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Joined: May 2005
Posts: 706
T
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Sorry I missed your earlier challenge, Neil. I don't bump into the aluminum issue because I personally never use it.

On the 200-amp issue...Algonquin, IL, Village of Lakewood, IL, and in distant memory I believe Palatine, IL. In this area I won't quote a 100-amp service without calling the local building department.

Dave

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Joined: Jul 2004
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My point was, nobody got a permit to plug in the window shaker. You generally have to get a permit for a central system.


Greg Fretwell
Joined: Aug 2005
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Brownsville, Texas, I just looked at a service a few days ago at a house to which my g/f moved into with her family. 100A Cutler Hammer main with a 100A circuit breaker (feeding a subpanel), a 40A circuit breaker (this one feeding a 50A SQ D subpanel for a range, added later), and a 60A circuit breaker. Needless to say, there's 4 big bedrooms, a big living room, big dining room, LOTS of receptacles, LOTS of lighting, lots of electrical consumption. And oh yeah, central A/C.

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I've measured slowly increasing loads on 20-amp circuits which didn't trip the breaker until 24-25 amps. I've always wondered how much current is really required to trip a main on a slowly increasing load, & if the wire would handle it.

Dave

Joined: Jan 2003
Posts: 4,391
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Moderator
Dave if you take a look at the trip curves for breakers it can be surprising.

A 20 amp breaker (or fuse) may carry 24-25 amps forever and still meet all UL requirements.

Thermal trips are just not that accurate and the ambient temp has an influence on the breaker.

Don't worry about the wire, it can take it, 12 AWG is rated 25 to 30 amps anyway and the tables do not run on the edge of safety.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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I believe that is the reason for 240.4(D) anyway. Larger circuits are usually designed for a specific load but circuits with receptacles on them get loaded by the end user until the breaker trips and then they unplug the clock.


Greg Fretwell
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I understand how this works on 20-amp circuits, Bob. What I'm wondering about is how much current will a 100-amp or 200-amp main allow to pass without tripping. This type of loading (or overloading) seems likely with the addition of several sub-panels as additional loads are added to the main distribution panel.

Dave

Joined: Jan 2003
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Dave I looked up the trip curve for a basic Sqaure D 250 amp 240 volt 2 pole breaker.

It looks like it may carry 1.25 it's rated current from about 8 minutes to forever. (About 312 amps) It will carry 500 amps from 1 to 5 minutes.

I just do not see this as a worry, UL and the NEC are well aware of all these issues.

I doubt highly that properly terminated 250 amp condutors are going to burst into flames at 312 amps forever or 500 amps in 5 minutes.

Don't get me wrong if I see readings approaching 80% of a feeder or branch circuit rating I will be trying to get the customer to upgrade.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: May 2005
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I appreciate you looking that up Bob. I think the key words are "properly terminated".

It's All About The Connections.

(Lately I'm seeing loose neutrals and duplex receptacles with areas missing).

Dave

Joined: Mar 2004
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Speaking of connections. I had a remodell today and got into the panel. Always give them a good looking over. The main lugs had never been tightend. House was build in the 70's . One lug I put atleast 6 turns on . The other lug looked fine but the threads had seized. This is approx a 1500 square foot house with AC. By alrights it should have gone out by now. But visually looks ok. No signs of burn . To top it off before they installed breakers the mudder was kind enough to texture the inside of the panel.

Back on topic take this house for example. Someone correct me on this if I'm wrong going from memory. Lets do some math.

1500 sq foot home @ 3 VA =4500VA
first 3000va @ 100% 1500@35% = 525 VA

Total VA for house 3525VA
Range 8000VA
Dryer 5000VA
AC 5000VA
----------------------------
total VA 21,525
21,525 VA/240V = 89 Amps

So whats wrong with a 125Amp or 150Amp service. 125Amp service is a typical installation around here for me. We'll put on a 200Amp if there is a possibility of a hot tub or something in the future.

My question may still come out false.

Neil

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