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#8420 - 03/20/02 06:00 PM Breaker size??
ayrton Offline

Registered: 12/21/01
Posts: 205
Loc: Pa
I need to size my breaker in the MDP for an HVAC installation. There will be four ten ton split system units. Two on one floor and two on the floor above. the condensors will be roof top. 208/120 3phase is the system.
Now this is to be on the third and fourth floor. I am planning to run conduit up from the substation in the basement to a sub panel on the third floor to feed the new units. How do I calculate for my breaker size to the subpanel?

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#8421 - 03/20/02 06:06 PM Re: Breaker size??
electure Offline


Registered: 12/24/00
Posts: 4226
Loc: Fullerton, CA USA
Largest unit @ 125% + the remainder of the units @ 100%,just like a bank of motors. If the evap coils are on this panel also, don't forget 'em.
(OOPS, that was for the conductor size!!)

[This message has been edited by electure (edited 03-20-2002).]

#8422 - 03/20/02 07:50 PM Re: Breaker size??
Redsy Offline

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
I'm thinking for Inverse Time CBs, per Art. 430.62 and table 430-52--
250% largest FLA, plus sum of all other FLAs.

I don't imagine a compressor of that size is hermetic, but in that case, 440-22 would apply.

If I'm wrong, I think it's a step in the right direction.

#8423 - 03/27/02 03:00 PM Re: Breaker size??
Dan Neeser Offline

Registered: 03/12/02
Posts: 29
Loc: St. Louis, MO, USA
I believe Redsy is correct. Size the feeder circuit breaker at not more than:

- Largest motor FLA (per Table 430.52) X 250% (or the next standard size up)


- The full-load current of all other motors.

An additional consideration that I would recommend is to add a fusible disconnect at the HVAC unit. This provides a local disconnect for the unit to facilitate maintenance requirements and also may be needed if the unit requires protection from fuses...applying products according to their listing and labeling - NEC 110.3(B).

Some HVAC units indicate max overcurrent device, which can be protected by a fuse or circuit breaker. In this case a non-fused disconnect would work as the local disconnect(provided the fault current is not above 10kA, since most non-fused disconnects are not rated above that).

However, many indicate max fuse size. If this is indicated, fuse protection in accordance with the label must be provided. Many engineers I talk with always specify a fused disconnect at the HVAC for this reason.

#8424 - 03/27/02 03:25 PM Re: Breaker size??
ayrton Offline

Registered: 12/21/01
Posts: 205
Loc: Pa
Thanks for the help guys. After I wrote the question I went looking in Articles 430 and got confused. There is so much information and sometimes I read to much into the text.
You have LRA, and FLA. Which do you go by?
Figuring one unit is easy, you just look at the name plate and the max fuse is normally a given. I am terrible at calculations and am always second guessing myself. Last thing I want to do is bid a job, do it and than down the road come to find out my calc was wrong!
Thanks again for the advise! Turns out I will need a 400a subpanel to handle these loads and some small 120v control.

#8425 - 03/31/02 09:07 AM Re: Breaker size??
golf junkie Offline

Registered: 04/22/01
Posts: 511
Loc: York, NE
message deleted

[This message has been edited by golf junkie (edited 03-31-2002).]

#8426 - 03/31/02 02:35 PM Re: Breaker size??
JohnnyB Offline

Registered: 02/25/02
Posts: 33
Loc: Chicago IL USA
Another thing, there are HVAC rated breakers as opposed to regular ones and of course time delay fuses. As a HVAC tech I strongly suggest a dissconnect at each of the units, I would think it to be required.

As for LRA and FLA, one is Locked Rotor Amps and the other is Full Load Amps, I would think LRA to be used in calc since the compressors will draw it for a second or two, right?

#8427 - 03/31/02 07:26 PM Re: Breaker size??
wolfdog Offline

Registered: 10/24/01
Posts: 135
Loc: Dallas,TX
Hello JohnnyB, Didn't expect to run into you here.

#8428 - 04/01/02 04:20 AM Re: Breaker size??
Redsy Offline

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
It's early, and I'm goin' out on a limb but,
I believe FLA is normally used to size feeders & feeder OCPDs.
I believe LRA is used to size discos for hermetic units by using Table 430-151(B) to back-calculate for a HP rating based on LRA.
Anyone else?

#8429 - 04/01/02 05:40 AM Re: Breaker size??
smurf Offline

Registered: 03/31/02
Posts: 31
Loc: Benton, Ar. USA
Right on, Redsy. Won't anyone cut your limb for that.


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