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6/2 NM and 5kw heat

chris kennedy

chris kennedy

Sawhorse
Joined
Oct 24, 2009
Messages
1,538
Location
Miami Fla
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?
 
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?
 
jar546 said:
So why the 6/2? Seems oversized.
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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.
 
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.
 
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.
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What about upsizing the EG?

250.122(B)

?
 
jar546 said:
What is the size of the EG conductor for NM cable:10/2=?= #10

8/2=? = # 10

6/2=? = # 10

That is the point
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# 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.
 
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.
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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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jar546 said:
It only says if they are increased in size. It does not specify the reason, therefore the EG must be upsized.
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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
 
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



​
A 240-volt, single-phase, 250-ampere load is supplied from a 300-ampere breaker located in a panelboard 500 ft away. The conductors are 250-kcmil copper, installed in rigid nonmetallic conduit, with a 4 AWG copper equipment grounding conductor. If the conductors are increased to 350 kcmil, what is the minimum size for the equipment grounding conductor based on the proportional-increase requirement?



Solution



STEP 1.



Calculate the size ratio of the new conductors to the existing conductors:




document.php


​
STEP 2.



Calculate the cross-sectional area of the new equipment grounding conductor:




document.php


​
According to Chapter 9, Table 8, 4 AWG, the size of the existing grounding conductor, has a cross-sectional area of 41,740 circular mils.



STEP 3.



Determine the size of the new equipment grounding conductor. Again, referring to Chapter 9, Table 8, we find that 58,436 circular mils is larger than 3 AWG. The next larger size is 66,360 circular mils, which converts to a 2 AWG copper equipment grounding conductor.
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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.
 
jar546 said:
In the case here. The actual size needed is 8/2 because of the load being 30A of continuous load.
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30 amps is the maximum overcurrent protection (MOP) not the load.
 
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
 
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
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The Minumum Circuit Ampacity(MCA) is what you would use.
 
jar546 said:
Was the equipment ground upsized too?
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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.
 
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.
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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.
Click to expand...
 
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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