conductor sizing

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I HAVE A MOTOR STARTER PANEL I AM BUILDING.IT HAS A 110 AMP FUSED DISCONNECT.FROM THE FUSED DISCONNECT ON THE PANEL I AM GOING TO A
DISTRIBUTION BLOCK.FROM THE DISTRIBUTION BLOCK I AM GOING TO EIGHT 3 POLE FUSE BLOCKS.SHOULD THE SIZE OF THE WIRE BETWEEN THE DIST. BLOCK
AND THE 3 POLE FUSE BLOCKS BE SIZED FOR 110 AMPS OR JUST THE AMOUNT OF AMPS THAT IS DETERMINED BY THE FUSE ON THE 3 POLE FUSE BLOCK.OR IS
THERE SOME OTHER RULE.THE DISTANCE BETWEEN ANY OF THESE POINTS IS LESS THAN 10 FT IF THAT MATTERS.EVERYTHING IS IS ONE BOX.THANKS FOR ANY
HELP
 
The fuses and conductors should be selected based on the requirements of the individual motors, not the full available 110A. If the
individual fuse blocks are to feed VFDs, the conductor ampacity and fuse ratings will be determined by the max HP rating of each VFD.

Proper sizing can be quite involved, and since mistakes can be costly and dangerous, please refer to Article 430 of the National Electric
Code - "Motors, Motor Circuits, Controllers". (As I am writing this, I am refering to the NEC 1990 Handbook, by Early et al.)

The March 2000 edition of Allen Bradely's Industrial Controls catalog has a good writeup on selection and sizing of motor disconnects and
related equipment on page 4-79 that includes examples of motor and combination (motor and non-motor)loads, and other useful information.

Be cautious, and good luck!
 
F

Ferdinal Rais

Yes. The 110 Amps fuse is intended to protect the circuits downstream, including the wires. So, all wires/conductors upstream the 3 poles
fuse blocks shall be sized for minimum 110 Amps.

FR
 
B
National Electrical Code(NEC) Article 430, "Motors, Motor Circuits, and Controllers," is the governing NEC article for motors. In particular, if the wires in question are feeder taps then NEC(2002) 430.28, "Feeder Taps,"
would appear to be applicable. 430.25, "Multimotor and Combination-Load
Equipment," 430.53, "Several Motors or Loads on One Branch Circuit," and
430.112, "Motors Served by Single Disconnecting Means" may also be of interest in regards to this installation's design. There may be other
sections of Article 430 and the NEC that may specifically apply to this installation but without detailed information on the design, it is difficult to identify which other sections may specifically apply.

Bill Mostia
===========================================
William(Bill) L. Mostia, Jr. PE
Independent I & E Consultant
WLM Engineering Co.
P.O. Box 1129
Kemah, TX 77565
[email protected]
281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.
 
Check out NEC section 240-21. It covers location of overcurrent devices for conductors. Normally the conductors are protected where they
receive their supply, but there are 7 very specific exceptions. In the text is a description of the exception where you do not have to run
a large gauge wire into a fuse and a smaller one out.

Don
 
G

Gary H. Lucas

Sorry, but this is not true. The NEC has rules concerning a 'tap' off of a
feeder. The basic concept is that the current carrying capacity of a
conductor is related to BOTH the cross section area AND the length. A very
short conductor has a very low resistance, and a very low voltage drop, so
it can carry a very high current. By limiting the length of the conductor
you can insure that the small conductor will carry a fault current for a
sufficient time to trip the large overcurrent device.

A practical example: My dad (electrical contractor) wanted to ensure that
our hand power tools had good ground wires. We hooked a 6000 watt 120 volt
load from the ground wire to the case of a power tool that we had purposely
cut all the strands of the ground wire except ONE, using a sharp knife.
This was to simulate a wire breaking over time from bending where the
strands fail one at a time. The single strand carried the load with NO
signs of overheating! In order to burn it off we had to apply a dead short,
which burned the strand off and tripped the 20 amp breaker.

Gary H. Lucas
 
B
I have always seen the wires coming out of a power distribution block sized for their respective load. Technically, they should be sized for 110 amps, but it is not very practical. To be on the safe side, keep the wires as short as possible.

Bill
 
R

Randy DeMars

See NFPA 79 section 8.4 for information on fusing in such cases. There is also a similar rule somewhere in the European standards. If you need help finding that let me know.
 
E

Eric M. Klintworth

Mr Mostia is correct.
Further, it sounds like the control panel would fall under NFPA 79 (in the USA). NFPA 79 is even a bit more liberal than the NEC (aka NFPA 70). The (slightly dated) version I have allows the conductors between the splitter block and the fuses to be sized for "the sum of the maximum
load currents applied" as long as the conductors are under 10 ft long and do not extend beyond the control panel enclosure. All these conditions are met by the original poster, so he could just size his wires to be protected by the fuses.

I would recommend getting a copy of NFPA 79. Its available at "www.nfpa.org":http://www.nfpa.org

Eric M. Klintworth, PE

Columbus, Ohio
-------------------------------
 
T

Trea Parsons

Read article 430-28 Feeder Taps I believe this would apply or at the very least would be a good rule of thumb
 
> Sorry, but this is not true. The NEC has rules concerning a 'tap' off
> of a feeder. The basic concept is that the current carrying capacity
> of a conductor is related to BOTH the cross section area AND the
> length. A very short conductor has a very low resistance, and a very
> low voltage drop, so it can carry a very high current. By limiting the
> length of the conductor you can insure that the small conductor will
> carry a fault current for a sufficient time to trip the large
> overcurrent device.

There appears to be some confusion here with Ohm's law and the ability of a conductor to carry current. Ohm's law says that the lower the
resistance for a given voltage, the higher the current that will flow but makes no statement regarding if the conductor can actually carry the
current. A fuse is an example of a conductor that is not capable of flowing particular currents(overload and fault) even though Ohm's law may
say that the current will flow.

The short circuit withstand of a particular conductor(based on ICEA methods) is a function of the current squared times time (I^2 x t) and
the cable properties(conductor area and maximum temperature ratings). A discussion of short time rating of cables can be found at:

http://www.mikeholt.com/Newsletters/Conductor SC Protection-Long R2=.PDF(Note that is URL is long and your browser font may make it two lines which you will then have to reconstruct the URL.)

The reasons behind the various tap rules in my opinion are a bit murky. In some cases, they are required to be quite short but in others they are
allowed to be quite long. The constant factors seem to be that the tap conductors be protected either by physical means and/or by length (shorter the tap, the less likely something will happen to cause a fault), protected from downstream overloads, and under some conditions(typically the longer ones) the tap conductor sizes are related to the feeder protection or ampacity.

Bill Mostia
- - - - -
William(Bill) L. Mostia, Jr. PE
Independent I&E Consultant
WLM Engineering Co.
P.O. Box 1129
Kemah, TX 77565
[email protected]
281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.
 
G

Gary H. Lucas

No confusion on my part! Conductor heating is a function of voltage drop in the conductor times the current = power, and the ability of that conductor to dissipate that heat. Even fuses carry large overcurrents for short periods.

Gary H. Lucas
 
Please note that while NFPA 79, "Electrical Standard for Industrial Machinery," is commonly applied to a wide class of machinery, its scope is
actually more limited than the general definition of "machinery." One should review the definition of "industrial machinery" in the definition section of NFPA 79 before determining if NFPA 79 is a code requirement.

Please also note that my previous note referred to NEC code sections on this topic rather than giving an direct answer because tap rules are complex and without knowing the complete design hard to apply via an e-mail note. In general, if a tap rule applies you are allowed to have lesser tap conductors than the feeder conductors without direct overcurrent protection of the tap
conductors but there are application rules and requirements that have to be followed. For those of you that would like some additional light reading on this subject, you might also review NEC 240 "Overcurrent Protection," Section VIII "Supervised Industrial Installations".

Bill Mostia
===========================================
William(Bill) L. Mostia, Jr. PE
Independent I & E Consultant
WLM Engineering Co.
P.O. Box 1129
Kemah, TX 77565
281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.
 
G

Gord Graham P.E.C.

Does the USA not have any rules about non electrical people doing electrical work that will endanger the general public or the personel
where this installation will end up? We sure do in CANADA Easy reading material to answer this question quickly
Canadian Electrical code
Rule 14-100 (b) (i) (ii) (iii) (iv)
Hope you don't mind Canadian input
If you would like me to send you a scanned copy of our rule let me know.
 
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