Air handler nameplate requirements

[DaveBowden]

Can anyone help me understand why an air handler with a 4.6kw heat strip and a blower motor with a 4.3 amp FLA would need a circuit with a minimum ampacity of 32.5 amps and a 35 amp breaker?
The 2 story addition we just roughed in called for a 5kw heat strip in the air handler and we fed it with 10/2 romex (like we always do for a 5kw load). Now we get to go back and replace the 10/2 with 8/2. I'm not too happy about it, but I can't see any way around it other than trying to get the AC guy to change the stick on label with the requirements on it.

 

[augie47]

agree that the math doesn't add up...
perhaps a call to the manufacturer or distributor

 

[cowboyjwc]

Well the short answer would be, because 440.6(A) says that's the way you do it. But that's just an inspectors point of view.

 

[Strife]

Even with the 4.6KW heat and the 4.2A motor at 1.25 it only comes to 25A. First thing that comes to mind is are you sure the MCA is not for something like 7.5 KW heater? It seems lately a lot of manufacturers put a generic label on the product and it's our job to find the rating based on the size of heat used. I've seen several AHU being able to take 2.5KW, 5KW and 7.5KW heaters. And the label was based on one of them.

Can anyone help me understand why an air handler with a 4.6kw heat strip and a blower motor with a 4.3 amp FLA would need a circuit with a minimum ampacity of 32.5 amps and a 35 amp breaker?
The 2 story addition we just roughed in called for a 5kw heat strip in the air handler and we fed it with 10/2 romex (like we always do for a 5kw load). Now we get to go back and replace the 10/2 with 8/2. I'm not too happy about it, but I can't see any way around it other than trying to get the AC guy to change the stick on label with the requirements on it.

 

[Cavie]

Can anyone help me understand why an air handler with a 4.6kw heat strip and a blower motor with a 4.3 amp FLA would need a circuit with a minimum ampacity of 32.5 amps and a 35 amp breaker?
The 2 story addition we just roughed in called for a 5kw heat strip in the air handler and we fed it with 10/2 romex (like we always do for a 5kw load). Now we get to go back and replace the 10/2 with 8/2. I'm not too happy about it, but I can't see any way around it other than trying to get the AC guy to change the stick on label with the requirements on it.

The label is correct. It's a 208 volt unit. Very common problem I find in condo buildings. Many people don't think of it.

 

[augie47]

very good, Cavie !
Math does work for 208.

 

[iwire]

very good, Cavie !
Math does work for 208.

Ditto that ....

 

[Dennis Alwon]

Yesterday I spent 20 minutes trying 208V and I cannot get it to work with 32.5. That was the first thing I thought also but I still can't make it work. Someone show me the math.

I took 4600/208= 22
22+4.3=26.3 * 1.25 = 32.8

If I just use 1.25 * 22 amps I get 27.5
27.5+4.3 = 31.8

I even did conversion using ohms law-- what am I doing wrong.

 

[augie47]

try calculating the motor at 1.25 also.

 

[Dennis Alwon]

try calculating the motor at 1.25 also.

I did that in the first one and got 32.8

 

[DaveBowden]

Actually, the numbers in my OP were for 230 volts on the nameplate. There were different ratings on the nameplate for 208 volts, but I didn't write them down because they didn't apply.

 

[Dennis Alwon]

Actually, the numbers in my OP were for 230 volts on the nameplate. There were different ratings on the nameplate for 208 volts, but I didn't write them down because they didn't apply.

Now that comes to 31.5 amps.

4600 at 230 v means a resistance of 11.5.

So at 240 volts we have a unit heater of 5009. 5009 +4.3 amps * 1.25 = 31.5

 

[Dennis Alwon]

Now that comes to 31.5 amps.

4600 at 230 v means a resistance of 11.5.

So at 240 volts we have a unit heater of 5009. 5009 +4.3 amps * 1.25 = 31.5

But they are saying 32.5 at 230V which makes no sense

 

[jumper]

Dennis,

A 5 kW heater at 230v is 4.6 kW.

Use 5 kW at 230v + motor and see what you get.

 

[Dennis Alwon]

Dennis,

A 5 kW heater at 230v is 4.6 kW.

Use 5 kW at 230v + motor and see what you get.

Look at the post above. The op said it is 230V 4600 watts

 

[Riograndeelectric]

this always confuses me.
I try and stick with the manufactures name plate circuit requirements.

Question?

if the AHU or HVAC indicates the minimum and maximum fuse size I take this to mean the unit has have a fused disconnect switch.
I assume the manufacture has taken in the 1.25 multiplier in there determination of min and Max fuse size is this correct.

 

[Dennis Alwon]

this always confuses me.
I try and stick with the manufactures name plate circuit requirements.

Question?

if the AHU or HVAC indicates the minimum and maximum fuse size I take this to mean the unit has have a fused disconnect switch.
I assume the manufacture has taken in the 1.25 multiplier in there determination of min and Max fuse size is this correct.

We all agree you must use the manufacturers nameplate but it seems like something is in error.

 

[Mgraw]

I went to a couple of AHU manufacturers websites and checked the mca to their heater and motor amp ratings x 1.25. In every case the mca was higher by an amp or two.

 

[GUNNING]

we got the motor and the heat strips what about the control board consumption?

Also the discrepancy of the calculated voltage as stated on the device ,208 0r 230, vs the required 240v standard of the nec.

 

[Mgraw]

For the AHUs I looked at the FLA of the motor was @230v. But the MCA for the motor and heater was @208/240v.