Breaker Rating Larger Than Feeder Ampacity

[Sunny_92]

I came across the following circuit on a design that was engineered by another company:

480V Switchboard
|
175A Rated Thermal-Magnetic Circuit Breaker, Fixed LT Trip, Adjustable Instantaneous Trip
|
(1) set of #2 CU conductors in conduit, 40 feet
|
50 HP Soft Starter
|
(1) set of #2 CU conductors in conduit
|
Disconnect Switch
|
(1) set of #2 CU conductors in conduit
|
50 HP Motor

According to 2008 NEC Table 310.16, #2 CU is good for 115A if we assume 75 deg C terminations, so it would typically need to be protected by a 125A or smaller breaker. However, it is protected by a 175A breaker in this case. Is this a code violation?

Since this is a motor feeder, does Table 430.52 allow the #2 conductors to be fed from a 175A breaker in this situation?

 

[kwired]

The fact a motor is involved is what allows it to have a 175 amp breaker and still be code compliant. 65FLA x 2.5 = 162.5 - round up to next higher standard device is 175.

Since there is a soft starter involved it likely would never trip a 80 amp maybe even 70 amp breaker though.

 

[petersonra]

Since this is a motor feeder, does Table 430.52 allow the #2 conductors to be fed from a 175A breaker in this situation?

Most power circuits the CB provides both short circuit and overload protection.

In motor circuits, the CB generally only provides short circuit protection and the motor overload protection (in your case something likely built into the soft starter) protects the conductors from overload while also protecting the motor from overload. i would point out that IMO you are still required to use table 250.122 to size the EGC if you are using a wire type EGC.

 

[Sunny_92]

Got it, thank you both for confirming my suspicion!

 

[ggunn]

According to 2008 NEC Table 310.16, #2 CU is good for 115A if we assume 75 deg C terminations, so it would typically need to be protected by a 125A or smaller breaker. However, it is protected by a 175A breaker in this case. Is this a code violation?

If the conductors are 90 degree rated, derate their 90 degree ampacity for conditions of use and compare it to the breaker rating. The 75 degree ampacity is not relevant to this evaluation.

 

[kwired]

If the conductors are 90 degree rated, derate their 90 degree ampacity for conditions of use and compare it to the breaker rating. The 75 degree ampacity is not relevant to this evaluation.

I have no idea what you just tried to say there.

Typical motor application your minimum conductor ampacity needs to be 125% of the FLA in the tables at the end of art 430.

Typical motor application the terminal temp rating will be 75C and therefore the minimum conductor ampacity will need to come from 75C ampacity column.

You can use 90 C ampacity for adjustments, (ambient temp, #conductors in raceway) but if adjusted values result in smaller conductor you still must at least have the 75C conductor size as your minimum for the sake of the termination temperature.

OP's application could have been 4AWG copper before any adjustments were made or before any voltage drop considerations were taken, and still been on a 175 amp breaker.

 

[Smart $]

I have no idea what you just tried to say there.
...

I think what he is saying is because the minimum size conductor for a motor circuit must have an ampacity of 125% of motor FLA... after the application of adjustment and correction factors (result being lower typically), that when using a 90°C-rated conductor it will [in most cases] be as large as or larger than required for 75°C termination.

 

[ggunn]

I have no idea what you just tried to say there.Typical motor application your minimum conductor ampacity needs to be 125% of the FLA in the tables at the end of art 430. Typical motor application the terminal temp rating will be 75C and therefore the minimum conductor ampacity will need to come from 75C ampacity column. You can use 90 C ampacity for adjustments, (ambient temp, #conductors in raceway) but if adjusted values result in smaller conductor you still must at least have the 75C conductor size as your minimum for the sake of the termination temperature. OP's application could have been 4AWG copper before any adjustments were made or before any voltage drop considerations were taken, and still been on a 175 amp breaker.

What I am saying is that strictly looking at OCPD and conductor sizes, what makes a conductor adequately protected by OCP is if the ampacity appropriate for the conductors' insulation, derated for conditions of use, is greater than the next standard size down OCP from the one being used. Derating for terminal protection is not part of the calculation.

For example, in the PV (solar) arena where everything is continuous use, we set the inverter interconnection OCPD at the next size up from 125% of the inverter nameplate current. We evaluate the 90 degree conductors we use two ways: one is 90 degree ampacity derated for conditions of use and the other is 75 degree ampacity derated for continuous use. Both these derated ampacities must be equal to or greater than the inverter nameplate rating, but only the 90 degree conditions of use ampacity must be greater than the next size down OCPD from the one set by the inverter current.

That's all I am saying. I know next to nothing about motors. They are those thingies that spin, aren't they?

 

[Smart $]

What I am saying...

Well, though my assessment above is correct, I certainly presumed incorrectly on what you meant. :blink:

 

[Jraef]

SPECIFICALLY in a motor circuit, and only in a motor circuit, IF you have an motor Thermal OverLoad (TOL) device, it REPLACES the need for the thermal (L) trip in a circuit breaker. So even if the breaker has one, it becomes redundant and irrelevant; the TOL becomes not only the motor protective device, but also that of the cables. So if you have a 50HP motor and have sized the conductors as per article 430 pertaining to using the motor FLA tables, then those cables are protected by whatever is providing the motor TOL protection. In your case, that is the Soft Starter. It must have programmable TOL protection or a separate TOL relay. Once you have that, the breaker can be sized only to provide the Short Circuit protection (I). An "MCP" breaker in an FVNR starter does exactly that; it has magnetic-only trips, no thermals. But manufacturers can no longer use MCPs on Soft Starters and VFDs, so you will always get a thermal-mag breaker now, even though the thermal is irrelevant.

 

[Sahib]

I think what he is saying is because the minimum size conductor for a motor circuit must have an ampacity of 125% of motor FLA... after the application of adjustment and correction factors (result being lower typically), that when using a 90°C-rated conductor it will [in most cases] be as large as or larger than required for 75°C termination.

Is it not that the 90 degree conductor after derating is actually smaller than 75 degree conductor and so more economical to use?

 

[ActionDave]

What I am saying is that strictly looking at OCPD and conductor sizes, what makes a conductor adequately protected by OCP is if the ampacity appropriate for the conductors' insulation, derated for conditions of use, is greater than the next standard size down OCP from the one being used. Derating for terminal protection is not part of the calculation......

That's all I am saying. I know next to nothing about motors. They are those thingies that spin, aren't they?

Motors are different.

If you want a full explanation read Jraef's post. It was almost as exciting as the eclipse.

 

[kwired]

Is it not that the 90 degree conductor after derating is actually smaller than 75 degree conductor and so more economical to use?

90 deg conductor will be smaller, but you still must size the conductor to 75 deg ampacity if it lands on a 75 deg rated terminal.

nearly all terminations we currently use in general light and power distribution are 75 deg.

This leads us to having to use a conductor sized per the 75 deg ampacity column as a minimum conductor size, but if we need to make further adjustments we can calculate those adjustments using the 90 deg ampacity of the conductor insulation, when only a 70% adjustment is the end result, the size that was selected at 75 deg is still larger conductor and must be used. If after making adjustments the 90 deg conductor results in a larger conductor then you must use that larger conductor as the minimum size allowed.

 

[ggunn]

Is it not that the 90 degree conductor after derating is actually smaller than 75 degree conductor and so more economical to use?

Yes, maybe, but it is irrelevant. Derating the ampacity for continuous use at 75 degrees has nothing to do with the 90 degree ampacity of the wire.

 

[Smart $]

Is it not that the 90 degree conductor after derating is actually smaller than 75 degree conductor and so more economical to use?

90 deg conductor will be smaller, but you still must size the conductor to 75 deg ampacity if it lands on a 75 deg rated terminal.

nearly all terminations we currently use in general light and power distribution are 75 deg.
...

Yes, maybe, but it is irrelevant. Derating the ampacity for continuous use at 75 degrees has nothing to do with the 90 degree ampacity of the wire.

You are all looking at typical installations. I'm talking motor circuits only. A typical installation requires the termination to be determined using 125% of continuous plus 100% noncontinuous. A motor circuit is actually neither but for this purpose is considered a noncontinuous load, so 100% motor FLA is used to make the terminal temperature limitation determination.

Now consider typical installation minimum conductor ampacities are figured at only 100% of the calculated load. Motor circuit minimum conductor ampacity must be determined using 125% of motor FLA. The only conductor which has a 90°C rating of 125% greater than its 75°C Table value is #14. All other [larger] sizes have less than a 125% ratio. This forces a conductor (other than #14) to be one size larger than required for a 75°C termination in most cases.

 

[Sunny_92]

Now consider two scenarios with a different circuit seen below.

480V Switchboard
|
1000A Rated MCCB w/ adjustible LSIG electronic trip unit
|
(3) sets of #350 CU conductors in conduit, 30 feet (allowable ampacity = 930A)
|
1200A MCC

Scenario 1:
125% of largest motor FLA + 100% of the sum of the FLA of the remaining motors on the MCC = 634 A
Rating of largest branch PD in MCC + the sum of the FLA of the remaining motors on the MCC = 684 A

Scenario 2:
125% of largest motor FLA + 100% of the sum of the FLA of the remaining motors on the MCC = 849 A
Rating of largest branch PD in MCC + the sum of the FLA of the remaining motors on the MCC = 1074 A

430.62 leads me to believe that in scenario 1 the 1000A breaker must have the LT pickup trimmed back to 930A max, while in scenario 2 the 1000A breaker can be left with the LT pickup at 1000A.

Is this correct or am I missing something?

 

[Smart $]

Now consider two scenarios with a different circuit seen below.

480V Switchboard
|
1000A Rated MCCB w/ adjustible LSIG electronic trip unit
|
(3) sets of #350 CU conductors in conduit, 30 feet (allowable ampacity = 930A)
|
1200A MCC

Scenario 1:
125% of largest motor FLA + 100% of the sum of the FLA of the remaining motors on the MCC = 634 A
Rating of largest branch PD in MCC + the sum of the FLA of the remaining motors on the MCC = 684 A

Scenario 2:
125% of largest motor FLA + 100% of the sum of the FLA of the remaining motors on the MCC = 849 A
Rating of largest branch PD in MCC + the sum of the FLA of the remaining motors on the MCC = 1074 A

430.62 leads me to believe that in scenario 1 the 1000A breaker must have the LT pickup trimmed back to 930A max, while in scenario 2 the 1000A breaker can be left with the LT pickup at 1000A.

Is this correct or am I missing something?

Appears correct to me, but you could go with a lower LT setting in S1. You are taking advantage of 430.62(B) in S1 with an LT setting of 930A.

 

[Jraef]

Appears correct to me, but you could go with a lower LT setting in S1. You are taking advantage of 430.62(B) in S1 with an LT setting of 930A.

And for a lot of breakers the trip settings are not that granular. You are going to have a setting of 900A or 1000A, not 930A and the NEC allows you to use the next size up, so 1000A is acceptable.

 

[Smart $]

And for a lot of breakers the trip settings are not that granular. You are going to have a setting of 900A or 1000A, not 930A and the NEC allows you to use the next size up, so 1000A is acceptable.

Next size up not permitted above 800A [240.4(C)].