derating MC

[binney]

ADJUSTMENT FACTORS

310.15 exception #5: Adjustment factors shall not apply to Type AC cable or to type MC cable without an overall outer jacket under the following conditions:

(a) each cable has not more than three current carrying conductors.

(b) conductors are #12

c) not more than 20 current carrying conductors are bundled, stacked or supported on ?bridle rings?

What is MC without an overall jacket?

Is this the typical MC I get when I ask for MC at a supply house?

I use a lot of MC and I?ve always kept my bundles to 9 current carrying to avoid the 50% adjustment factor required by 310.15B2a.

If this applies to the MC I use on a regular basis, this would save me a boat load of headaches.

 

[iwire]

Re: derating MC

Originally posted by binney:

What is MC without an overall jacket?

Is this the typical MC I get when I ask for MC at a supply house?

Yes it is the typical MC cable used in dry locations.

MC with an overall jacket is a lot like liquid tight flexible conduit. It has a plastic jacket and can be used in wet locations, around here it is commonly used in parking garages.

 

[pierre]

Re: derating MC

You can actually install up to 20 current carrying-conductors and not have to adjust ampacity for MC or AC cables.

 

[shortcircuit2]

Re: derating MC

How would I figure the allowable ampacity of twelve MC 12-2 120volt branch circuits bundled through an area exposed to temperatures of 160 degrees fahrenheit?

shortcircuit2

 

[iwire]

Re: derating MC

Originally posted by shortcircuit2:
How would I figure the allowable ampacity of twelve MC 12-2 120volt branch circuits bundled through an area exposed to temperatures of 160 degrees fahrenheit?

shortcircuit2

With a ambient temperature of 160 F, 310.16 rates 12 AWG THHN (MC) at 12.3 amps.

12 - 12/2 MCs would be 24 CCCs (current carrying conductors)

24 CCCs gets a 45% downward adjustment per Table 310.15(B)(2)(a).

12.3 x .45 = 5.5 amps.

So your 12 AWG would be virtually worthless with a rating of 5.5 amps.

For 20 amp branch circuits with a bundle of 24 CCCs and an ambient temp of 160 you will need to use 3 AWG.

[ March 13, 2005, 12:24 PM: Message edited by: iwire ]

 

[George Stolz]

Re: derating MC

Assuming MC with 90? conductors, (since they'd have to be installed in that ambient temperature):

#12 CU @ 90? is good for 30 amps (310.16)

30.0 x .41 = 12.3 amps (ambient correction)
12.3 x .60 = 7.38 amps (310.15(B)(2)(a)(ex. 5))

Can't guarantee I'm right, but that's the answer I get.

 

[George Stolz]

Re: derating MC

Morning, Bob!

Originally posted by iwire:
24 CCCs gets a 45% downward adjustment per Table 310.15(B)(2)(a).

12.3 x .45 = 5.5 amps.

Table 310.15(B)(2)(a) only applies if all 24 CCC's are contained in the same cable, no? Edit: No, stupid (me), read the damn text for once!

[ March 13, 2005, 12:33 PM: Message edited by: georgestolz ]

 

[iwire]

Re: derating MC

George I think you got it right, I think I blew it with my 45% downward adjustment.

5.5 amp or 7.3 amps either way the 12 AWG is worthless for lighting and power circuits.

 

[George Stolz]

Re: derating MC

Is it just me, or is the derating section a little muddy?

 

[pierre]

Re: derating MC

160 degrees F is darn hot - what would this application be for?


George
what do you find so "muddy"?

 

[George Stolz]

Re: derating MC

Well, for one, Bob is at least twice as smart as I am and got it wrong. For two, the answers were 2 amps apart. This entire page is full of exceptions and odds and ends, seems like approximate values could be had for less hoop-jumping.

IMO.

 

[shortcircuit2]

Re: derating MC

pierre...the question was hypothetical...I just wanted to point out that the ambient temperature of locations that cables are installed has an effect on the cables current carrying capacity.

Heres another one...

If we have...

Ten 12/3 MC multiwire branch circuits feeding 120 fluorescent lighting fixtures and the load on each circuit is 12 amps, then...

1. What is the allowable ampacity if they are bundled for 22 feet as they branch out of a panel?

2. What is the allowable ampacity if they are NOT bundled but pass through a boiler room for 8 feet where they are exposed to tempatures in excess of 160 degrees fahrenheit?

(edited to add that the supply is 208/120 volt 3-phase)

shortcircuit2

[ March 13, 2005, 08:59 PM: Message edited by: shortcircuit2 ]

 

[don_resqcapt19]

Re: derating MC

shortcircuit2,
Is the supply single phase or 3 phase?
Don

 

[shortcircuit2]

Re: derating MC

Man...Don you are sharp. I missed one of the key parts of those questions, didn't I...

For the above questions, figure that the circuits feed off a 208/120 volt 3-phase supply.

Don...maybe we can see some other posts before yours?

shortcircuit2

[ March 13, 2005, 09:02 PM: Message edited by: shortcircuit2 ]

 

[pierre]

Re: derating MC

Hint - 310.15(B)(2)(a) & (4)(c)

 

[shortcircuit2]

Re: derating MC

question #1...18amps

question #2...30amps


shortcircuit2

 

[iwire]

Re: derating MC

Originally posted by shortcircuit2:
question #1...18amps

I do not agree, I come up with 13.5 amps

Originally posted by shortcircuit2:
question #2...30amps

I agree if the total circuit length is longer than 80 feet.

Originally posted by shortcircuit2:

If we have...

Ten 12/3 MC multiwire branch circuits feeding 120 fluorescent lighting fixtures and the load on each circuit is 12 amps, then...

1. What is the allowable ampacity if they are bundled for 22 feet as they branch out of a panel?

Ten 12/3 MC multiwire branch circuits from a 208Y/120 supply is 30 current carrying conductors which requires a 45% downward adjustment per Table 310.15(B)(2)(a) resulting in an ampacity of 13.5 amps.

Had the supply been 240/120 you would normally have 20 current carrying conductors with a resulting ampacity of 15 amps. However fluorescent lighting is a non-linear load so even with a 240/120 supply you still have 30 current carrying conductors.

Originally posted by shortcircuit2:
2. What is the allowable ampacity if they are NOT bundled but pass through a boiler room for 8 feet where they are exposed to tempatures in excess of 160 degrees fahrenheit?

This question is lacking the total length of the circuit.

If the total length of the circuit is 79 feet or less the ampacity of the 12/3 cable will be 12.3 amps.

If the total length of the circuit is 80 feet or more the ampacity of the 12/3 cable will be 30 amps although 240.4(D) and 110.14 will limit us to 20 amps.

How did I do?

Bob

Edited in the info about non-linear loads.

[ March 18, 2005, 08:49 AM: Message edited by: iwire ]

 

[shortcircuit2]

Re: derating MC

Hi iwire...My interpretation...

Question #1...Has 30 current-carrying conductors due to the nonlinear loads associated with the fluorescent lighting, and the neutral of the 3-phase wye-connected 208/120 volt multiwire branch circuit must be counted as a current-carrying conductor...310.15(B)(4)(c)

Then I applied the rule in 310.15(B)(2) exception #5, which applies to MC cable bundled with more than 20 CCC, which told me to apply a 60% derating adjustment factor...

So, #10 MC cable=30amps x .60 = 18 amps.

2) I see that I didn't include the circuit length, which is a concern for applying the correct formula for derating as instructed by 310.15(A)(2) exception...

You done good iwire...how did I do?

shortcircuit2

 

[shortcircuit2]

Re: derating MC

Iwire...Massachusetts has a revision to 310.15(B)(2)(a) that deletes exception#5 and replaces it with a different ex#5...which you have applied correctly to question #1

shortcircuit2

 

[iwire]

Re: derating MC

Originally posted by shortcircuit2:
Iwire...Massachusetts has a revision to 310.15(B)(2)(a) that deletes exception#5 and replaces it with a different ex#5...which you have applied correctly to question #1

I just plain blew question 1 , that is twice now I did not remember to use the 60% from 310.15(B)(2) exception #5 and went right to the standard derating table.

Do you agree with the results I posted for question 2?

Bob