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Re: [TowerTalk] ampacity, overcurrent protection, etc.

To: Jim Lux <>
Subject: Re: [TowerTalk] ampacity, overcurrent protection, etc.
From: Gary Schafer <>
Date: Tue, 25 Jan 2005 18:17:02 -0500
List-post: <>
Hi Jim,
I have to take issue with you on some of this:

Jim Lux wrote:

The rules in the NEC for conductor sizing and overcurrent protection are sort of complex, and one does not necessarily depend on the other. For code fiends, 240-3 has the OCP rules. Once you get away from conventional residential construction, the rules allow all sorts of things, assuming there's a rational basis for what you want to do.

One does depend on the other. There is sometimes more leeway. You can sometimes run more current on a given conductor size than normal but you can still can not way undersize a breaker on a given size wire.

Conductors get sized based on 1) Expected loads 2) Permissible Voltage Drop 3) Temperature ratings

This is standard.

You can use a smaller wire for a given current, if it's rated for the temperature it will rise to in use, for instance. (Last 10 years have added requirements that the devices the wire is connected to has to also be rated for the higher temperature).

This is also standard.

Don't discount the possibility that the heating limit for wires varies depending on the environment. Jamming 10 wires in a conduit will result in a lower ampacity rating than single wires strung in open air.

Also standard for any installation.

Overcurrent Protection is based on:
1) Expected size and type of load (in general, 125% of continuous+100% of noncontinuous load)
2) Type of wiring and its ampacity

Over current protection based on the load size still doesn't negate the proper wire size for the load and breaker.

More specifically: "
Overcurrent protection for conductors and equipment is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in conductors or conductor insulation.

Over current protection must in addition be small enough to trip the breaker before the wire burns open.

So, if you can make a case that your AWG 20 wire won't be excessively or dangerously heated by that 400 service, you can protect it accordingly. Maybe you've got your wires cooled by liquid nitrogen (which would also help with the voltage drop problem).

There's a whole raft of exceptions to the ampacity/OCP correlation..
For instance:
Motor circuits
Phase Converters
Devices rated 800A or less (just in case you were running that 100kW transmitter)
transformer secondary conductors

Article 225 covers the rules for "outside branch circuits and feeders" which has different rules than normal interior wiring.

Then, there's probably a raft of special stuff in the 500s Certainly, Art 530 (Motion picture and Television Studios) has all sorts of weird stuff permitted.

Finally, your local AHJ (Authority Having Jurisdiction) might allow all sorts of odd things for certain uses (i.e. farm buildings).

There is also a provision in the code that allows an undersize line and breaker to feed the kitchen stove. It seems a very stupid thing to do though but it is legal. The idea is that you will not have all the burners and oven on at the same time. But if you should turn them all on at the same time (cooking thanksgiving dinner) you may have nuisance breaker interruptions. Go figure.

But the bottom line is that I think you would be hard pressed to find any exception in the code that would allow for a breaker to be oversized to the extent that was being discussed. #10 wire with a 50 amp breaker!
Keep in mind that the panel feeding that line is in the house so part of that line falls under what the house is subject to. The other end of the line at the barn is inside (I assume) and falls under a structure related code also.

Anybody can do what ever they want to do but giving that type of advice here may get someone in trouble in more ways than one.

Gary  K4FMX


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