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[TowerTalk] ampacity, overcurrent protection, etc.

To: towertalk <>
Subject: [TowerTalk] ampacity, overcurrent protection, etc.
From: Jim Lux <>
Date: Tue, 25 Jan 2005 14:00:44 -0800
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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.

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

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).

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.

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

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.

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).


See: for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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