Good vent actually

Posted by John T on April 27, 2012 at 07:42:38 from (216.249.82.117):

In Reply to: Vent on electrical ratings posted by David G on April 27, 2012 at 06:54:00:

VERY GOOD Great Post

A) I (long retired EE) have also advised the NEC calls out MINIMUM acceptable standards to protect life and property but that DOES NOT MEAN one cant exceed those standards. In fact, if things ever got "close" I, as a conservative designer, would opt for an upgrade or even bigger wire etc just to err on the safe and conservative side, thats just how Im WIRED lol

B) YES its NOT what anyone here (including yours truly) says or thinks that matters regardless if their opinion is lay or trained professional, ITS WHAT THE LOCAL AUTHORITY SAYS that counts the most. HOWEVER, if the local authority suggests something LESS then what the NEC calls for as a minimum standard THAT WOULD MAKE ME NERVOUS...........

C) As far as protecting motors and motor branch circuits YES YOURE RIGHT. The purpose of a circuit breaker in the motor branch circuit is to protect the feeders. The purpose of the thermal overload protection device (perhaps integral to motor or inside a motor starter or combination strter) IS TO PROTECT THE MOTOR FROM OVERHEAT AND OVERCURRENT. NOTE there are certain limited situations and exceptions in which the circuit breaker may be sized bigger then normal if necessary to allow the motor to start.

NOWWWWWWWWWW AS FAR AS WIRE SIZES they way I approached it as a conservative designer was as follows:

1) You FIRST compute the max continuous load

2) You size the wire so the load is no greater then 80% of the max continuous load

3) You size the overcurrent protection device to match and protect the wires ampacity i.e. if you use 20 amp rated wire, you use no bigger then a 20 amp breaker WELL DUHHHHHHHH

AND

You calculate VOLTAGE DROP and if the voltage drop is excessive then you increase the wire size unitl the drop is acceptable.

SUBJECT TO SOME EXCEPTIONS FOR WELDERS AND DUTY CYCLES ETC

NOTE if you do all the above and the wire is properly protected and the voltage drop is within limits, all will work and theres really no need to go wild and double or triple the wire sizes like some lay persons may suggest, but hey if that turns your crank or floats your boat or your dad did it that way HEY IM NOT STOPPING YOU its your house and your money so go for it. Im ONLY saying if you apply the NEC minimum standards and compute loads and dont exceed 80% of the max load current in the wires rating and if voltage drop is within limits THAT WILL WORK FINE AND SATISFY THE NEC AND MOST LOCAL AUTHORITY but feel free to do as yall please and double or triple wire if you have a need..

PS many people look at short hand versions or incomplete wire ampacity charts and dont realize those ratings depend on MANY VARIABLES SUCH AS............. Temperature,,,,,,,,,,Conductors in free air or enclosed,,,,,,,,,If enclosed (or buried in earth) how many conductors in what enclosures,,,,,,,,,,The class of insulation

Sooooooooooo you have been so advised

FUN chat, great post, thanks

John T

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