motor cable

[Jk1996]

say if you have a motor circuit fed by 100amp fuses then then the overload set at 75 amps does this mean the motor cable should be rated for 100amps or should it be rated at 75amps?

 

[itsmoked]

75 plus the obligatory overage. "100% of non-continuous loads and 125% any continuous load"

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

It's been a while, but I don't think that overloads exist as far as conductor sizing is concerned. You have the fuse and you have the conductor. But on the other hand, if you're talking overloads it's probably a motor circuit and the rules for motor circuits are entirely unlike the rules for any other type of circuit and generally the only determinant is the motor horsepower.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Gr8blu]

IMO - the fuses set the rating for the circuit. Just because you have an arbitrary setpoint that is lower does not relieve you of the responsibility to design to what the circuit can take. In this case, treat it like a 100 A circuit (based on fuses) ... plus any overloads required by the local jurisdiction.

Converting energy to motion for more than half a century

 

[che12345]

Dear Mr. Jk1996 (Electrical)(OP)4 May 22 21:47
"... say if you have a motor circuit fed by 100amp fuses then then the overload set at 75 amps does this mean the motor cable should be rated for 100amps or should it be rated at 75amps? "
1. Standards/Regulation reference: IEC 60364-x, BS 7671, NEC ... etc.
2. The fuse is for protection of the cable on (over-current e.g. short-circuit current) NOT for protection of (over-load).
3. The fundamental relationship for Fuse:
a) Ib<In<0.9Iz, b) I2<1.6Iz [see IEC 60364-x]
Where Ib = load current, In = nominal current of Fuse, Iz =effective conductor carrying capacity, I2 = current causing operation of Fuse.
4. From above 3. a) In = 100A fuse, In< 0.9Iz. That is Iz> 100A/0.9 = 111.1 A
5. As an indication [see BS 7671]: Copper , PVC insulation, ambient 30degC, conductor operating at 70Deg C, Open clipped direct or lay on a non-metallic surface... 25mmsq is 114A, or 35mmsq is 141A.
Che Kuan Yau (Singapore)

 

[itsmoked]

Specifically for motors that have overloads you can actually have a breaker or fuse that's much higher than the conductor rating. The reason is because the load is ONLY an overload protected motor. If you have other things involved that aren't overload protected then no, the breaker/fuse must protect the conductor from overload also.

Lots of VFD data sheets describe the wire size and then specify a fuse or breaker that might make your hair stand on end. However, the overload function completely prevents the specified conductor from overheating while the conductor will easily support the breaker rating long enough for the breaker to safely trip in the event of a short circuit.

Thanks Che! Exactly.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Here the cable is sized to 125% of the motor FLA as taken from a table in the electrical code. Fuses can be 175% rounded up. Breakers can be 250% rounded up.

But, you probably should be following the electrical code or standard where you are located in the world.

 

[che12345]

@ Dear Mr. itsmoked (Electrical)5 May 22 11:47
"... Specifically for motors that have overloads you can actually have a breaker or fuse that's much higher than the conductor rating. The reason is because the load is ONLY an overload protected motor. If you have other things involved that aren't overload protected then no, the breaker/fuse must protect the conductor from overload also...."
1. I presumed the arrangement to be from (Fuse 100A), Cable* to (starter with over-load protection) and Cable** to the motor terminal.
2. The Fuse shall protect both lengths of cable* and cable** against [short-circuit current], NOT [over-loading current]. The fuse shall blow before the cables are (over-heated) by [short-circuit current]. The conductor size is limited by the Breaker/Fuse size, not the thermal over-load rating. The Cables can be >> than the thermal over-load setting or motor full-load rating, but the In<0.92Iz per IEC.
Che Kuan Yau (Singapore)

 

[che12345]

@ Dear Mr. LionelHutz (Electrical)5 May 22 12:16
"... Here the cable is sized to 125% of the motor FLA as taken from a table in the electrical code. Fuses can be 175% rounded up. Breakers can be 250% rounded up. But, you probably should be following the electrical code or standard where you are located in the world.".
1. Yes, I am in full agreement with learned Mr. LionelHutz advice taken from the NEC.
Note: In the US, NEC is the Law. Comply it.
2. I try to work out based on 100A fuse, what is the minimum cable size.
Based on a) Cable size > 1.25 Motor FLA, b) Fuse > 1.75 Motor FLA.
With Fuse rated 100A, the motor FLA < 100A/1.75 = 57.14 A
Cable size > 1.75 motor FLA = 1.75 x 57.14A =99.99 A
Per NEC table 75Deg C cable at ambient 30Deg C rated 100 A, cable size is AWG 3 = approx 26.6 mmsq.
Note: the nearest higher IEC size is 35mmsq.
3. Observation: Both IEC and NEC methods are in agreement.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. Jk1996 (Electrical)(OP)
@ Dear Mr. LionelHutz (Electrical)5 May 22 12:16
Corrigendum
My apology, the following is incorrect:
" With Fuse rated 100A, the motor FLA < 100A/1.75 = 57.14 A
Cable size > 1.75 motor FLA = 1.75 x 57.14A =99.99 A
Per NEC table 75Deg C cable at ambient 30Deg C rated 100 A, cable size is AWG 3 = approx 26.6 mmsq. Note: the nearest higher IEC size is 35mmsq ".
It should be:
" With Fuse rated 100A, the motor FLA < 100A/1.75 = 57.14 A
Cable size > 1.25 motor FLA = 1.25 x 57.14A =71.42 A
Per NEC table 75Deg C cable at ambient 30Deg C rated 85 A, cable size is AWG 4 = approx 21.15 mmsq. Note: the nearest higher IEC size is 25mmsq "
Che Kuan Yau (Singapore)

 

[LionelHutz]

Except 57.14A isn't a standard motor current rating.

52A = 40hp@460V/50hp@575V or 62A = 60hp@575V or 65A = 50hp@460V are three possible FLAs that could use 100A fuses.

The fuses don't have to be 175% rounded up and the breakers don't have to be 250% rounded up. We mostly use 150% for both.

 

[jraef]

As Lionel was saying, for determining conductor size, you do not use the motor nameplate FLA, you must use the FLC value in the NEC table in article 430 based on the HP rating of the motor . It’s not a choice, it’s a rule.



" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

@ Dear Mr. jraef (Electrical)7 May 22 07:41
".... for determining conductor size, you do not use the motor nameplate FLA, you must use the FLC value in the NEC table in article 430 based on the HP rating of the motor . It’s not a choice, it’s a rule..."
1. Agreed. However, it is essential to be aware of the limitations and take note of the Exception and motors built for especially low speeds or high torques...... the nameplate current rating shall be used.
2. NEC 430-6 (a) (1) " the values shown in Tables 430-147....150 shall be used to determine the ampacity of conductor.... , instead of the actual current rating marked on the motor nameplate..... Exception No. 3: For a listed motor-operated appliance that is marked horsepower and full-load current, the motor full-load current marked on the nameplate of the appliance shall be used instead of the horsepower rating on the appliance to determine......"
3. NEC Table 430-149 " ..... Motors built for especially low speed or high torques may require more running current, multispeed motors will have full-load current vary with speed, in which case the nameplate rating shall be used...."
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. Jk1996 (Electrical)(OP)
@ all NEC user community

1. Reference: As per NEC NEC 430-6 (a) (1) " the values shown in Tables 430-147....150 shall be used to determine the ampacity of conductor.... , instead of the actual current rating marked on the motor nameplate..... Exception No. 3: For a listed motor-operated appliance that is marked horsepower and full-load current, the motor full-load current marked on the nameplate of the appliance shall be used instead of the horsepower rating on the appliance to determine......"
NEC Table 430-149 " ..... Motors built for especially low speed or high torques may require more running current, multispeed motors will have full-load current vary with speed, in which case the nameplate rating shall be used...."
2. Following is my personal opinion. All learned advice are greatly appreciated and welcome.
2.1 The values shown in Table 430-147.....150 are assigning a "fictitious" current value based on standard horse power rating, taking the phases, voltage rating only.
2.2 If the actual current rating marked on the nameplate is taken:
a) the value would be the "actual" input current value which would had taken into account of the Power (hp/kW), whether it is a single or three-phase, rated Voltage, rated
Frequency, rated Speed (RPM), rated Power-factor, rated Efficiency etc.
b) it would replace Table 430-147 ....150, which would be superfluous.
Che Kuan Yau (Singapore)

 

[LionelHutz]

Exception 3 is for an appliance. The definition of an appliance is “utilization equipment, generally other than industrial, that is normally built in standardized sizes or types and is installed or connected as a unit to perform one or more functions, such as clothes washing, air conditioning, food mixing, deep frying, and so forth.

 

[Jk1996]

So am I right in thinking for a motor circuit the fuse is purely for short circuit protection. Overloads on the cable will be protected by the overload set to 70Amps so the cable can be sized to the 70 amps not to 100amp?

 

[che12345]

Dear Mr. Jk1996 (Electrical)(OP)2 Jun 22 17:54
"....#1. am I right in thinking for a motor circuit the fuse is purely for short circuit protection. #2. Overloads on the cable will be protected by the overload set to 70Amps so the cable can be sized to the 70 amps not to 100amp? ...".
General: a) The usual motor circuit would comprise of two lengths of cables : cable #1 from 100A fuse to the starter with a thermal over-load set at 75A. From the thermal over-load, another length cable #2 to the motor.
b) Now, consider that the 100A fuse must protect cable #1 and #2 from short-circuit. Note: For Type 2 coordination (per IEC), the fuse must also protects the thermal over-load from damage by short-circuit current along cable #2, including the motor.
#1. Yes
#2. No. The thermal over-load does NOT protects cable #2 from short-circuit current. The fuse must protect it including the thermal-overload for type 2 coordination per IEC.
Che Kuan Yau (Singapore)