NEC - Conductor sizing for motor and cable protection

[OCPD]

Hello All,

I have a question on NEC cable protection for motor fed via ATS. As the conductor and upstream protective device settings for motor is sized differently than regular circuit.

A 600A upstream breaker feeds 400A ATS via 1 set of 500kcmil Copper cable (380A). The load side of ATS is connected to 300kVA chiller. The nameplate of the chiller says Min. Circuit Ampacity of 345A and Max. Circuit Breaker Size of 800A. My question is since the ATS feeds the motor which per nameplate is good thru 400A only (as per the sticker inside) is this still good per NEC.

My thoughts are the ATS is not protected by the upstream 600A breaker. I wanted to make sure I am thinking correctly since NEC on motors always stumps me.

 

[dkidd]

A 600A upstream breaker feeds 400A ATS

You have a problem right there. The ATS needs to be protected at 400A.

 

[Dennis Alwon]

I agree with dkidd - if the breaker feeding the ats is 600 amp then the equipment must be rated 600 amps

408.36 Overcurrent Protection. In addition to the requirement
of 408.30, a panelboard shall be protected by an
overcurrent protective device having a rating not greater
than that of the panelboard. This overcurrent protective device
shall be located within or at any point on the supply
side of the panelboard.

 

[infinity]

What if the ATS were a 400 amp disconnect switch would it then be code complaint? Seems like the 600 amp OCPD is just for starting current and with an MCA of 345 nothing will ever happen to the ATS.

 

[texie]

Hello All,

I have a question on NEC cable protection for motor fed via ATS. As the conductor and upstream protective device settings for motor is sized differently than regular circuit.

A 600A upstream breaker feeds 400A ATS via 1 set of 500kcmil Copper cable (380A). The load side of ATS is connected to 300kVA chiller. The nameplate of the chiller says Min. Circuit Ampacity of 345A and Max. Circuit Breaker Size of 800A. My question is since the ATS feeds the motor which per nameplate is good thru 400A only (as per the sticker inside) is this still good per NEC.

My thoughts are the ATS is not protected by the upstream 600A breaker. I wanted to make sure I am thinking correctly since NEC on motors always stumps me.

An ATS presents unique OCPD requirements and OCP must be provided in accordance with the manufacturers instructions as that is the way they are tested and listed. There should be a label inside or in the documentation listing the allowable size and type of OCPD allowed for the switch and the conditions of use depending on the OCPD. For example, it may require an exact brand and model and size for certain withstand ratings.
I would be very surprised if the switch is not be labeled for a 600 amp breaker. It may even list an 800 as OK under the right conditions.
I think I would be more concerned about the chiller manufacturer coming back on me if that 600 amp breaker ever tripped on startup since that 600 breaker is 25% smaller than the max listed for the chiller.

 

[OCPD]

An ATS presents unique OCPD requirements and OCP must be provided in accordance with the manufacturers instructions as that is the way they are tested and listed. There should be a label inside or in the documentation listing the allowable size and type of OCPD allowed for the switch and the conditions of use depending on the OCPD. For example, it may require an exact brand and model and size for certain withstand ratings.
I would be very surprised if the switch is not be labeled for a 600 amp breaker. It may even list an 800 as OK under the right conditions.
I think I would be more concerned about the chiller manufacturer coming back on me if that 600 amp breaker ever tripped on startup since that 600 breaker is 25% smaller than the max listed for the chiller.

Now when I think about your comment and look at the label it looks the breaker should be good. The breaker upstream is a Siemens SHMD6 (LTPU set at 600A). Since the motor load is less than switch rating. The circuit with upstream 600A breaker with 1 set of 500kcmil feeder to 400A ATS which has motor load with (MCA=345A and Max Circuit breaker size as 800A should be fine. Am I correct?

I have attached the image of the ATS label. The amperage listed next to the breakers on the left is that the maximum upstream device size?