Motor Fuse / Conductor size issue. 2

[snoogie]

I am looking for a litle guidance about somthing that is driving me crazy whilst trying to size a motor conductor/fuse combination (Using NEC Code).
I have a 75HP AC motor (regular NEMA frame 1.15 SF)with seperate O/L protection so the fuse is SC and GF protection only and I want Type 1 coordination. So I use the FLA from the NEC tables = 96A, my branch circuit conductor MINIMUM size must be at least 125% FLA capable = 120A min so 1AWG (75degree C rating) However the application is for "heavy" start so i can increase the fuse the to 225% (Time delay RK5 Type)so potentially I can use a 2.25 x 96 = 216A say 200A fuse. The question is can my conductor still remain as 1AWG OR should the conductor always be rated higher than the capacity of fuse rating. I am thinking that I can use the 1AWG as the idea is the fuse will blow before the cunductor has time to melt (as the code/fuse/cable insulation has all been carefully planned to work that way (ha!)) and I cannot find a statement that says "the conductor size must be larger than the fuse capacity". But my real feeling is that it should be.
Another related issue is I see a lot of the larger cables in control panels (like from main disconnects and distribution blocks) that are the high temperature (200deg c)type that look like they have been sized directly with the current capacity in mind and not the fact that the terminal max temp capacity maybe only 75degc. (ie neglecting the fact that the terminal needs the copper content of a lower temp rated cable todisspiate the heat at the terminal hence a larger cable size regardless of capacity) Has anyone out there been caught out by this ?

 

[aolalde]

NEC table 430-152 states the Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground Fault protective devices.
300% of FLC for none time delay Fuse.
175% of FLC for Time delay Fuse.

This is for GF or SC protection.

The ampacities of insulated conductors are intended for Continuous Duty (130 Amperes for #1 AWG). The conductor is protected by the motor overload protection.

 

[GGOSS]

Hello snoogie,

Although I am not an expert in this area, I would have thought that in order to achieve Type 1, deviating from the fuse type and size specified by the contactor manufacturer would result in an uncoordinated system.

I guess what I'm trying to say is that cable size may not be your only issue. You need to consider the contactor as well.

Regards,
GGOSS

 

[snoogie]

GGOSS,There is another statement in the code, that says if other devices are specifically marked with maximums that they override the selection, soit should be considered as you say.

Do others agree with aolalde ? In that the overload is going to protect the cable so the 1AWG should be sufficient. (I know thats where the 125% minimum size rule comes from as the extra 25% is to cover the ovelroad setting.
It sounds good to me. The cable is rated to take the "actual" max running load (plus some) and fuse will be fast enough to protect against the instantaneous fault current.

 

[dpc]

In the NEC, the motor feeders are sized based on 125% of motor FLA. There is no direct relationship between the motor feeder conductor ampacity and the required motor short circuit protection.

So, as long as conductors and overcurrent protection are sized per Article 430, you're in compliance with NEC.

 

[rbulsara]

adolade is correct.

So is dpc, except I would not say that there is no relationship between the conductor and the breaker for SC protection.

But that fact the at the 125% rated conductors are adequately protected for short circuit currents by 250% rated breakers. (If you plot cable damge curves vs. the breaker time current curve, you will find that the damage curve is well beyond or above the breaker curve) The sustained overload is protected by the overload relay in the motor controllers as adolade indicated.