hhsting
Senior Member
- Location
- Glen bunie, md, us
- Occupation
- Junior plan reviewer
I have RTU whose condensing unit has MCA of 30 amps and MOCP of 40 amps, 208V three phase. Can I use 240V, 30 amps disconnect current rating, three phase per code?
HVAC Disconnect sizing
- Thread starter hhsting
- Start date
- Status
kwired
Electron manager
- Location
- NE Nebraska
Likely yes. Keep in mind that MCA is 125% of compressor rated amps plus all other loads, therefore unit likely only draws maximum of 21-25 amps.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I think he is talking about using 240 volts on a 208 unit. Technically that is not compliant and not good for the unit
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Sounds like the 240 volts is related to the rating of the disconnect.
That's how I interpret it too. I would say yes you could.
hhsting
Senior Member
- Location
- Glen bunie, md, us
- Occupation
- Junior plan reviewer
240V is the disco voltage rating and 208V is service system voltage in which the disco is installed. Question is about hvac condensing unit nameplate mininum circuit amps (MCA) rating can be used as current rating of the hvac condensing unit disco.
ramsy
Roger Ruhle dba NoFixNoPay
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
I believe those non-fuseable disconnects should be => MAX FUSE rating per nameplate, not Minimum Circuit Ampacity.
hhsting
Senior Member
- Location
- Glen bunie, md, us
- Occupation
- Junior plan reviewer
packersparky
Senior Member
- Location
- Wisconsin
- Occupation
- Inspector
It doesn't. It says the disconnecting means shall be at least 115 percent of the nameplate rated-load current or branch circuitselection current, whichever is greater.
Part II. Disconnecting Means
440.11 General. The provisions of Part II are intended to
require disconnecting means capable of disconnecting airconditioning
and refrigerating equipment, including motorcompressors
and controllers from the circuit conductors.
440.12 Rating and Interrupting Capacity.
(A) Hermetic Refrigerant Motor-Compressor. A disconnecting
means serving a hermetic refrigerant motor-compressor
shall be selected on the basis of the nameplate rated-load
current or branch-circuit selection current, whichever is
greater, and locked-rotor current, respectively, of the motorcompressor
as follows.
(1) Ampere Rating. The ampere rating shall be at least
115 percent of the nameplate rated-load current or branch circuit
selection current, whichever is greater.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Is "branch circuit selection current" the MCA? How could the rated load current ever be greater than the branch circuit selection current?
- Location
- Connecticut
- Occupation
- Engineer
hhsting
Senior Member
- Location
- Glen bunie, md, us
- Occupation
- Junior plan reviewer
So what can be used to size current rating of HVAC disconnect then nameplate MCA or MOCP based on that code article nameplate rated load current or branch circuit selection current whichever greater?
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- Location
- Connecticut
- Occupation
- Engineer
Neither. You need the rated load current or branch circuit selection current to properly size the disconnect, though either the MCA or MOCP for a single compressor would be larger the required disconnect ampere rating.
Also, note that the quoted section 440.12(A) only applies to a single hermetic refrigerant motor compressor. 440.12(B) would apply to combination loads.
kwired
Electron manager
- Location
- NE Nebraska
See 440.2 for definition. It basically only applies if the overload protection allows it to run beyond the rated current.
From what I am seeing one must determine equivalent horsepower rating and then determine if that exceeds the rating of the switch.
Unless other information comes up, I still stick with my original reply in this thread based upon the fact the MCA is 30 amps and that MCA already has 125% of the compressor rated current factored into it - it is likely going to convert to a horsepower rating less than the 30 amp switch is rated for.
I have also brought this observation up a couple times recently in related questions on non fused disconnect sizing - from past experiences on installs involving 30 amp Square D heavy duty switches (600 volt) and a 20 HP 480 volt motor. If you were to install a fused switch, you need to go with a 60 amp switch because a 30 amp fuse isn't enough to assure it will start without blowing a fuse. But if using a non fused switch on same load - the 30 amp switch is rated for 20 HP motors. Interrupting rating of the switch is the main issue here. Sort of no different than using a 15 amp snap switch on a 20 amp circuit - fine as long as it isn't switching more than 15 amps.
hhsting
Senior Member
- Location
- Glen bunie, md, us
- Occupation
- Junior plan reviewer
Ok so branch circuit selection current is it always higher than rated load current and MCA always higher than branch circuit selection current?
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kwired
Electron manager
- Location
- NE Nebraska
See definition in 440.2 - as worded BCSC should always be higher than rated load current.
440.4(C) requires BCSC to be marked on the nameplate if motor overload protection is such that a BCSC will exist. So one shouldn't need to worry about it unless it is marked with a BCSC.
MCA is a determination of minimum size conductor necessary. Some may have different methods of determining exactly what that is, but the general rule is it is 125% of largest motor (compressor) rated current plus all other loads that can run simultaneously. IDK if they would use 125% of BCSC instead of rated current on units where this applies or not. I can only recall seeing BCSC on a nameplate a very limited number of times.
- Location
- Connecticut
- Occupation
- Engineer
Yes and yes.
- Location
- Connecticut
- Occupation
- Engineer
Yes, MCA would be based on 125% of the BCSC when a BCSC is given.
hhsting
Senior Member
- Location
- Glen bunie, md, us
- Occupation
- Junior plan reviewer
So MCA > BCSC > Rated load current. Whats the harm in using MCA as HVAC disconnect current rating?
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kwired
Electron manager
- Location
- NE Nebraska
You still must be under HP rating of the disconnect with your equivalent HP - but likely will be in most cases, especially if using a "heavy duty" safety switch.
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