|
Posted by Bud on August 4, 2005, 11:29 am
Chris Lewis wrote:
>
>>This is odd, could you explain that to me? I am an automotive systems
>>engineer and I design the wiring for automobiles. The length of wire
>>has nothing to do with the fuse rating. The device on the end of the
>>wire also has nothing to do with the fuse rating.
>
>
>
>>Well ok, some devices can terminate a few different sizes of wire, and
>>this is a physical thing, nothing to do with current amount but with
>>diameter, etc.
>
>
>
>>So why does wire length matter for breaker size in a home?
>
>
> It doesn't matter directly, but there are some indirect effects.
>
> First of all, and this applies to your engineering, the fuse/breaker
> should be no larger than the current carrying capacity of the wire.
>
Rules change a little for motor circuits. In general the wire has to
have a rating 125% of the motor rating. Because of the high starting
current of motors, time delay fuses/circuit breakers can have a rating
of 175% of the motor rating; this is for short circuit protection. For
overload protection motor starters with a narrow range overload trip
unit or thermal trip units in the motor are used. For low duty-cycle
welders the fuse can be much larger than the wire rating. (Don't use
higher ratings if you don't know what code requirements are and what you
are doing.)
> Second of all, in AC power wiring, devices are tested and approved
> for connection to a maximum circuit ampacity. So, you can't put a
> 15A 120V device on a 60A circuit.
>
Home A/C units will be marked with a maximum overcurrent device size. If
it says maximum fuse size, the device must be a fuse.
> And thirdly, which is where wire length comes into play: certain
> devices (especially A/Cs and other large motor-powered devices) have
> very large startup surges. The wiring has to be large enough so
> that coupled with wire length, there isn't excessive voltage drop to
> the device. The more the voltage drops, the more the startup surge
> is prolonged. Coupled with the fact that breakers and fuses have
> time-delay factors built in, it's entirely possible for a large
> startup motor load (like a central A/C) to trip a breaker on 100' of
> wire when it wouldn't trip the breaker on 10' of the _same_ wire.
>
> Often the wire size has to be a size or two larger than the ampacity
> would dictate to reduce voltage drop. With A/Cs and certain other
> loads, not only the wire sometimes has to be larger than you'd expect
> (with long runs), the breaker has to be somewhat larger than the
> steady-state draw of the A/C would suggest.
>
> Central A/Cs often have somewhat oversize wires and breakers, with
> a fuse local to the device closer to the A/C's continuous draw.
>
| 
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML Sitemap