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Old 10-16-2010, 02:07 AM   #1
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I'm wanting to run service, underground, to a detached garage. I currently have 100 amp service in the house, via breaker panel, and am wondering if a 50amp service to the garage would be sufficient. In the garage, will be an air compressor, and a welder, as well as general lighting. Can anyone tell me how to determine what the ampacity of these machines would require? I know there's a nameplate on them, but I'm not sure which ampreage rating I should go by: output current?

What would be the recomendations on wire size for the service, assuming 50 amp; will 8 AWG be sufficient there? I have 50' of 8/3 with ground. It's Romex, so I assume that I could run the cable through PVC underground, would that be correct?

Do I need to install two ground rods at the garage, and at what depth should they be driven?

My main concern right now is with the ampacity of the cable i want to use (8/3 w/g). The NEC chart I looked at online gave different ampacities for the cable based on temperature, and I need an explanation as to what the significant difference would be between the ddifferent temp ratings.

Thanks in advance,

~ssa

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Old 10-16-2010, 02:22 AM   #2
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First of all, you can NOT install type NM cable into an underground conduit. Forget about trying to use that cable for this project.

Let's take a look at that welder. Does it have a cord and plug attached? Many single phase welders have a NEMA6-50P cord attached, which requires a 50 Amp 240 Volt circuit and receptacle. Others only require a 120 Volt circuit. So we need to determine the size of the unit, and its power requirements.

Next, the air compressor. Standard cord and plug? Or direct-wire large unit?

INPUT voltage and amps are what we need to look at. Nameplate information is critical here.

Do NOT rely on any bogus HP claims on the air compressor unit. If the hp rating is blank on the motor's nameplate, then it is overrated, and we need to look at the Amps as well.

Out buildings that require ground rods usually have 2 rods buried a minimum of 8 feet, placed at least 6 feet apart.

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Old 10-16-2010, 02:31 AM   #3
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kb, thanks for the quick reply. I will need to take a look at the air compressor and welder tomorrow; it's at another location, so I don't have the info redily available to me.

As to the underground cable, what will I need for this; UF? And what method used to install it? I assume it will not run in conduit then?
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Old 10-16-2010, 02:37 AM   #4
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Originally Posted by sirsparksalot View Post
I'm wanting to run service, underground, to a detached garage. I currently have 100 amp service in the house, via breaker panel, and am wondering if a 50amp service to the garage would be sufficient. In the garage, will be an air compressor, and a welder, as well as general lighting. Can anyone tell me how to determine what the ampacity of these machines would require? I know there's a nameplate on them, but I'm not sure which ampreage rating I should go by: output current?
For the welder genrally input rating that what I count never the output at all and this is a wirefeed welder or stick welder ? that will make the diffrence.

For the air compressor look at the motor nameplate and also if dual voltage set up you have to decided which way you want to wired on 120 or 240 volts and I will really recomend this to be on own circuit.

For genral lighting you can have it on either 15 or 20 amp circuit depending on the conductor size if you have 2.5mm˛{#14 AWG } it have to be on 15 amp while the 4.0mm˛ { #12 AWG } use on 20 amp circuit.

ditto with genral recetpales and I will advise ya to run at least two circuits on this one and yeah all the 120 volts item are required to be on RCD { GFCI } unless overridden with local codes { check with them first before you do anything }

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What would be the recomendations on wire size for the service, assuming 50 amp; will 8 AWG be sufficient there? I have 50' of 8/3 with ground. It's Romex, so I assume that I could run the cable through PVC underground, would that be correct?
If 10mm˛triplex { #8-3 } NM you are only restricted to 40 amp on copper conductor and no you can not run the NM in the conduit at all espcally with underground run that is a wet location anyway.

The only way you can run full 50 amp is run 16mm˛ { #6 AWG } in both THHN / THWN and NM cables then it will be not a issue at all

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Do I need to install two ground rods at the garage, and at what depth should they be driven?
Oui you do need two ground rods and you need them at full length { typically 8 footers } and drive them all the way down to the ground and sink it lower and keep the rods at least 2 meter { little over 6 feet } apart and you can use 10mm˛ or 16mm˛ bare or green conductor { I rather run 16mm˛ and some inspectors can get picky on this one }

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My main concern right now is with the ampacity of the cable i want to use (8/3 w/g). The NEC chart I looked at online gave different ampacities for the cable based on temperature, and I need an explanation as to what the significant difference would be between the ddifferent temp ratings.

Thanks in advance,

~ssa
There are quite few diffrent way to deal with the ampacity rating and all the NM and UF cables are only rated at 60°C so use the 60°C chart not other two { 75°C or 90°C }

The THHN/THWN can use 75°C rating

The 90°C rating is only used for derating if you have more than few conductos in the conduit or bundling etc etc

Oh yeah .,, before I forget anything else you will need minum of 1 inch conduit I know you can use 3/4 inch but 1 inch is easier to pull.

And on the subpanel make sure you buy the grounding bussbar due some of them don't have it and also remove the bonding screw or strap one of the two. and keep the netural and ground seperated on subpanel.

If more question just holler.
Merci.
Marc
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Old 10-16-2010, 12:11 PM   #5
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Marc,

Thank you. I think I understand. Can you tell me the safety issues against running the NM through conduit underground, and what do I need to run and how? Should I run UF underground w/ no conduit? I'd really like to know the safety concerns.

Thanks.
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Old 10-16-2010, 01:20 PM   #6
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The electrical code prohibits NM cable usage in wet areas, even in coduit. UF can be used without conduit except where it emerges from the ground. In that case it needs the protection of conduit.
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Old 10-17-2010, 01:35 AM   #7
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I will be using UF-B for the run to the garage.

Understood on NEC's no use of NM-B underground, even within conduit. Still, however, I don't understand how the cable could be damaged by water if it's enclosed in PVC, considering that the PVC run would go all the way from inside the house (attached to the breaker panel), underground, and attached to the subpanel inside the garage. How would water be able to get in the PVC?

Nevertheless, the text quoted below indicates a minimum burial depth of 24 inches IF it's buried under parts where heavy traffic would be encountered. My installation will be under the yard, with only pedestrian traffic, so am I correct that I could bury it at 18 inches?

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PVC Conduit

When UF Cable is installed in PVC conduit and buried, the minimum burial depths are less. For example, this type of installation in soil with only pedestrian traffic has a minimal burial depth of 18 inches. This is 6 inches shallower than what would be required [24"] without the conduit. When installed under a concrete floor that has a minimum of two inches of concrete, the mimimum burial depth is 12 inches. Again this is 6 inches less than what would be required without the conduit, The minimum burial depth for installations under streets, highways, roads, alleys, driveways, and parking lots is 24 inches. This is the same requirement as would be enforced without a conduit. The minumun depth for a one- or two-family dwelling driveway is 18 inches, Again, this is the same as what would be required without the conduit.
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Old 10-17-2010, 02:17 AM   #8
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Quote:
Originally Posted by sirsparksalot View Post
I will be using UF-B for the run to the garage.

Understood on NEC's no use of NM-B underground, even within conduit. Still, however, I don't understand how the cable could be damaged by water if it's enclosed in PVC, considering that the PVC run would go all the way from inside the house (attached to the breaker panel), underground, and attached to the subpanel inside the garage. How would water be able to get in the PVC?

Nevertheless, the text quoted below indicates a minimum burial depth of 24 inches IF it's buried under parts where heavy traffic would be encountered. My installation will be under the yard, with only pedestrian traffic, so am I correct that I could bury it at 18 inches?
The quick way the conduit can get water is condesation or poor covered fitting and they will stay in the lowest part of the whole conduit.

The conduit you can buried them at 18 inches unless stated by local codes.

Merci.
Marc
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Old 10-17-2010, 02:25 AM   #9
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thanks Marc. I think I'm on my way
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Old 10-17-2010, 09:42 PM   #10
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OK, we've decided to update the service to 200 amps, and are installing a new Breaker panel in the house.

We're running 6/3 w/g from a 50 amp breaker in the house to a 100 amp Main lug for the garage. My question refers to this Main Lug Only panel. It is rated at 100 amps, and we will have a 50 amp breaker from there for the Welder (dedicated circuit). We also will want lighting and a few recepticles, however, so I need to know how to make that work. Should I install a separate 20 amp breaker in the Main Lug for the lighting and recepticles? This seems weird to me, as even though the main lug is rated for 100 amps, it's supply is only 50, and obviously, we won't be able to use the welder while having lights on at the same time (a defeated purpose, it seems).

We've already spent money on the 6/3, and now it seems that was a waste because I think we're going to need at least 70 amps out there, requiring bigger cable.

Does any of this make sense? I know it doesn't, please help!
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Old 10-17-2010, 11:59 PM   #11
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You can able run the welder and lights on the same time in the garage without major issue but expect some flicker to show up a little with it during welding running { some will flicker a bit and some don't flicker much at all }

Tell me did you brought 50 foot worth of 6/3 NM??

If you did just save it you will need for welder circuit in couple spots.{ or electric heater in case you go that route }

And with 100 amp subpanel in garage it is not a issue you can have serveral circuits from it and the last decent sized garage I did install simauir to your requirement it was 8 circuits total.

To run full 100 amp from the house to garage you will need 50mm˛(#1 Alum) or 35mm˛(#2 copper ) in conduit otherwise you can get by using 35mm˛ Alum with 90 amp breaker to save couple extra bucks right there.

For the conduit on this size you will need at least 1.25 inch or larger conduit { I will recomened 1.5 or larger }

Make sure you get grounding bussbar for your subpanel in garage breaker box and keep the ground and netural seperated.

Merci.
Marc
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Old 10-18-2010, 12:33 AM   #12
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You can able run the welder and lights on the same time in the garage without major issue but expect some flicker to show up a little with it during welding running { some will flicker a bit and some don't flicker much at all }

Tell me did you brought 50 foot worth of 6/3 NM??

If you did just save it you will need for welder circuit in couple spots.{ or electric heater in case you go that route }

And with 100 amp subpanel in garage it is not a issue you can have serveral circuits from it and the last decent sized garage I did install simauir to your requirement it was 8 circuits total.

To run full 100 amp from the house to garage you will need 50mm˛(#1 Alum) or 35mm˛(#2 copper ) in conduit otherwise you can get by using 35mm˛ Alum with 90 amp breaker to save couple extra bucks right there.

For the conduit on this size you will need at least 1.25 inch or larger conduit { I will recomened 1.5 or larger }

Make sure you get grounding bussbar for your subpanel in garage breaker box and keep the ground and netural seperated.

Merci.
Marc
Marc, That welder had a nameplate rating of 50 amps, so I'm just wondering if all will really work. Also the guy at Home Depot calling himself a licensed electician said that the welder would be an inductive type motor, and that therefore it had to be derated 20%, meaning that I should cover it with 60 amps and #4awg. But then he also said that I could run NM-B in PVC underground, so I take his advice w/ grain of salt. This is gonna be a costly experience, I'm afraid, but it has to be done now, as the work has already started.

Also, the Main Lug. It has no main breaker, so I assume that the only disconnects will be within the house in the main panel, and with the individual breakers in the sub panel? Do I need to install a separate disconnect somewhere?
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Old 10-18-2010, 12:50 AM   #13
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Quote:
Originally Posted by sirsparksalot View Post
Marc, That welder had a nameplate rating of 50 amps, so I'm just wondering if all will really work. Also the guy at Home Depot calling himself a licensed electician said that the welder would be an inductive type motor, and that therefore it had to be derated 20%, meaning that I should cover it with 60 amps and #4awg. But then he also said that I could run NM-B in PVC underground, so I take his advice w/ grain of salt. This is gonna be a costly experience, I'm afraid, but it has to be done now, as the work has already started.

Also, the Main Lug. It has no main breaker, so I assume that the only disconnects will be within the house in the main panel, and with the individual breakers in the sub panel? Do I need to install a separate disconnect somewhere?
First thing I will tell ya I never trust the Big Orange guy at all due majtory of them is not really qualifed at all and super rare you will snag a real qualifed person whom know the electrical system very well..

Now for welder as long the input current is 50 amp that is not a issue I have ran into this set up many time { the real current useage will float about 35 or so amp depending on setting on buzzbox welder }

What size main lug box you have there and please run the model number for it so one of us will know what is legit and what not and all detached garage you will need a common disconnect source aka main breaker if you get over 6 throws.

If you did not open the box of the load centre before you do that please tell us the model number and we will tell you what is the correct one to use. Normally I will buy a load centre with main breaker { it useally cheaper than MLO }

As Far for NM in the conduit for underground run that is not allowed peroid so you can see why I never trust the big box guys at all they will not always give you a correct info.

You will have to follow the listing above what I mention to order to get this set up legit.

Merci.
Marc
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Old 10-18-2010, 01:15 AM   #14
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What size main lug box you have there and please run the model number for it so one of us will know what is legit and what not
100 amp MLO, HOM612L100FCP
http://www.hardwareandtools.com/Squa...r-6720981.html

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... and all detached garage you will need a common disconnect source aka main breaker if you get over 6 throws.
What is meant by 6 throws? Is this 6 breakers?

Quote:
If you did not open the box of the load centre before you do that please tell us the model number and we will tell you what is the correct one to use. Normally I will buy a load centre with main breaker { it useally cheaper than MLO }
I actually have the old 100 amp load center from the house, because we replaced it with 200 amp. Could I use that in the garage even though the feed would be less than the ampacity of the Main Breaker? I'm thinking the feed from the house to the garage will be about 70amp, while the main breaker would be 100 and would never trip.
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Old 10-18-2010, 01:24 AM   #15
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I have see that verison pretty often that is ok with small numbers of circuits.



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What is meant by 6 throws? Is this 6 breakers?
6 handle no matter if single or multipole verison.



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I actually have the old 100 amp load center from the house, because we replaced it with 200 amp. Could I use that in the garage even though the feed would be less than the ampacity of the Main Breaker? I'm thinking the feed from the house to the garage will be about 70amp, while the main breaker would be 100 and would never trip.


Why not use that and just buy a grounding bus bar and that is good to go as long it is very good shape and you can sized like 90 amp conductor size as I mention above the whole thing will not trip at all if sized properly and that main breaker will act like disconnect switch due you will have more than 6 throw anyway.

Merci.
Marc

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