Temp. derating, adjustment, correction factors.

[Toros]

Hi
Commercial inspector wants us
to show calc. for roof mounted 200A feeder of air condition sub panel with total load of 110A
.
feeder: 2" EMT, (3) 3/0 THHN, Cu + EQ.GR (off a 75 deg. terminal , 200A/3p cir. breaker)

if it was a more than three hot conductors, then I would calculate so;

200A x .75 (per 310.16) = 150A
i am still fine, no need to increase the conductor size.

What do you think??

Thank you

 

[infinity]

What is the MCA of the unit?

 

[Toros]

What is the MCA of the unit?

that sub panel supplies (4) 4-tons AC units (each: 9000va @ 240V, 3 ph)

 

[jumper]

that sub panel supplies (4) 4-tons AC units (each: 9000va @ 240V, 3 ph)

I get that as 86.6A.

36000/240/1.732.

edit: forgot continuous. x125%=108.2.:slaphead:

 

[Toros]

I get that as 86.6A.

36000/240/1.732.

I know that
The question is,
can I Temp. derate that @ .75 even though this a 3-wire feeder exposed in the sun ?????

 

[jumper]

Looks right to me.

FYI, adjustment factor for 4-6 CCCs is .8, not .75, not that it would be used here though.

 

[jumper]

I know that
The question is,
can I Temp. derate that @ .75 even though this a 3-wire feeder exposed in the sun ?????

How did you arrive at .75? Roof adder for temp.

Assuming you used charts:
What ambient temp did you use? What height off roof is conduit?

 

[Toros]

How did you arrive at .75? Roof adder for temp.

Assuming you used charts:
What ambient temp did you use? What height off roof is conduit?

I have (3) #3/0 copper wire plus EQ.GD
in a 2" EMT 140' long feeding a sub panel used for AC units (with total load of 140A)
@ Temp. of 123-131 (F) in Los Angeles, valley

conduit run across the roof elevated; 1" through red woods

 

[jumper]

I have (3) #3/0 copper wire plus EQ.GD
in a 2" EMT 140' long feeding a sub panel used for AC units (with total load of 140A)
@ Temp. of 123-131 (F) in Los Angeles, valley

conduit run across the roof elevated; 1" through red woods

If you are only 1” above the roof you have to add 40F more. That puts you at .5 instead of .76.

This whole roof adder thing got deleted in the 2017.

Why is the load now 140A and not 110A?

 

[Toros]

If you are only 1” above the roof you have to add 40F more. That puts you at .5 instead of .76.

This whole roof adder thing got deleted in the 2017.

Why is the load now 140A and not 110A?

ok
tanx jumper
the chart says correction factors based on more than three conductors
mine is three- hot conductor feeder
how is that....??????

 

[jumper]

ok
tanx jumper
the chart says correction factors based on more than three conductors
mine is three- hot conductor feeder
how is that....??????

I think you are transposing headers. The roof adder stuff does not say 3 or more CCCs, that is a different chart.

Reread them.

 

[Toros]

I think you are transposing headers. The roof adder stuff does not say 3 or more CCCs, that is a different chart.

Reread them.

REREADING WHAT??
NEVER READ ABOUT ROOF ADDER
WHERE CAN I READ BOUT IT????

 

[infinity]

What code cycle are you under? For the 2017 if you're 1" above the roof there is no adder. Under the 2014 NEC if you use XHHW there is no adder either.

 

[Dennis Alwon]

I assume you are using 90C wire so you can de-rate from 225 amps value of the 90C column. As long as the final overcurrent protective device is not larger than 200 or whatever the temp. rating is after de-rating. Start at 225 not 200 amps

 

[infinity]

I assume you are using 90C wire so you can de-rate from 225 amps value of the 90C column. As long as the final overcurrent protective device is not larger than 200 or whatever the temp. rating is after de-rating. Start at 225 not 200 amps

Since this is feeding 4 AC units the OCPD can be larger than the conductor size if they used just a feeder but since there is a sub-panel at the end I agree with you.

So assuming that the feeder is the correct size for the load the only issue is do the #3/0 conductors have an ampacity of 200 amps after adjustment.

 

[Dennis Alwon]

How can you have a feeder without a panel?

I believe he can take the largest motor and calculate the largest breaker allowed for that motor and then add the fla of the other motors. Not enough info given...

430.62 Rating or Setting — Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor
load(s) and consisting of conductor sizes based on 430.24 shall
be provided with a protective device having a rating or setting
not greater than the largest rating or setting of the branchcircuit
short-circuit and ground-fault protective device for any
motor supplied by the feeder [based on the maximum permitted
value for the specific type of a protective device in accordance
with 430.52, or 440.22(A) for hermetic refrigerant motorcompressors],
plus the sum of the full-load currents of the
other motors of the group.
Where the same rating or setting of the branch-circuit shortcircuit
and ground-fault protective device is used on two or
more of the branch circuits supplied by the feeder, one of the
protective devices shall be considered the largest for the above
calculations.

 

[infinity]

How can you have a feeder without a panel?

You run a feeder and tap off to a disconnect for each AC unit. Common practice for AC units and motors.

I never would have thought to do that. You then have to follow tap rules.

 

[kwired]

You run a feeder and tap off to a disconnect for each AC unit. Common practice for AC units and motors.

yes, but I don't see why that should make any difference other then the panel is simpler to add more load to if it still has open spaces.

 

[jumper]

I still do not get what 3 CCCs has to do the the temp adjustment.

I totally forget the whole adder thing since it was mostly ignored anyways.

 

[infinity]

yes, but I don't see why that should make any difference other then the panel is simpler to add more load to if it still has open spaces.

Can the feeder for multiple units or motors have an OCPD larger than the feeder conductor ampacity to compensate for starting current?