Air Handler With Two Disconnects

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mkgrady

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Massachusetts
I need to wire an air handler in a home that has built in emergency heat strips. The nameplate on the unit is wrong. It shows the MCA at 5 amps and 240 volts. It appears there are three 5KW strips so the real MCA would be over 60 amps without even counting the fan motor. Even the wiring diagram on the unit doesn't show any heat strips.

What has me stumped is the built in disconnects. There are two 60 amp 2 pole circuit breakers that feed the unit. Not sure why overcorrect protection is built in like that or why there are two breakers. I'm trying to anticipate how to wire the thing while I wait for the HVAC contractor to supply the real MCA and a wiring diagram. I doubt it needs two 60 amp circuits run to it but I really don't know what to expect. Can somebody take some mystery out of this for me?
 
Generally those breakers come as part of the heater kit. If there are 2 60 amp breakers it is probably 20 kw heat and would require 2 60 amp circuits.
 
Heat strips are added in by the hvac contractor. They use the same base unit and depending on the size needed they install the proper kw strip. 2- 60 amp breakers is usually for a 20 kw unit. If the load is over 60 amps then the load must be split up. For a 15kw unit you usually see a 60 amp and a 30 amp breaker.

Be careful-- some units will only accept one circuit to it while others need 2 circuits. I usually pull a larger wire and install a disconnect with the appropriate loads. This will allow me to install either one or two circuits
 
Why are you not supplying us with the make and model of the machine so we can look it up?

It is quite common that a heat pump's air handler will have auxiliary heating strips that may or may not be field installed. Either way, it is common that there will be two disconnects.

Without knowing what machine you have, I can't get on the same page as to what you are looking at.
 
MAC702 said:
Why are you not supplying us with the make and model of the machine so we can look it up?

It is quite common that a heat pump's air handler will have auxiliary heating strips that may or may not be field installed. Either way, it is common that there will be two disconnects.

Without knowing what machine you have, I can't get on the same page as to what you are looking at.
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The make is Heil. The model as labeled can't be correct because the nameplate has a MCA of 5 amps. I didnt make note of what the incorrect nameplate model number is but I received an email that indicates FCM4X36***L+WALLCON. When I look that up on the web I get nothing.
 
I've never seen an incorrect nameplate on a furnace or air handler before. I'll have to take your word for it that there's no useful information on it for us to go on.
 
MAC702 said:
I've never seen an incorrect nameplate on a furnace or air handler before. I'll have to take your word for it that there's no useful information on it for us to go on.
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I know almost nothing about this but Dennis indicated the HVAC contractor installs the heaters in the field. I'm guessing once they do so they are required to affix a new nameplate and wiring diagram. I have asked the contractor to provide both. In the mean time I'm trying to get a little education about how this all works, and I'm glad that is happening.
 
mkgrady said:
I know almost nothing about this but Dennis indicated the HVAC contractor installs the heaters in the field. I'm guessing once they do so they are required to affix a new nameplate and wiring diagram. ...
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I've field-installed lots of auxiliary heat strips in heat pump air handlers. In each case, there is simply a box to check on the existing nameplate to indicate which heat strip assembly was installed. I've also worked on myriad machines already installed and most have neglected to even check the box.
 
I suppose its possible the air handler is only a fan, with the AC being elsewhere, thus the MCA of 5A. As for the heat strips, they are field installed, with varying numbers of heaters. I dont have a NEC in front of me, but I seem to remember that heater circuits are limited to 60A; more than that requires an additional circuit(s). Max heating load on a 60A breaker is 48A or ~11.5kW.

For residential ones with AC and back-up emergency electric heat, we generally run a 10/2 NM and a 6/2 NM. There can be 2 (or more) sets of lugs in the unit; the smaller ones for the 10/2 for the AC, and the larger ones for the 6/2 and heat strips. If there are no discos or breakers in the unit, just run 2 of them outside (the $6 non-fused Midwest or GE ones are common here).
 
mkgrady said:
The make is Heil. The model as labeled can't be correct because the nameplate has a MCA of 5 amps. I didnt make note of what the incorrect nameplate model number is but I received an email that indicates FCM4X36***L+WALLCON. When I look that up on the web I get nothing.
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FCM4X36 is a good model number for a Heil fan coil. What we really need is the model number of the heater kit installed in it.
 
MAC702 said:
I've field-installed lots of auxiliary heat strips in heat pump air handlers. In each case, there is simply a box to check on the existing nameplate to indicate which heat strip assembly was installed. I've also worked on myriad machines already installed and most have neglected to even check the box.
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This is what I was thinking. I thought there was a code section that required this marking. The check box is more accurately on a sticker in most cases (minor detail) and it usually lists the MOCP including the 5 amp fan described above, at both 208 volts and a column for 240 volts. Most Engineers require a single point connection. I have seen a separate feed for heat, but those are usually mounted in the duct beyond the unit.
 
I have a 15kw unit in my home and it is setup for 2 circuits. I ran a larger feeder to a 4 cir. panel and then I installed a 60 and a 30 amp branch circuit to the unit.
 
MAC702 said:
I've also worked on myriad machines already installed and most have neglected to even check the box.
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I have seen plenty of new installs that were inspected and the box was not marked.

I find it strange that the mechanical inspector does not notice this. The electrical inspector will want the box checked but I don't see it as our job as we are not the one's that install the heat strips.
 
mkgrady said:
The make is Heil. The model as labeled can't be correct because the nameplate has a MCA of 5 amps. I didnt make note of what the incorrect nameplate model number is but I received an email that indicates FCM4X36***L+WALLCON. When I look that up on the web I get nothing.
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I have seen both a label that has data for every possible heat kit listed for the unit and "somebody" is supposed to mark which one was used as well as stick on labels that come with the heat kit and you are supposed to affix it to the unit, it is supposed to replace the label for the fan coil only.

These days you generally don't purchase a unit with heat already installed, you select a fan coil unit and a heat kit that is listed to go into that fan coil unit and assemble them in the field.

growler said:
I have seen plenty of new installs that were inspected and the box was not marked.

I find it strange that the mechanical inspector does not notice this. The electrical inspector will want the box checked but I don't see it as our job as we are not the one's that install the heat strips.
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Marking the box does not change what is or is not installed. If any doubts I check what is installed regardless of what is or isn't marked.

That said about 99.9% of those I see are 4.8 kW (@ 240 volts) per heating element installed, so just counting heat elements installed gives you a pretty good indication of rating before you give much thought to exact details. Pretty rare to see other then 4.8 kW single phase elements and also pretty rare to see a heat strip kit intended for three phase supply, though I have run into a couple here and there.
 
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