direction on MOCP (maximum overcurrent protection)

[Coastalguy1]

I would like some direction on MOCP-
Here is an example-
I have a 350ton chiller-480v 3ph
manufacturer data- 386 MCA, 301.8FLA, 600A MOCP
I understand the 600amp is the maximum overcurrent protection but don't I take the FLA- 301.8A x 125%= 377.25A which would allow me to use a 400amp fuse or breaker?
Thank you,

 

[don_resqcapt19]

You can use any size OPCD that is equal to or smaller than the maximum listed on the nameplate. If you go too small the unit may trip the OCPD on start up. You do need to use wire with an ampacity of at least 386 amps no matter what size OCPD you use.

 

[Coastalguy1]

Thank you Don. Got it on the wire size.
:thumbsup:

 

[jumper]

You can use any size OPCD that is equal to or smaller than the maximum listed on the nameplate. If you go too small the unit may trip the OCPD on start up. You do need to use wire with an ampacity of at least 386 amps no matter what size OCPD you use.

Question: Given the size of this unit, let's call this circuit a feeder:

Would not not this:

215.3 Overcurrent Protection. Feeders shall be protected
against overcurrent in accordance with the provisions of
Part I of Article 240. Where a feeder supplies continuous
loads or any combination of continuous and noncontinuous
loads, the rating of the overcurrent device shall not be less
than the noncontinuous load plus 125 percent of the continuous
load.

determine a minimum size?

 

[don_resqcapt19]

Why would it be a feeder?...size of the circuit has nothing to do with the question of feeder or branch circuit.

 

[jumper]

Why would it be a feeder?...size of the circuit has nothing to do with the question of feeder or branch circuit.

Okay, we make it a branch circuit HVAC circuit. Is there a minimum size OCPD?

210.20 Overcurrent Protection. Branch-circuit conductors
and equipment shall be protected by overcurrent protective
devices that have a rating or setting that complies
with 210.20(A) through (D).
(A) Continuous and Noncontinuous Loads. Where a
branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the rating
of the overcurrent device shall not be less than the noncontinuous
load plus 125 percent of the continuous load.

don_resqcapt19

You can use any size OPCD that is equal to or smaller than the maximum listed on the nameplate.

 

[infinity]

Okay, we make it a branch circuit HVAC circuit. Is there a minimum size OCPD?

210.20 Overcurrent Protection. Branch-circuit conductors
and equipment shall be protected by overcurrent protective
devices that have a rating or setting that complies
with 210.20(A) through (D).
(A) Continuous and Noncontinuous Loads. Where a
branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the rating
of the overcurrent device shall not be less than the noncontinuous
load plus 125 percent of the continuous load.

IMO the minimum size is based on the MCA. If the unit has a MCA of 100 amps and I use a 100 amp conductor on a 80 amp OCPD then the circuit ampacity is 80 amps not 100.

 

[jumper]

IMO the minimum size is based on the MCA. If the unit has a MCA of 100 amps and I use a 100 amp conductor on a 80 amp OCPD then the circuit ampacity is 80 amps not 100.

That was kinda my point.

When Don said this: "You can use any size OPCD that is equal to or smaller than the maximum listed on the nameplate." I was saying that I disagree in part.

IMHO, I believe that a more accurate statement would be:

"You can use any size OPCD that is equal to or smaller than the maximum listed on the nameplate, but not less than the nameplate MCA".

 

[don_resqcapt19]

That was kinda my point.

When Don said this: "You can use any size OPCD that is equal to or smaller than the maximum listed on the nameplate." I was saying that I disagree in part.

IMHO, I believe that a more accurate statement would be:

"You can use any size OPCD that is equal to or smaller than the maximum listed on the nameplate, but not less than the nameplate MCA".

Good point.

Yes, that would be correct as the size of a branch circuit is based on the rating of the OCPD, so that would require a OCPD with a rating equal to or greater than the MCA for the minimum.

 

[GoldDigger]

Good point.

Yes, that would be correct as the size of a branch circuit is based on the rating of the OCPD, so that would require a OCPD with a rating equal to or greater than the MCA for the minimum.

And Mike Holt asserts, with accompanying documentation, that if the circuit conductor size supports the MCA and the motor has integral overload protection, then the OCPD size can be greater than that otherwise required to protect the circuit conductors (in the absence of motor overload protection), up to the maximum OCPD size specified by the manufacturer.

 

[don_resqcapt19]

And Mike Holt asserts, with accompanying documentation, that if the circuit conductor size supports the MCA and the motor has integral overload protection, then the OCPD size can be greater than that otherwise required to protect the circuit conductors (in the absence of motor overload protection), up to the maximum OCPD size specified by the manufacturer.

Yes, the code is clear about that. You can size the wire to the MCA and the OCPD to the MOP.