200A Service - City says 2/0, Builders Supply says 3/0...

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  #1  
Old 07-19-05, 03:43 PM
BobP
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200A Service - City says 2/0, Builders Supply says 3/0...

OK, so the city of Saint Paul, MN has a handout available for download regarding residential service installation. It requires 2/0 copper with 1-1/2" EMT for a 200A service. My local Home Depot claims that 3/0 copper is the minimum. What's up here? I've looked at various tables and to me it appears that I need 3/0 to meet the 200A threshold. Why would the city only require 2/0 for 200A?

BobP
 
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Old 07-19-05, 04:19 PM
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The city probably allows you to use slightly undersized conductors on the assumption that a residential service is never going to draw 200A continuously. There may be peaks and spikes up to 200A, but sustained current will be much lower, so a smaller wire is perfectly safe.

Also, I see you're in Minnesota where ambient temperatures are relatively low in summer and down right cold in winter, thus increasing conductor ampacity. They have probably accounted for this.

They're only providing a minimum -- you can use the larger 3/0 conductors if you want to spend a little more. I don't think it's a safety issue though; you'll see a marginally smaller voltage drop with the larger cable yielding better efficiency.
 
  #3  
Old 07-19-05, 04:28 PM
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NEC table 310.16 is the one to go to for ampacity of conductors for _most_ applications. Using this table, the ampacity of 3/0 Copper is 175A (using 75C terminations, no ambient heating issues, etc.) For most applications, if you were using a 200A breaker, you would use 3/0 wire.

However the NEC has an exception for the _main_ power feed to a home. This is in section 310.15(b)(6). Basically this is an alternate table that only applies to this _particular_ application, which you can use to size your wires. Basically it lets you use wires which are one or two sizes smaller than 310.16 would suggest for a given service size.

This is a recognition that _in a home_, a 200A service will only be used at 200A a very small fraction of time, if ever. Because of these load characteristics, the NEC permits a conductor of one ampacity to be protected with a breaker of higher amp rating.

For a number of other _specific_ applications (motor loads and welders, for example), the NEC lets you do this. For some types of motor and welder loads, you might see #10 wire protected with a 60A breaker. This is safe only when the specific characteristics of the load are considered, and the NEC has this permission only for a few types of load. You can only use these exceptions for the particular loads that the rules describe.

For example, using table 310.15(b)(6) is allowed for the main power feeder to a home, but you couldn't use this table to size the conductors between the main panel and a subpanel, and you couldn't protect a #10 conductor with a 60A breaker for a heater.

-Jon
 
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Old 07-19-05, 06:43 PM
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Originally Posted by BobP
My local Home Depot claims that 3/0 copper is the minimum. What's up here?
What's up? NEVER listen to a home center employee. That's what.

Read what Winnie wrote. You use 310.15(b)(6) for dwelling services and feeders.

Everything he wrote is true, all except for the sub-panel part. In my area, and many others, we DO use 310.15(b)(6) for sub-panels.
Buuuut, that's a subject for a different 5 page thread.
 
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Old 07-20-05, 07:06 AM
BobP
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Thanks to all!! I found the table that speaks to residential service and feeders and it's much more clear to me now. This forum is great!

BobP
 
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Old 07-20-05, 02:40 PM
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Originally Posted by Speedy Petey
What's up? NEVER listen to a home center employee. That's what.
Listening to them is fun, doing what they say is a whole other ballgame. I would suspect someone, someplace has wired there whole house with 14g THHN because the "BOX STORE electrician" said its the way to go.

A friend of mine didn't listen to me and let someone wire there new house (he was an alarm tech, he knows how to wire things). Inspector made them remove every piece of 14g wire on circuits fed by 12g wire. Every switch in the house was 14g. They were slightly mad..
 
  #7  
Old 07-22-05, 08:33 AM
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Originally Posted by Hellrazor
Listening to them is fun, doing what they say is a whole other ballgame. I would suspect someone, someplace has wired there whole house with 14g THHN because the "BOX STORE electrician" said its the way to go.

A friend of mine didn't listen to me and let someone wire there new house (he was an alarm tech, he knows how to wire things). Inspector made them remove every piece of 14g wire on circuits fed by 12g wire. Every switch in the house was 14g. They were slightly mad..
What's wrong with having a branch be of lower gauge wiring then the main feed? For example, having 12 gauge run to a junction box and then run off there with 14 gauge wiring to individual lights.
 
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Old 07-22-05, 08:45 AM
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Originally Posted by sx460
What's wrong with having a branch be of lower gauge wiring then the main feed? For example, having 12 gauge run to a junction box and then run off there with 14 gauge wiring to individual lights.
What's wrong is that this (assuming a 20 amp breaker) is that it:

1) is a fire hazard.

2) is a code violation.
 
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Old 07-22-05, 01:13 PM
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Originally Posted by racraft
What's wrong is that this (assuming a 20 amp breaker) is that it:

1) is a fire hazard.

2) is a code violation.
Good point, I guess my logic was backwards.
 
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Old 07-22-05, 05:30 PM
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ok......leave it to me to kink the math...lol....


Now if that 12 AWG is run to a junction which is then feeding a 14AWG for lets say a post light and you are sizing the 12 AWG in regards to voltage drop then you actualy OCP the circuit for 15 AMPS because you have to protect for the smallest wire in the circuit ( weak link ) you can do it.......(this is a specific application only.....no one in their right mind would do 12 AWG to 14 AWG in a house otherwise...atleast no electrician I know as it would to be be considered shotty.)

Man I love Electricity.......Lets all get shocked a few times and then do the math.....

Disclaimer: ElectricalMan in no way shape or form condones actually shocking yourself and doing math at the same time. The shocking ones self is only used to envoke humor on a hot day.
 
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Old 07-22-05, 05:31 PM
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Originally Posted by sx460
What's wrong with having a branch be of lower gauge wiring then the main feed? For example, having 12 gauge run to a junction box and then run off there with 14 gauge wiring to individual lights.
And in the case of #12 feeding #14 on a 15 amp breaker?

It's bad practice and just plain poor workmanship.
Not to mention a waste of good #12 wire.
 
  #12  
Old 07-22-05, 08:59 PM
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The funny thing is that the riser conductors and meter socket to panel conductors are 2/0 but the utility has, at best, 1/0 aluminum conductors. There's no fusing or breakers between the main lugs of your panel and the utility primary fuse on their transformer (unless you are served by a CSP transformer with a secondary breaker). In any event, the weak link is not your 2/0 or 3/0, it's the utility-owned service conductors. I guess that's your fusible link.
 
  #13  
Old 07-23-05, 07:29 AM
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I believe that it is fundamentally important to differentiate between:
1) physics
2) good practices
3) rules

1) Physics.

When current goes through a wire, the wire will heat up. Put enough current through a wire, and it will heat up to the point of glowing and eventually melting. But for home wiring, the limiting factor is not the wire itself melting, but instead the insulation getting too hot and failing.

The temperature of a conductor is set by a combination of factors: the current flowing through the conductor, the resistance of the conductor (based upon material and size), and the heat dissipation capability of everything surrounding the conductor. Pass more current through a conductor, and it will get hotter. Blow cool air over the conductor, and it will get cooler. Wrap thermal insulation around the conductor, and it will get hotter. Put the conductor next to a heat source, and it will get hotter, etc.

The requirement for protecting a conductor it to keep it cool enough that it will last for a good long time.

When a conductor is rated at 90C, this does not mean that it is perfectly fin up to 89C, but that at 91C it will melt and fail. Materials that have such a precise melting point are quite rare. Instead the 90C rating means that if the conductor is maintained at 90C it will have a statistically long operating life. Some samples might fail quickly...other samples might function 'forever', and still other samples might function quite happily at 110C. 90C is a statistical aspect of the insulation.

Code does permit a careful calculation of all of the aspect involved in heating a wire, setting the ampacity that way, at least for 'feeders', and possibly for branch circuits. This has to be done under 'engineering supervision', and in all but rare instances this engineering calculation would be far more expensive than the materials saved...certified professional engineers cost as much a lawyers per hour

So instead code has a set of tables that let you calculate reasonable ampacities for conductors. These tables make assumptions about the thermal insulation found around the cables, and other assumptions about how the conductors will be used, and then they add a pretty big safety factor. Unless you are doing something like engineering the main power feeders for a Las Vegas casino (ooops, they got that one wrong), the tables are the way to go.

One place where the residential DIYer will encounter different ampacities from the NEC tables: the power company uses different tables to calculate the ampacity of conductors used to supply your home. There are several features for the difference (note, I am inferring these; I've never worked for a power company...maybe someone with more experience can confirm): 1) the feeder goes through the open air, and thus has better cooling. 2) the power company makes different assumptions about the actual current that a home will draw. 3) Because the cables are on the outside of the house, failure is more allowable in a statistical sense, so the conductors can be permitted to run hotter. Net result is that the power company will assign higher ampacities to the same conductors.

2) Good practise.

There is disagreement on the mixing of wire sizes as good or bad practise. It is clear that the _code_ only requires that branch circuit wiring be protected by looking at the smallest wire in the circuit, and providing a circuit breaker that protects for that. In other words, you could have a mix of #8, #10, #12, and #14 on a single branch circuit, and if you protect it with a 15A breaker, then you meet code requirements. Do this sort of job neatly, and it would probably pass inspection. But this is the sort of thing that would make an inspector say 'hmmm, inexperienced' and check everything in greater detail.

It is often a sign of bad workmanship if you do this, but there are situations where mixing conductor sizes is a good idea. For example, if you have a long run between the panel and the room where the circuit is utilized, it might make sense for reasons of voltage drop to use a #10 or #12 'home run', but then to use #14 for the actual wiring inside the room. This is _good practise_ because it helps to minimize voltage drop.

Additionally, while the branch circuit wiring itself must be sized as above, there are 'tap rules' to do permit the use of smaller sized conductors on these circuits. The application is pretty technical, and is based again upon statistical experience with failures; I would never attempt to design my own tap for residential wiring, but there is a place where home DIYers do encounter a version of these 'tap rules'. Many fixtures and light dimmers come with wires on the device rather than screw terminals. These wire are clearly smaller than the branch circuit wiring, yet are acceptable for use. These wires are of short, well defined length, and have been evaluated by UL to be suitably protected when on normal branch circuits.

3) rules.

Rules are a distillation of years of experience of many electricians, coming up with a codified list of what works. If a particular practise is statistically associated with failures or fires or loss of life, then that practise get eliminated from the code. For some things, the practise initially appears 'safe enough', but over time it is learned that you see failures. An example is the shared neutral/ground for electric ranges. For other things, development of technology makes a particular safety requirement easier and cheaper to implement, shifting the cost/benefit ratio in favor of greater safety. More GFCIs and higher temperature rated conductors are now required because it is cheaper to do this today than 30 years ago.

Don't mistake rules for physics. If you put 14ga wire on a 20A breaker, it will _probably_ work. But it has a greater _chance_ of failure than if you put that 14ga wire on a 15A breaker, and this has been learned over time by the experience of a large number of people. Most of the electric code is _not_ about keeping things working correctly. Instead it is about providing safety when something goes wrong...and electricity is such a good tool that things go wrong pretty rarely.

-Jon
 
  #14  
Old 07-23-05, 07:30 AM
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The reason the utility overhead lines are so small is that; 1) they are in free air and 2) the utility folllows their own code, not the NEC. They can basically do what they want as they write their own rules.
 
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