ECN Forum Forums General Topics of the Trade General Discussion Area (MOP)/Min or Max. overcurrent protection??

One thing I find that often gets overlooked when using the equipment manufactures minimum branch circuit rating is voltage drop due to the length of conductors. With things like rooftop units, you often end up with hundreds of feet of wire which could lead to under voltage issues and hard starting for motor/compressors.

Scott, this raises the question in my mind; how did they come up with 65.68 MCA and 110 MOP using the figures you stated. They just made a mistake on the MCA or the MOP?
(65.68 x 175% = 114.94 amps)Should they have went up to a 125 amp MOP instead of 110? If I'm understanding you correctly, we have to go by the manufacture's requirements even though the code may allow us to not to have a disconnect if the unit has a marked "off/on" switch? I should have insisted on seeing the manual for the unit, which of course I will if I get the job. Hope I've got enough figured in to cover the unexpected:(
Thanks again for the help.

The engineers at the manufacturers shop come up with these numbers in accordance with the U/L marking guide for HVAC condensers.
They want a breaker big enough to reliably start the unit.
The 175% is just a general rule for motor loads.

KJay, thanks for the reminder. This unit I may be wiring in going on a possible 250ft. run. The #4 I'm figuring is good for 85 amps(75 degree column). Hope that will be sufficient. Very good thing to remember though. Can't always go by just what's on the nameplate, and I'm finding out on industrial wiring and commercial, a person can go bankrupt real quick if you don't do the right math:(

If you assume a 80% load, based on min circuit amps of 65 you get 52a normal running current and that translates to a 8v drop over 250' of #4. (under 2% at 480v)

This is not true if the end use equipment is not a single motor and most 'packaged' units are not. 450.52 only applies to single motor circuits, I only mention this because I've seen this section applied to the wrong equipment too many times.

To further complicate things some engineers and AHJ's will view the circuit from the disconnect to the mechanical unit as a 'motor' feeder, but not the circuit from the circuit breaker to the disconnect and they will size or require the circuit between the breaker and switch to be sized per the breaker size, 110A (#2's) or 150A (1/0's) in the event it's a single motor device).

If you use an un-fused switch at the equipment then the breaker can be no larger than 110A (per the equipment label) and the feeder between the breaker can remain #4's (for a single motor).