Two Circuits to a Garage

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Old 11-27-07, 07:04 AM
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Two Circuits to a Garage

Can someone explain to me why having 2 circuits from a house to a detached garage is a code violation? I've never been able to get an explanation as to why this is is a code violation / unsafe. Can anyone explain the reason for this?
 
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Old 11-27-07, 08:09 AM
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225.30 Number of Supplies

Where more than one building or other structure is on the same property and under single management, each additional building or other structure that is served by a branch circuit or feeder on the load side of the service disconnecting means shall be supplied by only one feeder or branch circuit, unless...


(Followed by numerous exceptions) The explanation for this refers to the similar requirements for a single service as laid out in 230.2

As for the reason, I'll just point you to this explanation which lists some: (Scroll to question #5 if your browser does not automatically)

http://www.iaei.org/subscriber/foc/cmp4.htm#q5

-core
 
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Old 11-27-07, 08:36 AM
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So if I'm reading it right, the safety concern is that you have a single disconnect for an external building?

I don't understand the second part about the grounding, but it sounds like the existence of multiple circuits can create a safety hazard in an of itself, outside of the single point of disconnect. Can this be explained in a little more plain English for my simple mind? Does it mean that multiple paths can cause a shock hazard? If so, under what conditions does this exist?

Also, I seem to recall someone mentioning an exception for a 3 way switch of some sort?
 
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Old 11-27-07, 08:49 AM
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The exception is that you may add a circuit that is a lighting circuit only, and only when the light is to be controlled from the other building. In other words, you can have a light at the detached garage which is controlled completely or in part by a switch at the house. The circuit for the light is allowed as an exception to the rule. This exception is for safety. They don't want you to have walk to/from the garage in the dark.

The issue with grounding is very complex, and a discussion on grounding could easily fill numerous pages in a book. The gist of the rule is that you want a single electrical ground path between the buildings.
 
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Old 11-27-07, 09:26 AM
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The hazard is that the equipment grounds or grounded conductors (neutrals) of the two circuits could be at potential relative to each other. You could touch the ground of circuit A and the ground of circuit B and actually get shocked from that because the grounds are derived from separate circuits and possibly not interconnected.

This scenario becomes much more plausible if you consider two tools with metal frames, say a lathe and a welder, sitting next to each other in a workshop, each fed with separate circuits from the main building. The chances of a person touching both metal frames at the same time is pretty high. You would expect that both grounded frames are safe to touch, but if fed from different circuits, you could be shocked. Add to that the complication that you may have a concrete floor "reference ground" that is at a different potential than either of the tools.

The greater the distance from the main panel, the greater the potential for shock. Since outbuildings are usually a long distance away, the chances are increased when dealing with separate structures that each have separate foundations with different reference grounds. Requiring that buildings have only one feed and centralized bonding and grounding at the point of entrance helps to mitigate these problems. There's a lot of technical stuff to digest on the topic of grounding, but I hope this provides a little insight.
 
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Old 11-27-07, 10:01 AM
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Let me preface my remarks that I do believe codes should be followed and am not suggesting that this one is poorly thought out however....

The same unsafe conditions suggested for a detached structure can occur in any (attached) room in the primary structure. You could have multiple circuits on different breakers. Multiple grounds at different potentials. Further since in the real world many panels or either mislabeled or not labeled I'm sure fire fighters turn off all probable sources of power not just what is labeled.

You could explore the logic this question a bit by asking about a breezeway connecting a garage to a house. If only a roof is used does the garage still fall under the description of detached. What if a slab is poured for a floor of the breeze way? Is it still detached? Here it is not uncommon to have the main panel in a detached garage and all of the many feeders for the house running through the breezeway roof to the main house. Couldn't it be argued that the garage is the main structure and the house detached? If a breezeway in fact makes it attached how could a simple wood structure change safety requirements?

Yes ,of course AHJ is the final word and perhaps this discussion is outside the scope of this forum but it does go to why people would have questions.
 
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Old 11-27-07, 10:16 AM
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I certainly agree with all of your points, Ray. The same problems can happen in a contiguous structure. I suspect that the chances are simply magnified with an outbuilding. Plus, the physical separation of the buildings makes a good obvious "break point" for the code to kick in.
 
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Old 11-29-07, 07:01 AM
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So if you use the light exception, it's not a concern since the switch is plastic and the ground is not an issue? Or do you still use a common ground between the garage and the house for the switch and everything else?

Also, if I'm understanding correctly, you can only have 1 switch in the house controlling the detached structure and not 2 (for say 2 separate spot lights)
 
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Old 11-29-07, 07:08 AM
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The code is all about safety.

The light exception is for safety. Those who made the code felt that a light was more important for safety than the extra circuit.

Since this light exception is only for a light (no receptacles, no appliances, etc.) the potential danger is limited.
 
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Old 11-29-07, 07:35 AM
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After following all these posts about detached buildings I decided to check mine out closely. I found I have two 20 amp circuits and no circuit box in the detached building. It is interesting that they went under the cement slab, pops up in the crawl space, back into the ground to the detached building. The building had a permit on it for structural but when I checked no electrical permit was issued. Looks like I need to do some studying planning and get up to standards. Of course I will run my plan through the experts, get a permit and do it right.
 
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Old 11-29-07, 09:35 AM
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Originally Posted by racraft View Post
The light exception is for safety. Those who made the code felt that a light was more important for safety than the extra circuit.
Is this limited to one switch only?

I know this is kind of digressing from my original question, but at this point I'm trying to figure out if I could have exterior spot lights on one switch and interior lights on another switch.

Edited to add: Does the answer change any if they are on the same circuit vs two circuits?
 
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Old 11-29-07, 10:16 AM
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One other lighting circuit can be used.
 
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Old 11-29-07, 10:31 AM
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So if 1 circuit originates in the house and is then controlled by 2 switches going to 2 separate lights, does that still count as 1 circuit? Or that fact that there are multiple current carrying conductors make it count as two?
 
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Old 11-29-07, 11:18 AM
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You can have one single extra line (hot/neutral/ground) to the outbuilding for a light.

Control your other lights from within the garage or use wireless.
 
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Old 11-29-07, 12:12 PM
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Thanks. That's exactly what I needed to know. It doesn't all make sense to me, but at least I know the rules to evaluate my options.
 
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Old 11-29-07, 01:13 PM
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I honestly didn't even know there was such an exception.

Got curious yesterday and looked a bit but couldn't find it. I did see 225.30(D) which allows for additional branch circuits for control of outside lighting from multiple sources, but this can't be what you guys are talking about.
 
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Old 11-29-07, 01:47 PM
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This what we are talking about. Specifically, the light needs to be on a three way switch setup with one switch at the house and the other at the garage.
 
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Old 11-29-07, 05:04 PM
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Originally Posted by michaelshortt View Post
After following all these posts about detached buildings I decided to check mine out closely. I found I have two 20 amp circuits and no circuit box in the detached building.
It is quite possible that you have a multi-wire branch circuit. For the purpose of the code section that limits the number of supplies to a garage, this counts as one circuit.

In terms of circuit capacity, you have the benefit of two 20A 120V circuits, and you have 2 separate breakers. But because of the shared neutral and egc, this doesn't violate the code section limiting the number of supplies.

You should have 'disconnecting means' in the garage, however. This could be as simple as a double pole switch.

-Jon
 
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Old 11-29-07, 06:30 PM
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Winnie, I do have two separate 12/2 wires with grounds. I think that would be the code violation.
 
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Old 11-30-07, 01:44 AM
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Originally Posted by racraft View Post
“The exception is that you may add a circuit that is a lighting circuit only, and only when the light is to be controlled from the other building.”

“Since this light exception is only for a light (no receptacles, no appliances, etc.)”

(only) “One other lighting circuit can be used.”

“You can have one single extra line (hot/neutral/ground) to the outbuilding for a light.”

“Specifically, the light needs to be on a three way switch setup with one switch at the house and the other at the garage.”

I see nothing that supports these statements as the only exception.

Section 225.30(D) is one of five exceptions that would allow additional feeders or branch circuits to an accessory structure or building. It has several provisions but the one that is most likely to apply to this question is; "for different uses". It gives an example; “such as control of outside lighting from multiple locations.”
 
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