[Top] [All Lists]

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

To: <>
Subject: Re: [TowerTalk] ampacity, overcurrent protection, etc.
From: "Jim Lux" <>
Date: Tue, 25 Jan 2005 18:31:04 -0800
List-post: <>
----- Original Message -----
From: "Gary Schafer" <>
To: "Jim Lux" <>
Cc: "towertalk" <>
Sent: Tuesday, January 25, 2005 3:17 PM
Subject: Re: [TowerTalk] ampacity, overcurrent protection, etc.

> Hi Jim,
> I have to take issue with you on some of this:
> >
> > 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.

Getting into a parsing thing here... OCP and conductor sizing both depend on
the load, but one does not necessarily 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.

Sure you can.. I can put a 1 Amp breaker on a 4/0 wire.. but I think you
meant the other way around.. running a lot of current through a small wire.
You can do that, but there's a whole lot of "exceptions" involved.
> >
> > 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.

Proper wire size for load and installation requirements, I'll buy.

The breaker protects the wire, but, wire size doesn't determine breaker as a
one for one relation, and vice versa..

> > 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.

Presumably, wire burning open is an "excessive temperature", but I hope you
or I never design a system where the expected operating point for the copper
conductor is 1000C...

Realistically, the breaker doesn't protect the wire, it protects the stuff
around the wire.  What protects the wire is the interrupting capability of
the breaker in a very high current fault.
> >
> > 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
> > etc.
> >
> > 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.

The code is a "minimum" standard, not a "design guideline" or even a "good
practice".. P.E.s and architects (and, I'll bet, contractors) run into this
all the time when the owner/developer asks why you're designing something
more than the bare code minimum.  If you're building a subdivision of 500
houses, the difference between AWG14 and AWG12 makes a significant
difference in cost, even if the lights do blink a bit more.

The code's designed to make sure the house doesn't burn down, not that it's

> 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!

>From an old code (1981, but I don't think it's changed much, and I don't
have my current code handy)

Article 530-18 Motion Picture Studios
(b) Feeders ....
The overcurrent device setting for each feeder shall not exceed 400 percent
of the ampacity of the feeder, as given in Tables....
(d) ... The rating of the fuses or setting of the circuit breaker shall not
be over 400 percent of the safe ampacity of the cables or cords as given in
Tables ....

Table 310-16 gives 40A for 85C and 90C AWG10 wire (in a conduit or cable or
earth) based on 30C ambient.
Table 310-17 gives 55A for 85C or 95C AWG10 insulated wire in free air.

So, based on 530-18, you could OCP that AWG10 romex at 160A in a movie
studio, if you were bold enough. (And, from first hand experience, there's
lots of folks in the industry who are that bold, if not bolder, to the point
of frightening)

But even at home, if you used "knob and tube" (uninsulated wires in free
air), you might be able to get away with AWG10 wire and a 50A OCP.  Maybe
not a good idea, but probably within the code.

> 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.

Hmmm. farm outbuildings are pretty unregulated even in Los Angeles county
(something that a friend of mine counted on back in the 80' a small
cabin to code with all the permits, then have a bunch of "outbuildings" with
all sorts of useful functions..)
Regulatory Life has changed a lot locally, so I wouldn't count on
duplicating this today.

> 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.
I think the relevant advice is:

Do sensible things.
If you do things that aren't sensible, but are "within the code", be sure
you've done your homework.

> 73
> Gary  K4FMX


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.

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>