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Old 07-31-2009, 08:24 AM   #1
 
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Service Conductor Sizing


I am replacing an old fuse panel with a new breaker panel. 100 Amp.
The current fuse panel is fed with #2Al. I doubt that the existing feeds will reach the lugs on a new panel so I'm assuming I will need to run new feeds from the meter base to the new panel.

I'm confused on what size I need. NEC 310.15(B) seems to specify #4 Cu but I've also seen references and been recommended, on this board even, to run #3Cu.

#4 would be much easier to pull so if I can use that it would be great.

I'm assuming #8 for the ground.

Thanks,
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Old 07-31-2009, 11:03 AM   #2
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Quote:
Originally Posted by Charlie1101 View Post
I am replacing an old fuse panel with a new breaker panel. 100 Amp.
The current fuse panel is fed with #2Al. I doubt that the existing feeds will reach the lugs on a new panel so I'm assuming I will need to run new feeds from the meter base to the new panel.

I'm confused on what size I need. NEC 310.15(B) seems to specify #4 Cu but I've also seen references and been recommended, on this board even, to run #3Cu.

#4 would be much easier to pull so if I can use that it would be great.

I'm assuming #8 for the ground.

Thanks,
What size is your existing service panel?

For the new 100 amp breaker panel you need #3 Cu or 1/0 Al. Depending on the AL wire used. #4 Cu is to small for the 100 amp service. #3 is just as easy to pull and what you need if you go with copper. Yes, #8 for the ground. Green in color.
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Old 07-31-2009, 11:19 AM   #3
 
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panel is currently 100A with #2 Al.

Why do you say #4 is too small? It seems to be allowed per NEC.
This is why I'm confused.

There is existing conduit to pull through but there is one short elbow which is my concern. I think it is 2" and it looks pretty full with the #2Al. No way am I going to try the 1/0 Al.

Thanks for the reply,
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Old 07-31-2009, 11:38 AM   #4
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For a 100A service feeder (meter to panel), you can use #4Cu or #2Al (see NEC 310.15(B)). It doesn't hurt to go bigger, but you don't have to.
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Old 07-31-2009, 01:19 PM   #5
 
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Thanks. I've done a load calc and it comes to around 78A. It looks like there is no reason to go bigger.

Any tips for getting that old #2 out of there or pulling the new cable? This is a little new to me.
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Old 07-31-2009, 01:39 PM   #6
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Quote:
Originally Posted by Charlie1101 View Post
Thanks. I've done a load calc and it comes to around 78A. It looks like there is no reason to go bigger.

Any tips for getting that old #2 out of there or pulling the new cable? This is a little new to me.
Is there a disconnect outside at the meter? If not, you are going to have to get your utility company involved to pull the meter. Even then, you will have to work inside an enclosure that is still hot.

How easy the old cable is to remove depends on how it was routed and how much demolition you want to perform to get it out. It could be real simple if the meter and panel are adjacent, it could be a real PITA if they are not.
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Old 07-31-2009, 03:27 PM   #7
 
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Yeah, the meter will be pulled. I'm aware it will still be hot.
I'm still holding out hope that I can somehow reuse the existing wires. I may have a little room to move the panel up.

The cable appears to go straight back to an elbow and then straight down about 5-6 ft to the panel. I'm most worried about that elbow.
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Old 07-31-2009, 04:47 PM   #8
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Debate about size of pipe/wire for new panel


Quote:
Originally Posted by Charlie1101 View Post
panel is currently 100A with #2 Al.

Why do you say #4 is too small? It seems to be allowed per NEC.
This is why I'm confused.

There is existing conduit to pull through but there is one short elbow which is my concern. I think it is 2" and it looks pretty full with the #2Al. No way am I going to try the 1/0 Al.

Thanks for the reply,
first, you can't use anything smaller than #3 Copper for 100Amps. Secondly, you "think" the elbow is 2". From my experience, I can tell you that #3 AWG will slide through 1 1/2" pipe! (No matter what) Don't Drink and Drive!!!
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Old 07-31-2009, 04:59 PM   #9
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OK, let's all step back and regroup. Table 310.15(B)(6) specifically allows #4 for a 100 A single phase residential service and service feeder.

After the service, #3 is required for any other 100 A circuit.
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Old 07-31-2009, 06:25 PM   #10
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Quote:
Originally Posted by spark plug View Post
first, you can't use anything smaller than #3 Copper for 100Amps. Secondly, you "think" the elbow is 2". From my experience, I can tell you that #3 AWG will slide through 1 1/2" pipe! (No matter what) Don't Drink and Drive!!!
The OP is not an electrician, so a trade size 1 " would look like 2" to him.
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Old 08-01-2009, 08:25 PM   #11
 
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thank you to those that have been helpful.

I have a copy of NEC and it seemed clear to me that #4 is allowed but I've seen so many contradictory postings online that it had me doubting my interpretation. I should probably just ask the inspector to be sure.

I was going off memory on the conduit size. It is 1 1/2", 2" OD.

Anyway, I went panel shopping and no way am I avoiding using the existing conductors. I saw a 30 space Square D QO panel that looked good.

I just joined this forum and it looks like a good place for help and input.
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Old 08-01-2009, 10:50 PM   #12
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Quote:
Originally Posted by Charlie1101 View Post
I am replacing an old fuse panel with a new breaker panel. 100 Amp.
The current fuse panel is fed with #2Al. I doubt that the existing feeds will reach the lugs on a new panel so I'm assuming I will need to run new feeds from the meter base to the new panel.

I'm confused on what size I need. NEC 310.15(B) seems to specify #4 Cu but I've also seen references and been recommended, on this board even, to run #3Cu.

#4 would be much easier to pull so if I can use that it would be great.

I'm assuming #8 for the ground.

Thanks,

I don't know why you don't just go all the way and upgrade to 200 amps. Panel changes in my opinion are only putting off the inevitable. Eventually you're gonna need the bigger service.
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Old 08-02-2009, 11:57 AM   #13
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Quote:
Originally Posted by InPhase277 View Post
OK, let's all step back and regroup. Table 310.15(B)(6) specifically allows #4 for a 100 A single phase residential service and service feeder.

After the service, #3 is required for any other 100 A circuit.
Table 310.15(B)(6)
I see that. What about 310.16. Seems to contradict itself. What am I missing InPhase?

310.15 says services and feeders, 3 wire. Is the difference 3 wire vs 4 wire? The last 100 amp feeder I ran, I used #3 Cu. Could I have run #4 instead?
Thanks John
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Old 08-02-2009, 12:09 PM   #14
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This has been discussed at great length on many boards.

Some areas allow the used of 310.15(B)(6) for services and feeders as the wording can be interpreted that way.
Some areas only allow it to be used for main services.

Why some think a main feeder can use #4cu for 100A but a feeder cannot is beyond me.
Luckily my area allows us to use this table.
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Old 08-02-2009, 01:10 PM   #15
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Originally Posted by J. V. View Post
Table 310.15(B)(6)
I see that. What about 310.16. Seems to contradict itself. What am I missing InPhase?

310.15 says services and feeders, 3 wire. Is the difference 3 wire vs 4 wire? The last 100 amp feeder I ran, I used #3 Cu. Could I have run #4 instead?
Thanks John
I suppose the intent is that a three wire feeder is between the metering equipment and the first means of disconnect, and it is supposed to be very short. A four wire feeder, on the other hand, is after the service equipment and is basically a large branch circuit and can be any length, so the sizing is restricted. That's just my interpretation. I guess it could be interpreted to also apply to a 3-wire subfeed to a detached building in the pre-2008 code, but I wouldn't do it due to the unlimited length it could be.
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