I'm checking a job that requires 480 volt 3 phase to a roof top heat pump unit for a gigantic industrial facility. On the name plate it has MCA 65.68 (minimum circuit amps) then it has MOP 110 amps. On other units for the maximum overcurrent protection, I've seen "Max. ?? amps", but on this one it looks like it could go either way Max. or Min. (MOP) What would be your input as to which way this should go. If I am calculting right according to the table for A/C and refrigerating equipment, it must be a "wound motor" and figured at 150% to get anywhere close to 110 amps if that is the max. And if you will refresh me on what breaker I would use at 110 amps since they don't make one for that. I think I would have to go down to 100 amps, but not too sure on this. I'm planning on using #4 thhn wire for the hots and the ground will be determined by the Max. overcurrent protection. Thanks for the input. Also there are (2) 480/277 volt panels side by side. One panel takes the big thermal magnectic breakers, and the other one takes regular size breakers. Will it matter which panel I use? Thanks again.
The 'MOP' or MOCP, is the maximum fuse or cb for the unit, based on the mfg nameplate. You may provide protection (CB/fuses) of any rating between 65.7 and 110.
As to the CB and panel...you have to check first which one has sufficient 'spare' amperage capacity for the new load. Time for a load calc or a recorder. When you determine which panel can handle the new load, that is the one ya get a CB for.
Minimum circuit ampacity is used to size the conductors, MOCP sizes the breaker. You can end up with some scary looking combinations but thus is the norm when you are working with motors. In your case you could use #4 cu and a 110a breaker That ".68a" pushed you over from #6
Thanks for the replies. I was pretty sure it meant maximum, but like I said most smaller units I've wire had it abbreviated a litte more,(max. 110). I checked on both type breakers today at 2 of my supply houses. The big thermal magnectic breaker (100 amp.) (almost as big as an I-Line type breaker)will cost me almost $ 600.00 without tax or mark up at one place and $700.00 at the other place. The smaller breaker that fits in an "NF" type Sq.D panel was almost as much ( about $40.00 less)!!! I knew they would be expensive, but not that much. Got to run around 220 ft. of conduit, around 250 ft. of #4 wire (3 phases x 250= 750ft) and 250 ft. of #8 ground wire or #6 ( depending on size of breaker I use, 100 or 110), around 25ft. in the air using a scissor lift, with earplugs, steel toe boots, safety vest, and harness for the lift., plus got to go through roof to unit. By the way, I know I have to have a disconnect for the unit on the roof, but this unit has a "on/off" switch mounted on the cover. Will this suffice the need for a disconnect or will I have to mount another one? Thanks again for the input. Gonna be a major job if I get it.
Another question: If I have to set a disconnect, which it looks like I will, sizing the disconnect requires taking 115% of the full load current or the branch dircuit selection current whichever is greater. So if I figure 110 amps as being the branch circuit selection current the HP would be 125 HP?? 3 phase 480 volts. I'm probably figuring this wrong. It's 2:45 am and I'm getting tired. Sorry if the calculation is wrong. Not thinking too good under the circumstances:)
Are there any exceptions about a built in switch as a disconnect?? It looks like just a "turn off and on" switch on the front of this unit. I've haven't seen one like this before. Is it possible that could be considered a disconnect for the unit? (480 volt 3 phase) About $400.00 difference in price if it can be. Thanks again. Steve..
Greg, I haven't had a chance to look at the manual. It was raining the day I got on the roof, and the job is about an hour away, but it did have an "off" marked at the switch. It's just a turn switch, (no knife blade style), so I'm sure it's operating a contactor inside the unit. I'm would guess that the manual would say it could be used as a disconnect, since there is no real feasible place on the unit to mount a disconnect. All the doors at the electrical access are removable, and it looks like the electrical is meant to come up from the roof inside the unit. Thanks again. I got to get this bid in today, so I'm hoping I got it all straight:)
Per the Projects I deal with, a common Electrical Design for Refrigeration Assemblies' Feeders / Branch Circuits is applied as follows:
Manufacturer Specs (using the same values as "SPARKYNC" listed):
Voltage and Phase: 460/3 MCA: 65.68A MOPD: 110A (Note: "Fuse or HACR Breaker" is almost never included with the Spec. Values from the MFG.)
1: MCA... ("MCA" = "Minimum Circuit Ampacity") The MCA is used to determine the Minimum Size of the Branch Circuit Conductors feeding the Equipment. In this case, #4 THHN CU. will be adequate for an MCA of 65.68A.
2: MOPD... ("MOPD" = "Maximum Over Current Protection Device") The listed MOPD will determine the Maximum Ampacity for the "Final Overcurrent Protection Device" on this Branch Circuit. Commonly, the Design parameters I employ, will have a Fusible Disconnect at the Equipment; regardless if the Equipment does or does not include Factory Installed Disconnecting means.
The Fuses at this Disconnect are sized per the MOPD listing. The MOPD value is 175% of the FUll Load Amperes (FLA) for the Equipment. This conforms with Section / Table 430.52, for Maximum Rating of a Dual Element (Time-Delay) Fuse.
Using the MOPD of 110 Amps as 175% of the FLA, the Equipment should be rated for 62.7 Amps as the highest draw.
The Fusible Disconnect (Safety Switch) will need to be sized to accommodate a 110 Amp Fuse, which means a 200 Amp Fusible Disconnect Switch - or more specific; 600 Volt - Heavy Duty, 200 Amp, 3 Pole, Fusible, EXO, NEMA 3R, with (3) 110 Amp RK5 600VAC Fuses.
MOPD - Part 2...
If the above Fused Disconnect is compliant and acceptable, the Branch Circuit Breaker may be sized to be Maximum 250% the FLA.
Using the FLA of 62.7A, the largest Circuit Breaker trip rating we may use is 150 Amps (62.7 x 2.5 = 156.75), so the (3)#4 THHN CU Branch Circuit Conductors may have -Ground Fault & Short-Circuit Protection- from a 150/3 MCCB. This is compliant to Section / Table 430.52, for Maximum size of an Inverse Time Circuit Breaker. (Note: References to Table 430.52 have been applied to the use of Squirrel-Cage Rotor Induction Motors. Wound Rotor and DC Motors have different values in this Table).
Hope these notes are of assistance.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!