# More Clarification on Detached Garage Sub-Panel



## wirenut1110 (Apr 26, 2008)

Sounds like a good plan without going into great detail. Just don't forget the ground rods. You may be able to use a 30 amp for your disconnect. I'm not sure if this counts as a 2 circuit but I'm sure someone will clarify.


----------



## theatretch85 (May 17, 2008)

```
225.39 Rating of Disconnect. The feeder or branch-circuit
disconnecting means shall have a rating of not less than the
load to be supplied, determined in accordance with Parts I
and II of Article 220 for branch circuits, Parts III or IV of
Article 220 for feeders, or Part V of Article 220 for farm
loads. In no case shall the rating be lower than specified in
225.39(A), (B), (C), or (D).
(A) One-Circuit Installation. For installations to supply
only limited loads of a single branch circuit, the branch
circuit disconnecting means shall have a rating of not less
than 15 amperes.
(B) Two-Circuit Installations. For installations consisting
of not more than two 2-wire branch circuits, the feeder or
branch-circuit disconnecting means shall have a rating of
not less than 30 amperes.
(C) One-Family Dwelling. For a one-family dwelling, the
feeder disconnecting means shall have a rating of not less
than 100 amperes, 3-wire.
(D) All Others. For all other installations, the feeder or
branch-circuit disconnecting means shall have a rating of
not less than 60 amperes.
```
I don't think this subpanel would fall under the two circuit installation. The big box stores around here the 30 amp double pole costs the same as the 60 amp double pole; I'd just get the 60 amp and install that in the detached structure and be done with it. It is certainly not a one family dwelling and therefore does not need to be 100 amps.


----------



## Stubbie (Jan 7, 2007)

You will need to put a hold down kit on the 60 amp breaker....:thumbsup: Here is an example if you are not aware of this requirement.


----------



## jamiedolan (Sep 2, 2008)

Stubbie said:


> You will need to put a hold down kit on the 60 amp breaker....:thumbsup: Here is an example if you are not aware of this requirement.


When an external disconnect is used prior to the panel, you can use main lugs, correct?

Jamie


----------



## J. V. (Jun 1, 2007)

jamiedolan said:


> When an external disconnect is used prior to the panel, you can use main lugs, correct?
> 
> Jamie


Yes, and then no main breaker is required. Yes, at least one ground rod. I would install two and space them 6' apart.


----------



## joed (Mar 13, 2005)

jamiedolan said:


> When an external disconnect is used prior to the panel, you can use main lugs, correct?
> 
> Jamie


If the external disconnect is not in the same building then as long as the panel can only hold a maximum of 6 breakers I believe you are correct.


----------



## jerryh3 (Dec 10, 2007)

joed said:


> If the external disconnect is not in the same building then as long as the panel can only hold a maximum of 6 breakers I believe you are correct.


I thinks he's referring to if the disconnect is on the outside of the detached structure.


----------



## 220/221 (Oct 9, 2007)

(


> D) All Others. For all other installations, the *feeder or*
> *branch-circuit disconnecting means *shall have a rating of
> not less than 60 amperes.


Interesting. I wonder what the reasoning is here?

Any theories?

While I'm at it, does this infer that the minimum circuit to *all others* is 60 amp?

The feeder disco would be at the main panel so.....?

Crap. If so, I've violated that more than a few times. For me, a typical sub would be #8's and a 50.




> (B) Two-Circuit Installations. For installations consisting
> of not more than two 2-wire branch circuits, the feeder or
> branch-circuit disconnecting means shall have a rating of
> not less than *30* amperes.


Does this say I can't run #12 wires for two circuits to a detached structure?

I'm confused.


----------



## WaldenL (Jan 9, 2009)

J. V. said:


> Yes, at least one ground rod. I would install two and space them 6' apart.


OK, I'm confused by the grounding rod comments. He's running 4-wire, so that's two hot, one neutral and one EGC, no? And as a sub-panel the neutral and grounding buses should _not_ be bonded. By adding a second ground path to the system aren't we messing it up?


----------



## Stubbie (Jan 7, 2007)

No....ground rods have nothing to do with the egc and ground fault paths.


----------



## WaldenL (Jan 9, 2009)

The ground rods are connected to the ground bus in the panel though, right? So the EGC path includes the local ground rods, but the neutral path doesn't as the two buses aren't bonded. EGC is fastest path to the earth, so the ground rods near the sub-panel provide that path. And that's why you don't bond the neutral and ground buses. Do I have it now?


----------



## joed (Mar 13, 2005)

Ground rods are for lightning not for actual circuit grounding.


----------



## Stubbie (Jan 7, 2007)

WaldenL said:


> The ground rods are connected to the ground bus in the panel though, right? So the EGC path includes the local ground rods, but the neutral path doesn't as the two buses aren't bonded. EGC is fastest path to the earth, so the ground rods near the sub-panel provide that path. And that's why you don't bond the neutral and ground buses. Do I have it now?


There is no fastest path to earth..... You are forgetting that current seeks the source (transformer) not earth. Though it may use 
earth if it is the *only* path. You are also forgetting that the path to earth through the GES and ground rods is at much higher impedance/resistance than over the egc. Current will always seek the source over the least impedance before using a higher impedance/resistance. A ground fault with earth as the only path to the source will likely not trip a breaker and in fact most times it won't. Not enough current can flow to trip the breaker due to the high impedance/resistance.

Ground rods provide no path for the egc they provide a path for high voltage.. high current.. events like lighting that are at levels way higher than residential voltages. It takes those kinds of events to get earth to be forced, if you will into, a conductor of appreciable amounts of current. 

The egc on the other hand is part of what is called the effective ground fault path which is an intentional low impedance path that we build to facilitate fault current getting to the source to allow enough current to flow to trip a breaker. 

So any fault that occurs at or on a branch circuit at a sub-panel fed by 4 wires will flow on the feeder equipment ground back to the service panel where it is bonded with the service neutral. Then out the service neutral to the center tap of the transformer (source). It will not take a path to ground when given this much lower impedance to flow on.

See the diagram below maybe it will make more sense.


----------



## 220/221 (Oct 9, 2007)

> (B) Two-Circuit Installations. For installations consisting
> of not more than two 2-wire branch circuits, the *feeder* or
> branch-circuit disconnecting means shall have a rating of
> not less than *30 amperes*.
> ...


 
Still confused. Anybody??

B = No 20 amp two circuit installations allowed?

D = No 50 amp circuits allowed?


----------



## chris75 (Aug 25, 2007)

220/221 said:


> Still confused. Anybody??
> 
> B = No 20 amp two circuit installations allowed?
> 
> D = No 50 amp circuits allowed?


 
Your looking at *disconnect* requirements. Not feeder requirements.

Re-read 225.39


----------



## Scuba_Dave (Jan 16, 2009)

Ok, so that refers you back to 220 III - which basically indicates sum of the loads as the deciding factor


----------



## theatretch85 (May 17, 2008)

220/221 said:


> The feeder disco would be at the main panel so.....?
> 
> Crap. If so, I've violated that more than a few times. For me, a typical sub would be #8's and a 50.



No. The OCPD (Over Current Protection Device) or your circuit breaker is at the main panel. This is sized to protect the wires. The Disconnect is installed at the detached structure and is basically just a simple switch to simply disconnect the service with one "switch". Most often a fused disconnect is used (circuit breaker). No matter whether you use a non-fused disconnect or a main breaker at the detached structure, point D is saying that the disconnect must be rated for NO LESS THAN 60 amps. 

Your feeder could be 40 or 50 amps with #8 or #6 wire, but your disconnect at the detached structure by code must be rated for no less than 60 amps. Anything greater than the OCPD at the source is just simply a disconnect and nothing more, since an overload would trip the OCPD in the main panel.


----------



## WaldenL (Jan 9, 2009)

Stubbie said:


> Ground rods provide no path for the egc they provide a path for high voltage.. high current.. events like lighting that are at levels way higher than residential voltages. It takes those kinds of events to get earth to be forced, if you will into, a conductor of appreciable amounts of current.


Damn! You know how there's that moment when you get it (um, the lightbulb comes on, since this is an electrical discussion) and then you look back and say "How did I have a problem with that before, it's so simple"... Well, that line (and the rest of your post) was that lightbulb! Damn that seems so obvious now. 

Appreciate the help!


----------

