Air handler MOP

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chris kennedy

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I have 8 single phase 208V AHU's. The heat strips are slash rated at 208/230V 5KW. Trying to figure MOP as the name plate does not specify.

R=E??P
R=230??5,000 or 10.6

P=E??R
P=208??10.6 or 4,082 watts at 208V

4,082?208=19.6A
19.6A?1.25=24.5A

Blower FLA =3.6A

24.5+3.6=28A

Right so far? Now do I multiply the 3.6 blower by 1.25 also as per 430.24?

3.6?1.25=4.5
24.5+4.5=29A

So 30A OCPD?
 
I would not multiply the blower times 1.25. I have done many 5 kw units at 240 volts and the nameplate always calls for 30 amp.
 
I will add that every 5kw resi unit I have seen does not have a 5kw element. It is usually smaller-- like 4.7kw or something. Maybe that is why 30 amps works.
 
Dennis Alwon said:
I will add that every 5kw resi unit I have seen does not have a 5kw element. It is usually smaller-- like 4.7kw or something. Maybe that is why 30 amps works.
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Bear in mind these will be operating at 208V. The slash rating is throwing me.

Looking for Maximum Overcurrent Protection.
 
Dennis Alwon said:
I thought you only need to use the larger of the loads.
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I don't believe that is how it is written, I think you will find it is the larger of the motor loads when there are multiple motor loads.



Dennis Alwon said:
I will add that every 5kw resi unit I have seen does not have a 5kw element. It is usually smaller-- like 4.7kw or something. Maybe that is why 30 amps works.
Click to expand...

Now that does not surprise me, 5 KW may be the entire rating including the motor. :smile:
 
Cavie said:
In resi units, the heater and blower are combined into one load. 5 KW heat 30 amp includes the blower.
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Thank you. That confirms what I was thinking.

I would bet that Chris' unit also is a total 5kw but will actually be a bit less with the 208 volt rating . I find it hard to believe there is not nameplate or perhaps this is an old unit.
 
Dennis Alwon said:
I find it hard to believe there is not nameplate or perhaps this is an old unit.
Click to expand...

These AHU's don't come with heat strips, it is a kit that is field installed. Name plate only lists blower info. I thought that the heat kits come with a new name plate to be placed over the original.

Also, I don't see where the MOP is required to be on the name plate.

IV. Marking of Heating Equipment

424.28 Nameplate.
(A) Marking Required. Each unit of fixed electric space-heating equipment shall be provided with a nameplate giving the identifying name and the normal rating in volts and watts or in volts and amperes.
Electric space-heating equipment intended for use on alternating current only or direct current only shall be marked to so indicate. The marking of equipment consisting of motors over hp and other loads shall specify the rating of the motor in volts, amperes, and frequency, and the heating load in volts and watts or in volts and amperes.

(B) Location. This nameplate shall be located so as to be visible or easily accessible after installation.

424.29 Marking of Heating Elements.
All heating elements that are replaceable in the field and are part of an electric heater shall be legibly marked with the ratings in volts and watts or in volts and amperes.
Click to expand...
 
Chris all the units we use have a nameplate with many choices. The hvac guy puts in the element and marks thru the appropriate choice for his heaters. This info also has the MCA , etc.
 
Dennis Alwon said:
Chris all the units we use have a nameplate with many choices. The hvac guy puts in the element and marks thru the appropriate choice for his heaters. This info also has the MCA , etc.
Click to expand...

Yes sir, thats what I'm used to seeing also, they mark the heater used with sharpie so you can see it for about a year.:roll: Not the case here.
 
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