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Old 12-20-2007, 07:55 PM   #1
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Gauge Wire and Breaker Discrepancy


Can someone please educate me?

I found several helpful links on how to pair up # 2 AWG Copper wiring to the correct breaker but depending on the author, I either need a 90, 95, or 100 Amp breaker. (See links below)

The only consistent information on # 2 AWG copper wiring is its usually rated at 125 Amps. It would seem to me any breaker under 125 Amps should be safe to use within reason (Yes, No, Maybe?). For my application of adding a sub panel to my workshop, I would prefer to use a 100 Amp breaker from my main service panel out to my workshop sub panel using the # 2 AWG copper wiring. Is this safe and within NEC 2002 code?

Any input would be greatly appreciated.

http://answers.yahoo.com/question/in...5124642AAJvNk7

http://www.greatinspector.com/faq-elec-wiresize.html

http://en.allexperts.com/q/Electrica...-SUB-PANEL.htm

Thanks,
Mac

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Old 12-20-2007, 09:20 PM   #2
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Gauge Wire and Breaker Discrepancy


Where did you get this 125a rating. As a branch circuit (your feeder) that #2 Cu is only rated for 90 amps. There are no 95 a brkrs.
Did you contact your local inspection authority. Do your homework ahead of time, and avoid aggravation, and possible hazardous installations.
Your local code (city ...whatever) might have a specific requirement.
I assume you are going to get the proper permits.

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Old 12-20-2007, 09:35 PM   #3
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Gauge Wire and Breaker Discrepancy


Quote:
Originally Posted by JGarth View Post
Where did you get this 125a rating. As a branch circuit (your feeder) that #2 Cu is only rated for 90 amps. There are no 95 a brkrs.
HUH??? Where did you get your information?

#2CU @ 60 deg C is rated at 95A. Because there is no 95 amp breaker we round UP to 100 amps.
This is pretty much for NM cable only since NM is about the only wiring method that uses the 60 deg column. Other wiring methods, including SER cable and pipe and wire, can use #3CU for 100 amps because they can use the 75 deg C column.

The code reference is:
240.4
(B) Devices Rated 800 Amperes or Less
The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.
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Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2008 & 2011 NEC.

Last edited by Speedy Petey; 12-20-2007 at 09:39 PM. Reason: Add code reference
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Old 12-20-2007, 09:50 PM   #4
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Gauge Wire and Breaker Discrepancy


The answer isn't a simple one. For your reference there are 2 tables located at this link..... go to the right column just below the conduit fill tables to conductor characteristics the first table listed is for conductor ampacities for insulated conductors (table 310.16) the second is a calculator for wire size and the 3rd is the ampacity table for main service conductors and main feeders for single family dwellings (table 310.15 (b)(6).

http://www.jhlarson.com/ind_tables/ind_table.htm

Your reference that #2 copper is rated 125 amps would be directed at the 310.15 b6 table. However without going into a long drawn out story this table is not applicable to your situation since a feed from the main panel to a sub-panel is not a "main feeder" carrying the total load of the dwelling with the service equipment and meter.
So the applicable table is the first table listed....NEC table 310.16. In your case you want 100 amps so if you look at the table left hand side 1st column you will see wire sizes and across the top you will see temp ratings and wire insulation types. You will also see the temperatures of 60 C, 75 C and 90 C. All the wires in the 90 C column can be used at the ampacities of the wires in the 60 C and 75 C columns. You will never use the ampacities listed in the 90C column in a residence. So that is a breif lay of the land on how you look at this table. These ampacities are used for various reasons along with temperature adjustments and deration adjustments as necessary.

In a nutshell #2 copper will be either 95 amps or 115 amps depending on some factors....those being type of wire insulation and is the feeder between panels a certain type of cable like UF-B or SER cable or individual wires in conduit like thhn/thwn. Most likely #2 copper will be individual wires in conduit and likely to be thhn/thwn rated temperature insulation but could be other types of wire or cables.
Next thing to know.... is the equipment that each end of the feeder rated for 60 C terminations or 75 C terminations or both. That information is obtained from the specifications sheet located on the circuit breaker panel door. Usually it will say rated for 60C/75 C termination. Or it will say 60 C terminations only. If you cannot identify which it is then 60 C termination must be used. So any ampacities you use will come from the 60 C column. If rated for both then your ampacities will come from the 75 c column when using individual wires or cables with insulation ratings that are listed in the 75C column or the 90 C coolumn. If your wire insulation is only listed in the 60C column then you can only use the ampacity of that column. Remember you can't use the ampacities listed in the 90 C column except for certain circumstances. Some cables must be used at 60C only. But with #2 copper those cables are probably not any that would have that restriction. So assuming your #2 copper is thhn/thwn or other insulation listed in the 75 C or 90 C column and you have 75C termination in both the sub-panel and the main breaker panel (service equipment) #2 copper thhn/thwn would be allowed at 115 amps. If only 60C cable types or termination rating then #2 copper thhn/thwn would be 95 amps.

The answer to your second question is yes you can feed the 100 amp rated panel with any breaker equal to or less than the panel rating if the feeder is sized at 100 amps or sized to the breaker. In the case if the #2 is rated 115 amps and 75C terminations then you could put a 100 amp breaker in the sub-panel or at both main and sub locations if the sub-panel has a 100 amp main breaker already installed. You cannot exceed the panel rating though even if the ampacity of the wire does...in this case 115 amps ampacity for #2 copper thhn/thwn would not allow you to install a 125 amp breaker which is the next breaker size after 110 amps. The breaker cannot exceed the panel rating. In this situation you would have #2 copper thhn/thwn rated 115 amps with a 100 amp circuit breaker for protection of the feeder. If you used 60 C termination then the #2 is rated 95 amps but there is no 95 amp breaker so you are allowed to go to the next size up which is 100 amps. So you would have #2 copper thhn/thwn at 95 amps and a 100 amp breaker.

To accurately give you the correct figures we need to know

Your sub-panel rating
Your wire size
Your insulation type
Individual wires in conduit or cable type....ie...ser, seu, uf-b, nm-b, thhn, use... etc
Termination temperature rating of both panels.

Then there is a host of other issues to go over when installing a sub-panel.

Hope this helped

Last edited by Stubbie; 12-21-2007 at 10:31 PM.
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Old 12-20-2007, 11:17 PM   #5
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Gauge Wire and Breaker Discrepancy


Stubbie,
Your post helps immensely! I think I understood most of it. I'm a little in shock to pick up on so much technical information. You are very good at explaining things.

Anyhoo... To give you the answers to your questions... I haven't actually purchased anything yet. I've been fishing around for the best information possible so I will have fewer headaches down the road. I can tell you that the information on my service panel is as follows: Main Ratings - 120-240 VAC 3-Wire 200 Ampere, and I could not find any information on the Termination Temperature Rating. However, it is a General Electric service panel installed in 1970 if this helps.

My main service panel used to have 2 50 amp breakers supplying power to an old AC/heating unit. After updating my AC/heating system, it only required 1 50 amp circuit, leaving me with an available 50 amp circuit. I decided I wanted to power up my workshop with something other than an extension cord. Based on research up to date, I have the potential to run wiring of the correct type from my service panel in the garage to a sub-panel in the workshop (which will require about 65 feet of wiring from panel to panel). My basic electrical needs for the sub-panel are.. 1 15 Amp circuit for lighting, 1 15 Amp dedicated circuit for a 5,000 BTU window unit AC (which is the smallest on the market to cool one small room in the shop), 1 20 Amp circuit for downstairs outlets, 1 20 Amp circuit for upstairs outlets for a grand total of 70 Amps. This should give me room to add a circuit or two later if I can squeeze 100 Amps from this project.

This is what I was thinking of doing but again, I am fishing around for good information before starting this project.
I would like to put a 100 Amp circuit breaker in the place of the 50 Amp circuit breaker on the main service panel.
From there, I thought I would run the appropriate gauge wire (# 2 AWG Copper - Yes, No, Maybe?) 65 feet to the workshop sub-panel. (I was told I may have to have a sub-panel of at least 125 Amp - Yes, No, Maybe?)
The wire will be buried in the ground inside conduit 30 inches deep (according to local code).
From the sub-panel, wire up the needed circuit as described above (total of 70 Amps with room to grow I hope).
I still haven't figured out if all 4 wires running between the panels have to be the same gauge (# 2 AWG Copper) or can I safely and legally use a smaller gauge wire for the neutral and ground? In other words, use # 2 for the 2 hot wires, a # 4 for the neutral, and a # 6 for the ground.

I wish I could answer your questions but of course I haven't bought anything yet. Could you give recommendations based on my information and your questions?

Thanks again for your terrific response,
Mac
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Old 12-21-2007, 07:48 AM   #6
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Gauge Wire and Breaker Discrepancy


You don't add the amperage of breakers in a panel to determine the subpanel amperage. That is determined by a load calculation. From what you describe, a 60A subpanel would be sufficient for your garage which would mean #6Cu would be all you need. Use three #6 THWN (two black, one white) and one #10 green in conduit or 6/3 w/g UF.
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Old 12-21-2007, 09:58 AM   #7
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Gauge Wire and Breaker Discrepancy


I'm in total agreement with househelper you only need a 60 amp feed to the sub panel. As HH said you do not add up the breakers to determine how much amperage the panel and project needs. You will find that could add up to much more than the rating of the panel. The logic here is if you add up the breakers you are saying that all the breakers will be at maximum load which is never the case. Load calculations are done to get a more realistic value for the amperage that will be imposed on the panel. Remember to that the amperage draw at the sub panel will be imposed on the main panel since it is the source component containing the feeder and breaker. So if you like you can give a load calculation a try to see what the load is on the main panel before you add the sub-panel to give you an idea of the 'reserve' power you have for the workshop. Here is a pretty good place to understand how to do that.......
http://www.selfhelpandmore.com/homew...calc/index.htm

I've never heard of a 30" requirement for an electrical trench unless it is the main service lateral to the dwelling.. Usually Sch 40 pvc is only required to be 18" deep for a feeder to a subpanel....so you need to check that 30" I believe you will find that to be incorrect.

Ok so here is what you need and househelper has already said this but I will add a bit to his post.

1 60 amp double pole breaker
1 6 space 12 circuit main lug only sub-panel rated 100 amps
2 black #6 awg copper thhn/thwn (hots)
1 white #6 awg copper thhn/thwn (neutral)
1 green #10 awg equipment ground wire.
All necessary conduit fittings and fasteners and glue
Use electrical 1" conduit

Install a pull string as you assemble your conduit and use it to pull the wires thru all at the same time once the conduit run is finished. If your going to have several bends then install pull point junction boxes as appropriate to facilitate pulling the wires . You are going to have the neutral and ground bars electrically isolated at the sub-panel. You will land the feeder neutral and the branch circuit neutrals on the neutral bar and all bare and green grounds on the ground bar. This is a diagram to show the important aspects of the installation but is not intended as a how to guide. It is accurate to the best of my understanding. Always verify with a qualified person when possible. All the 'green' shown at the main panel should already be in place except for the green ground wire for the sub-panel feeder. If you use full size breakers you will not be able to exceed the 6 hand rule for the required disconnect at the sub.... 12 circuits are only obtainable if you use tandem breakers ( not to be confused with double pole breakers). This will save you the cost of a main breaker in the sub-panel. Click image to enlarge.
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Gauge Wire and Breaker Discrepancy-4-wire-subpanel-detached.jpg  

Last edited by Stubbie; 12-21-2007 at 10:17 AM.
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Old 12-21-2007, 11:36 AM   #8
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Gauge Wire and Breaker Discrepancy


For the electricians. Is a grounding wire required when the sub panel is located in a seperate structure with no other metal paths between the buildings? I am getting ready to run new wires out to my garage and just want to buy what I need and nothing more. The previous owner ran 6/3 NM-B through schedule 40 underground and I want to replace this before it becomes a problem. BTW, there is a supplemental ground rod attached to the sub panel at the garage.
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Old 12-21-2007, 11:47 AM   #9
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Gauge Wire and Breaker Discrepancy


"For the electricians. Is a grounding wire required when the sub panel is located in a seperate structure with no other metal paths between the buildings? I am getting ready to run new wires out to my garage and just want to buy what I need and nothing more. The previous owner ran 6/3 NM-B through schedule 40 underground and I want to replace this before it becomes a problem. BTW, there is a supplemental ground rod attached to the sub panel at the garage."Today 10:58 AM

No the ground is not required. You just need to bond your neutral from the feeder to the grounding electrode conductor in the sub that runs to the ground rod
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Old 12-21-2007, 12:32 PM   #10
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Quote:
Originally Posted by RichyL View Post
"For the electricians. Is a grounding wire required when the sub panel is located in a seperate structure with no other metal paths between the buildings? I am getting ready to run new wires out to my garage and just want to buy what I need and nothing more. The previous owner ran 6/3 NM-B through schedule 40 underground and I want to replace this before it becomes a problem. BTW, there is a supplemental ground rod attached to the sub panel at the garage."Today 10:58 AM

No the ground is not required. You just need to bond your neutral from the feeder to the grounding electrode conductor in the sub that runs to the ground rod
But if you are going to do it this way (legal, but not recommended) you must use three insulated THWN wires of the same gauge (i.e. #6); you cannot use 6/2 cable.
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Old 12-21-2007, 12:43 PM   #11
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Gauge Wire and Breaker Discrepancy


Quote:
Originally Posted by RichyL View Post
"For the electricians. Is a grounding wire required when the sub panel is located in a seperate structure with no other metal paths between the buildings? I am getting ready to run new wires out to my garage and just want to buy what I need and nothing more. The previous owner ran 6/3 NM-B through schedule 40 underground and I want to replace this before it becomes a problem. BTW, there is a supplemental ground rod attached to the sub panel at the garage."Today 10:58 AM

No the ground is not required. You just need to bond your neutral from the feeder to the grounding electrode conductor in the sub that runs to the ground rod
Thanks. I have a similar setup that runs down to my pier for a boat lift, recep, and lights. Three #8's run to a T-handle disconnect. From there, there are 4 #10's L1,L2,N, and a ground going to a sub panel at the end of pier. The Ground wire is connected to a ground rod at the bulkhead. Should I remove the ground wire and just connect the three wires from the disconnect to the sub panel, bond the grounding bar to the sub box, and connect the nuetral to the ground rod?
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Old 12-21-2007, 12:50 PM   #12
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Gauge Wire and Breaker Discrepancy


Quote:
Originally Posted by HouseHelper View Post
But if you are going to do it this way (legal, but not recommended) you must use three insulated THWN wires of the same gauge (i.e. #6); you cannot use 6/2 cable.
I was going to use THWN for the new run. I might drop down to #8 wires with a 40A breaker to cut down costs a little. You said this method is not recommended. Is there an advantage(safer) running the fourth wire and isolating the ground bar?
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Old 12-21-2007, 12:58 PM   #13
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Originally Posted by jerryh3 View Post
I was going to use THWN for the new run. I might drop down to #8 wires with a 40A breaker to cut down costs a little. You said this method is not recommended. Is there an advantage(safer) running the fourth wire and isolating the ground bar?
It's safer and it gives you the option of adding phone, cable, or a metallic water line to the garage later. Not that it matters in this case, but the 2008 NEC requires a four wire feed to a detached building, removing the three wire option.
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Old 12-21-2007, 01:09 PM   #14
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HouseHelper and Stubbie,
Great response, thank you!

Sounds good to me! I want to run down to the hardware store and get started right away. However, I need a little more education to get over a few small hurdles.

1. I have read 4 electrical books on adding a sub-panel while visiting the hardware store and they all recommend pairing up # 6-3 wire with a 50 amp breaker not a 60 amp breaker. Help me get past this discrepancy!

2. # 6 wire is a good starting point for a sub-panel also according to what I've read. However, after 55 feet there will be a certain percentage voltage drop for every foot thereafter. My panels will need 65 feet of wire between them leaving me at 10 feet over the 55 feet voltage drop zone. If I understand this correctly, I will experience an approximately 5% - 30% voltage drop depending on several factors.

After reading these findings, I decided, with the help of one electrician, 2 hardware employees, and one wholesale electrical supply company employee, that I should try for some over-kill and go with something I could safely add more circuit to in the future. I was advised to use the # 2 AWG wire, 125 Amp sub-panel, and 1 100 Amp double pole breaker from the service panel (following the advice caused me to find the 100 Amp breaker discrepancy that I originally posted here).

However, before I started any research at all, I originally was going to use the existing 50 Amp double pole breaker from my service panel. From there, connect the #6-3 wire rated for direct bury (no conduit!) Run that to a 100 Amp indoor sub-panel and run all my circuits from there. Isn't it funny how things can evolve and get out of hand with a difference of opinion and tons of mis-information? If you guys can clarify the two hurdles I have, it would be greatly appreciated.

Finally the 30" bury was relayed to me by a local electrician in my area as well as the folks at the wholesale electrical supply store. I will call the city permit office to confirm.

Thanks again,
Mac

Last edited by broken_scout; 12-21-2007 at 05:03 PM.
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Old 12-21-2007, 01:25 PM   #15
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Gauge Wire and Breaker Discrepancy


The 6ga wire in NM or UF cable has an allowable amperage of 55A. Since there are no 55A breakers, you are allowed to use a 60A.

At 65ft, voltage drop should not be an issue.

Have you priced a 4 wire #2 Cu cable (or ~200ft of THWN?) If you really want more capacity, look into 2-2-2-4 Al cable protected by a 90A breaker.


Last edited by HouseHelper; 12-21-2007 at 01:25 PM. Reason: spelin!
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