Why wire is sized to MCA and not OCP?

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CelectricB

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Hi all,

I have a relatively simple question concerning wire size. Consider the screenshot below.
1635966254795.png
I understand that, per code, the wire may be sized to 125% FLA making it 200A. My question is, suppose the motor load malfunctioned or inrush current sustained too long, and it draws 210A, is this not an issue that we consider? It makes more sense to me that the wire be sized to OCP so as not to allow more amperage through the wire than it can supply. Thanks in advance!
 
The CB only handles short circuit problems. The overload device handles overloads. So the overload device protects the wiring from overloads.

You can make the wire larger than required if it makes you feel better.

The code allows the CB rating to be 250% of the motor FLC from the tables.
 
petersonra said:
The CB only handles short circuit problems. The overload device handles overloads. So the overload device protects the wiring from overloads.
Click to expand...

Just to expand a bit: the diagram shown in the original post is missing a very important bit, the motor overload/starter/controller.

The motor load must have an overload device just as petersonra states. You can consider this device a form of advanced overcurrent protection that is tailored to the overheating characteristics of a motor. This overcurrent protection will also protect the wire from overheating.

So the upstream breaker protects the system wire short circuit, and the downstream motor overload controller protects the wire from moderate overloads which would eventually overheat it. Moderate overloads could be caused by winding damage in the motor or by excessive mechanical loading.

If something strange happens such as a high resistance fault that causes excessive current flow above the ampacity of the wire but below the trip point of the breaker, then the breaker won't trip. IMHO such a fault would be extremely unlikely, and the heat at the fault location would quickly lead to a proper short which would trip the breaker.

Jon
 
Also the OP mentioned FLA, the NEC would require you to use the FLC from the tables in Article 430 based on the HP to size the conductors. This may bump the conductors up a size.
 
CelectricB said:
Hi all,

I have a relatively simple question concerning wire size. Consider the screenshot below.
View attachment 2558281
I understand that, per code, the wire may be sized to 125% FLA making it 200A. My question is, suppose the motor load malfunctioned or inrush current sustained too long, and it draws 210A, is this not an issue that we consider? It makes more sense to me that the wire be sized to OCP so as not to allow more amperage through the wire than it can supply. Thanks in advance!
Click to expand...
Even if you used a 200 amp breaker to protect the 200 amp conductors, it is very likely that a 200 breaker will carry 210 amps forever. It would be in compliance with the requirements of the UL 489 if that 200 amp breaker carried 268 amps forever.
 
To explain the reasoning behind all of this, it is expected that the wire sized per the NEC rules will take a certain amount of overload current while the protective devices, whatever they are, sense and react to it. This is part and parcel to how the ampacity tables are determined. It’s not as though at 200A it’s fine, but at 210A the insulation catches on fire.
 
From the above info I could envision a 3 phase 480 125 HP motor. I'd estimate 3/0 wire on a 250 amp breaker with motor overload setting around 175 amps.
 
petersonra said:
The CB only handles short circuit problems. The overload device handles overloads. So the overload device protects the wiring from overloads.

You can make the wire larger than required if it makes you feel better.

The code allows the CB rating to be 250% of the motor FLC from the tables.
Click to expand...
But if you oversize the wiring, you risk having to upsize the EGC even more.
 
tom baker said:
Some us us were taught”the overload protects the motor, the beaker protects the wire”.
Click to expand...
NEC 430 divides motor circuit protection into two parts; overload (430 Part III, starting at 430.31) and short circuit/GF (430 Part IV, starting at 430.51). This is different than Overcurrent Protection required for non-motor circuits. The overloads are the overload protection. The breaker is for short circuit (and ground fault). The circuit breaker can be much larger (250%) than the wire ampacity (125%), because it is short circuit/GF protection only, and has to hold shut for the motor inrush.

Many people get upset when a motor branch circuit has an ampacity way below the MOCP, but it is still possible to comply with code with that big of a mismatch.

Also, if 160A is the total load of more than one motor, you only need ampacity for 125% of the largest motor, plus 100% of all the rest. Likewise for the SC/GF protection.
 
"But if you oversize the wiring, you risk having to upsize the EGC even more. "

Can you elaborate on the EGC increase for wire size increase.
 
Thank you. I went through it again and see why the statement was made.
I was over thinking it since it was a
going from a 200 to s 250 OCD. This would automatically have an increase to a #4 over a #6.

Now if he left the 200 OCD and increased wire size I see how 250.122(B) would apply. 2014 NEC.

#6, 26240
#4, 41740
3/0, 167800
4/0, 212600. Increase 1.26102= 33089
250, 250000. Increase 1.4890=
39071
 
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