# 6/2 NM and 5kw heat



## chris kennedy (Jul 30, 2013)

Common situation here, EC runs 6/2 NM for AHU during rough. AHU is installed with 5kw heat and nameplate MOP of 30A. EC installs 30A breaker in panel with no disconnect at AHU as disco with no overcurrent protection comes installed in AHU.

Anyone see a problem with this install?


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## jwelectric (Jul 30, 2013)

No                                                                                                   .


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## jar546 (Jul 30, 2013)

There are possibly 3 different angles on this one depending on the type of conductors that the unit specifies, whether or not the 5k heater coil is included in the 30A nameplate and location of disconnect.  Is it one of them you are looking for?


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## chris kennedy (Jul 30, 2013)

Heat is included in MOP of 30A.


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## jar546 (Jul 30, 2013)

So why the 6/2?  Seems oversized.


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## rnapier (Jul 30, 2013)

jar546 said:
			
		

> So why the 6/2?  Seems oversized.


We have a number of sites where the electrician ran wires per plans that where to small for the units installed and it was a problem at finals so they now oversize the feeds to cover themselves.


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## jar546 (Jul 30, 2013)

Was the equipment ground upsized too?


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## gfretwell (Jul 30, 2013)

It is not unusual to see 6ga run to air handlers so they can install additional toaster wire strips.

They may not have even bought the system when they did the rough and they wanted the option.


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## jar546 (Jul 30, 2013)

gfretwell said:
			
		

> It is not unusual to see 6ga run to air handlers so they can install additional toaster wire strips.They may not have even bought the system when they did the rough and they wanted the option.


What about upsizing the EG?

250.122(B)

?


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## Gregg Harris (Jul 30, 2013)

jar546 said:
			
		

> What about upsizing the EG?250.122(B)
> 
> ?


It was a cable assembly of 6/2 the ground matches the ungrounded conductors


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## jar546 (Jul 31, 2013)

What is the size of the EG conductor for NM cable:

10/2=?

8/2=?

6/2=?

That is the point


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## Gregg Harris (Jul 31, 2013)

jar546 said:
			
		

> What is the size of the EG conductor for NM cable:10/2=?= #10
> 
> 8/2=? = # 10
> 
> ...


# 10 is the lucky number 250.122(B) would not apply in the original post. The conductors where not increased in size to compensate for voltage drop.


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## jar546 (Jul 31, 2013)

Gregg Harris said:
			
		

> # 10 is the lucky number 250.122(B) would not apply in the original post. The conductors where not increased in size to compensate for voltage drop.


It only says if they are increased in size.  It does not specify the reason, therefore the EG must be upsized.



> *Increased in Size*. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors.


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## Gregg Harris (Jul 31, 2013)

jar546 said:
			
		

> It only says if they are increased in size.  It does not specify the reason, therefore the EG must be upsized.


 How would it apply, there first must be a base criteria of length and temperature to require an increase. A # 6 was installed there is no increase

26,251 divided by 26,251 equals 1

10,383 times 1 equals 10,383

10,383 circular mil equals # 10 AWG


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## jar546 (Jul 31, 2013)

You have to look at the size that is required and the size that was installed.  The difference between the two conductors is the factor used to increase the size of the EG.



> Equipment grounding conductors on the load side of the service disconnecting means and overcurrent devices are sized based on the size of the feeder or branch-circuit overcurrent devices ahead of them. Where the ungrounded circuit conductors are increased in size to compensate for voltage drop or for any other reason related to proper circuit operation, the equipment grounding conductors must be increased proportionately.
> *Calculation Example  *
> 
> 
> ...


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## jar546 (Jul 31, 2013)

In the case here.  The actual size needed is 8/2 because of the load being 30A of continuous load.

The circ mils of #8 are 16,510

The circ mils of #6 are 26,240

The difference factor is 1.59

#8 comes with a #10 EG

The circ mils of #10 are 10,380

Multiplies by 1.59 (diff factor) of the ungrounded conductors = 16,504 or the equivalent of #8

So per the code, when you upsize for any reason, so must the EG.  If this was pulled in conduit, that is how they would size it.  NM does not allow that option.


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## rnapier (Jul 31, 2013)

jar546 said:
			
		

> In the case here.  The actual size needed is 8/2 because of the load being 30A of continuous load.


30 amps is the maximum overcurrent protection (MOP) not the load.


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## jar546 (Jul 31, 2013)

Chris damn you starting this thread and leaving.......LOL.


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## jar546 (Jul 31, 2013)

So we don't know the load to determine wire size?  Does the mfg spec MOP to match wire size or no because they can add more 5k heating elements?

 Even so, that still changes EG size if you up sized from 10 to 6


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## chris kennedy (Jul 31, 2013)

jar546 said:
			
		

> Chris damn you starting this thread and leaving.......LOL.


Just because I don't post a lot doesn't mean I'm not watching.


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## rnapier (Jul 31, 2013)

jar546 said:
			
		

> So we don't know the load to determine wire size?  Does the mfg spec MOP to match wire size or no because they can add more 5k heating elements? Even so, that still changes EG size if you up sized from 10 to 6


The Minumum Circuit Ampacity(MCA) is what you would use.


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## Dennis (Jul 31, 2013)

jar546 said:
			
		

> Was the equipment ground upsized too?


Doesn't need to be if you install a 60 amp breaker at the panel and install a small panel at the unit with 30 amp overcurrent protective device.


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## chris kennedy (Jul 31, 2013)

Dennis said:
			
		

> Doesn't need to be if you install a 60 amp breaker at the panel and install a small panel at the unit with 30 amp overcurrent protective device.


Agreed but not the exercise here.



			
				chris kennedy said:
			
		

> EC installs 30A breaker in panel with no disconnect at AHU as disco with no overcurrent protection comes installed in AHU.


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## Dennis (Aug 1, 2013)

That being the case then you cannot run NM cable because of the equipment grounding conductor issue. Install the disconnect as I stated and problem gone.

Unfortunately the equipment grounding conductor upsize is not usually an issue unless the run is very long.  Unfortunately again, the NEC does not give leeway there at all.


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## electriclese (Aug 12, 2013)

jar546 said:
			
		

> So we don't know the load to determine wire size?  Does the mfg spec MOP to match wire size or no because they can add more 5k heating elements? Even so, that still changes EG size if you up sized from 10 to 6


We use load to size OCPD and wire size is done according to OCPD and distance due to voltage drop.  No requirement to oversize for future although not a bad idea.

MOP is to account for inrush startup current. Compared to FLA rating.


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## electriclese (Aug 17, 2013)

Use table 250.122 for sizing equipment grounding conductors, what you might be getting mixed up with is 250.122(B) requires us that any time we increase wire size in ungrounded conductors for voltage drop we have to increase the EGC in size proportionately according to the circular mil area of the ungrounded conductors, this many times will leave us with the same size EGC as the ungrounded.


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## gfretwell (Aug 17, 2013)

This is certainly the unintended consequence of poorly worded code language.


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