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Author Topic: Is 4 0T necessary?  (Read 2576 times)

Ted Christensen

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Is 4 0T necessary?
« on: July 27, 2016, 12:15:02 pm »

I have a friene doing an event who is using a 200amp distro with 2OT with a mains on the distro of course being tied into a 400 amp service.  An inspector is saying we need to use 4 OT because its a 400 amp service and need a box to bump the service down to 200amps even though we are only really drawing 100 amps at most. Does this make sense?
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Tim McCulloch

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Re: Is 4 0T necessary?
« Reply #1 on: July 27, 2016, 12:29:39 pm »

I have a friene doing an event who is using a 200amp distro with 2OT with a mains on the distro of course being tied into a 400 amp service.  An inspector is saying we need to use 4 OT because its a 400 amp service and need a box to bump the service down to 200amps even though we are only really drawing 100 amps at most. Does this make sense?

Yes.  The 400 amp service has a 400 amp overcurrent protection device (breaker or fuses) and the feeder is rated for roughly 200 amps.

NEC Article 240 Overcurrent Protection

NEC 520.51(H) Supply Conductors.

The proposed fix is in compliance with 520.51(H)(4).  You use a short "tail" between the 400 amp service and a separate enclosed breaker or fused switch of 200 amps, then connect your 2/0 to the load side of that.

Not correctly protecting your feeder from overcurrent (short circuit due to physical damage or inadvertent overload on your load end) is why the inspector is not happy.  Be glad he didn't want to look in your amp racks or other stuff.

If there is a film/video grip service near you, you could probably rent exactly such a switch or breaker.  Even buying or building such a device will be cheaper than replacing your feeder cable.  And the inspector is right.  He's not making up stuff or giving you a wild interpretation.
« Last Edit: July 27, 2016, 12:31:59 pm by Tim McCulloch »
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Cailen Waddell

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Re: Is 4 0T necessary?
« Reply #2 on: July 27, 2016, 01:02:05 pm »

Tim is correct - and that jives with our rules at our PAC.  Our AHJ doesn't care but we do....


Sent from my iPhone using Tapatalk
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TJ (Tom) Cornish

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Re: Is 4 0T necessary?
« Reply #3 on: July 27, 2016, 03:43:04 pm »

I have a friene doing an event who is using a 200amp distro with 2OT with a mains on the distro of course being tied into a 400 amp service.  An inspector is saying we need to use 4 OT because its a 400 amp service and need a box to bump the service down to 200amps even though we are only really drawing 100 amps at most. Does this make sense?
If you can keep your distro within 25' of the service (cable length being 25' or shorter), you may be able to make use of the "tap rule": http://ecmweb.com/qampa/code-qa-55 and use 2/0 cable.

Assuming your distro needs to be farther than 25' from the service, you need 4/0 feeder or an intermediate 200A breaker as described by the other posters.
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Tim McCulloch

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Re: Is 4 0T necessary?
« Reply #4 on: July 27, 2016, 08:49:17 pm »

If you can keep your distro within 25' of the service (cable length being 25' or shorter), you may be able to make use of the "tap rule": http://ecmweb.com/qampa/code-qa-55 and use 2/0 cable.

Assuming your distro needs to be farther than 25' from the service, you need 4/0 feeder or an intermediate 200A breaker as described by the other posters.

520.51(H)(4) details how long the tail can be, for what inequality in ampacity.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Jonathan Johnson

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Re: Is 4 0T necessary?
« Reply #5 on: July 28, 2016, 03:14:31 pm »

Nitpicking about semantics here...

The wire size involved is properly written 4/0 AWG. As the electrical wire sizes (American Wire Gauge) go up, you've typically got 14, 12, 10, 8, 6, 4, 2, and 0. But what do you do when you get bigger than 0? You add zeroes. So now you have 00 and 0000. But since using those zeroes can be confusing on a written document, it's labeled 1/0, 2/0, 4/0 and so on. (Note that there can be intermediate wire sizes, like 9 AWG or 12.5 AWG, but those aren't typically found in electrical wiring.)

But when we say it, rather than saying four slash zero or worse, zero zero zero zero, we say four aught. That word aught is a word that isn't too common in American english; it means anything or everything. We really should say four naught, since naught is the word that actually means nothing or zero (naught is the opposite of aught). In fact, some electricians do say four naught, but that's not what I've typically heard. Probably because they are unfamiliar terms so easily misused. Common misuse of words creates confusion.

Of course, none of this helps answer the original question. I'm just being the grammar police today, that is all.
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Josh Millward

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Re: Is 4 0T necessary?
« Reply #6 on: July 28, 2016, 06:18:12 pm »

Nitpicking about semantics here...

The wire size involved is properly written 4/0 AWG. As the electrical wire sizes (American Wire Gauge) go up, you've typically got 14, 12, 10, 8, 6, 4, 2, and 0. But what do you do when you get bigger than 0? You add zeroes. So now you have 00 and 0000. But since using those zeroes can be confusing on a written document, it's labeled 1/0, 2/0, 4/0 and so on. (Note that there can be intermediate wire sizes, like 9 AWG or 12.5 AWG, but those aren't typically found in electrical wiring.)

But when we say it, rather than saying four slash zero or worse, zero zero zero zero, we say four aught. That word aught is a word that isn't too common in American english; it means anything or everything. We really should say four naught, since naught is the word that actually means nothing or zero (naught is the opposite of aught). In fact, some electricians do say four naught, but that's not what I've typically heard. Probably because they are unfamiliar terms so easily misused. Common misuse of words creates confusion.

Of course, none of this helps answer the original question. I'm just being the grammar police today, that is all.

THANK YOU.

It gets even more confusing when someone who is unfamiliar starts writing out like they think they hear it, "four ot" or "four ott" or any other similar version.

I had not previously considered the difference betwixt "aught" and "naught" though I see you are absolutely correct. I guess I've learned my second thing for the day. Thanks Jonathan!
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Robert Piascik

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Re: Is 4 0T necessary?
« Reply #7 on: August 11, 2016, 10:49:47 pm »


Of course, none of this helps answer the original question. I'm just being the grammar police today, that is all.



Well, let's hope your comments aren't for naught...

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Re: Is 4 0T necessary?
« Reply #7 on: August 11, 2016, 10:49:47 pm »


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