out building by your utility? Duke, here in Ohio, does it free as long as you keep service for 12 months. 50 amps isn't much for a shop; tools tend to reproduce. First you start with a TIG and Plasma cutter, next thing you know you have a blast furnace and you're making your own hydronic fittings.

PC7060

hot_rod said:

Thanks, would a 50A breaker in the house and a small sub panel with a ground stake be what the code requires?  Mainly a  single 50 receptacle in the shop for either welder, compressor or plasma cutter, a few lights and 120V receptacles.
Can the neutral be reduced? Or all 3 conductors need to be #6?

The 3rd wire will need to be the safety ground for 220.  if you want to support 110 in addition to 220 you will need a full size neutral plus a 4th ground wire  which can be as small as 10ga copper for 30-60A or 8ga for 70-100A. 

You will also need a 8’ ground rod as @mattmia2 said. I find a rotary hammer with attachment  drives the rod very easily.  

Zman

My understanding is that you would need (3)-#6 and (1)-#8 ground for a subpanel. If it is a straight run with just the sweeps on the ends you may be able to pull it. Have you been working out?
@EBEBRATT-Ed would know this for sure.

mattmia2

The ground can be bare which will help somewhat with fill. use pulling compound. Might need to use a small rope rather than just pull string

Zman

I think stranded wire for everything including the ground will be a must. Lots of lube. The heavy-duty flat pull rope will guaranty it won't break or cut into the sweeps.

EBEBRATT-Ed

For a 50A circuit from the house to a garage sub panel you can use a #10 equipment ground bare or with green insulation. (#10 is good for up to 60 amps.)

You can reduce the neutral but it has to be large enough for whatever 120 volt loads you have on the garage panel.

As far as the size wire for the hot legs #6 copper is good for 55a @60c and 65 a @75C
#8 copper 40a 55
#6 aluminum 40a 55a

If the breaker you come off of and the sub panel terminals are rated 75c you can size the wire at 75c and use the smaller wire.

Hold onto your wallet wire prices are up, hence the aluminum

When you get into the sub panel the neutrals and grounds get separated. grounds to a ground bar screwed to the panel, neutral bar is isolated from the panel neutrals go there, No bonding jumper in the panel (connection between neutral bar and ground....panel frame)

Looks like you will need a ground rod connected to the garage panel as this will be a feeder and not a branch circuit. I would use #8 minimum from the rod to the panel

Zman

Yup,
Like he said!

mattmia2

Is the ground to the ground rod #8 or #6? The ampacity probably dictates #10, but if it is below a certain size you have to protect it so that size that doesn't require protection becomes the practical minimum.

If you make the neutral smaller then you have to re-do your load calculations every time you add a 120v circuit to make sure the new circuit can't overload the neutral.

Zman

BTW, I think 50 amps will serve you well. I am always amazed by how much stuff you can run off a 50 amp construction turtle box. Unless you are getting into really heavy materials, welders and plasma cutters rarely pull more than 20 amp 240.

EBEBRATT-Ed

@mattmia2

Your right, #8 ground needs to be in a raceway, #6 needs protection "if subject to physical damage" so I use #4 even for a 100 amp service so I don't have to fool with a raceway or worry about it.

At least that's the way I read it. The code is more confusing than ever.

As far as the neutral goes it only carries the imbalance so if you had 4 20 amp circuits fully loaded the neutral could only see 40 amps max if you had 2 20s on each leg

Tim Potter

https://www.southwire.com/calculator-conduit

Handy conduit fill calculator, go with full size neutral but you can downsize the EGC (green or bare)

https://www.jadelearning.com/blog/sizing-equipment-grounding-conductors-table-250-122/

Tim

mattmia2

Pay attention to your state's requirements about where gfci and afci is required.

Alan (California Radiant) Forbes

Hot Rod: Don't you live on Know It All Lane?

hot_rod

Alan (California Radiant) Forbes said:

Hot Rod: Don't you live on Know It All Lane?

Nope, we moved off the lane a few months ago, back in Utah. So I had to ask

ChrisJ

I have a question that kind of relates to this thread.

How does the next size up breaker rule work with sub panels.

IE #6 romex is ok for 55A. When are we allowed to use a 60A breaker with it on a sub panel?

#6 thhn is rated 65A at 75C.  When can we use a 70A?

mattmia2

You can go up to the next standard size of fuse or breaker if the ampacity of the conductors is not a standard size. You should be sizing your conductors based on your load calculations. The breaker or fuse is really there to protect against a failure, not against an overload because you failed to do correct load calculations. The breaker or fuse won't protect against an overload under most conditions even if it matches the ampacity of the conductors.

There are some exceptions for 15-30 a circuits because the ampacity of those conductors is by decree in a rule of the code rather than from engineering calculations.

ChrisJ

mattmia2 said:

You can go up to the next standard size of fuse or breaker if the ampacity of the conductors is not a standard size. You should be sizing your conductors based on your load calculations. The breaker or fuse is really there to protect against a failure, not against an overload because you failed to do correct load calculations. The breaker or fuse won't protect against an overload under most conditions even if it matches the ampacity of the conductors. There are some exceptions for 15-30 a circuits because the ampacity of those conductors is by decree in a rule of the code rather than from engineering calculations.

So the main breaker in a panel doesn't protect the service against overload?

EBEBRATT-Ed

Yes @mattmia2

is correct. First you calculate the load on the conductors. The you derate the ampacity of the wires for two things if need be

#1 if the number of current carrying conductors exceeds 3 (grounds don't count , neutrals do sometimes) wires in the conduit or raceway you derate for that

#2 Derate for temperature. Heat is the wires enemy. So wires run on a roof etc or other hot location may need to be derated

Sometimes you have to derate for both issues

If the wire is rated 90 deg C you can use the 90C collumn to derate from.

Then there is the temp rating of the breaker terminals , panel terminals to consider.

As a general rule anything under 100 amps is considered to be 60 deg C anything over 100 amps is considered to be 75 deg C unless otherwise marked.


It is very confusing. So if someone says "what is the ampacity of a #4 copper wire". It all depends on the conditions it is installed under.

So once you determine the wire size you need after derating (if necessary) and you come up with the wire ampacity and it doesn't = a standard size breaker or fuse you can go p to the next size breaker.

And this doesn't apply to motor circuits which is another story

You can have an ac condensing unit with #14s run to it and the MFg calls for a 25amp breaker and it is fine

Tha'ts why the code book is as thick as the Boston yellow pages.................and getting worse

ChrisJ

@EBEBRATT-Ed I see nothing confusing there.

I used 75C for thhn as that's what QO breaker terminals and panels are rated for.

So after reading that my interpretation is nm-b #6 needing no other deratings is good for 55A and since that's not a standard size we can go to 60A for a breaker or fuse?

But what about my last question?  So the main breaker doesn't protect the service from overload?   

EBEBRATT-Ed

yes, you can use a 60 amp breaker on #6s in normal circumstances.

Services are sized by doing a load calculation. Most breakers can only handle a continuous load that is 80% of their rating.

Yes the breaker prevents overload but

The code gives you a gift on residential work.

Look up the rating of #4 copper. The code will let you use #4 on a 100 amp service or #2 aluminum.

On a 200 amp you use 4/0 Aluminum or 2/0 copper which are not rated 200 amps

They have decided for many years that when you calculate the service size by the code (for dwellings only) that the conductors only need to be 83% of the main breaker. This is due to diversity, everything isn't turned on at the same time.

Only applies to dwellings, not commercial or industrial

ChrisJ

@EBEBRATT-Ed
How are misc branch circuits used in calculating the load a sub panel? Appliances, drill presses, welders etc are easy, you just go by the name plate. But what about typical circuits with receptacles and no actual items permanently plugged in?

ratio

Residential code is a thing apart, it gives you demand factors to use for circuits based on where they're located.

I expect it'll (residential code) get pulled out of NFPA 70 (NEC) into its own separate doc one of these days.

Ha, I just looked, it is: NFPA 70A National Electrical Code® Requirements for One- and Two-Family Dwellings. Looks like it's still included in NFPA 70 too, though.

mattmia2

Look at the code, it tells you how to calculate the load. Most breakers will hold at 105% rating indefinitely so it is important to calculate continuous loads correctly. Because of the demand factor built in to the residential service conductor table, it is even more important to do the load calculations to prevent overloading the conductors than for other feeders.

Steve Minnich

Last month I did something similar. I added a 40 amp sub-panel to my attached garage/shop. Three #8s and a #10 ground servicing multiple saws, drill press, Refrigerator,  my boiler, ductless mini split, multiple outlets for three different benches, and an air compressor. 

EBEBRATT-Ed

@ChrisJ

part of the calculation for a dwelling unit which includes 1 family, 2 family and multifamly is to include 3 watts/square foot. This covers enough so nothing else need be added for lighting and general use receptacles that will be used for lighting, vacuum cleaners, radio, TV computers etc

That does not include other things that may be cord and plug connected like dishwashers, disposals refrigerators etc.