overcurrent protection location

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harley123064

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Name plate rating on a furnace calls out for max overcurrent protection on circuit 1 to be 60 amps max on circuit 2 to be 30 amp max. The furnace is fed with two number 6cu. with 60 amp breakers on ea. the furnace has two breakers on it. circuit 1 has a 60 amp. and circuit 2 has a 30 amp. is this correct or does the ocp. have to be at the panel?
 
augie47

augie47

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Tennessee
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State Electrical Inspector (Retired)
As far as overcurrent protection, you are fine (as long as the overcurrent protection list a breaker as acceptable)
A possible fly in the ointment could exist if that #6 is NM cable and the minimum circurt amps exceeds 55 amps.
 
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dnem

Senior Member
Location
Ohio
You need to ask this as 2 separate questions

harley123064 said:
Name plate rating on a furnace calls out for max overcurrent protection on circuit 1 to be 60 amps max on circuit 2 to be 30 amp max. ..... the furnace has two breakers on it. circuit 1 has a 60 amp. and circuit 2 has a 30 amp. is this correct or does the ocp. have to be at the panel?
Click to expand...

Where in the circuit the nameplate max OCPD is found doesn't matter.
It calls for a 60a max and a 30a max. . Furnace or panel doesn't matter. . You have them in the furnace so you're good.

harley123064 said:
The furnace is fed with two number 6cu. with 60 amp breakers on ea. ..... is this correct or does the ocp. have to be at the panel?
Click to expand...

Yes, the conductor OCPD must be at the source of the circuit.
Each #6 needs a 70a max at the source panel. . You have two 60a breakers in the panel so you're good.
 
D

dnem

Senior Member
Location
Ohio
augie47 said:
As far as overcurrent protection, you are fine (as long as the overcurrent protection list a breaker as acceptable)
A possible fly in the ointment could exist if that #6 is NM cable and the minimum circurt amps exceeds 55 amps.
Click to expand...

The Romex fly in the ointment is 334.80.
Even if Romex, the 60?C column is 55a. . 240.4(B): one up, 240.6(A): 55 bumps up to 60amps and the breakers are 60a, so it's good.
 
Dennis Alwon

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
dnem said:
The Romex fly in the ointment is 334.80.
Even if Romex, the 60?C column is 55a. . 240.4(B): one up, 240.6(A): 55 bumps up to 60amps and the breakers are 60a, so it's good.
Click to expand...
Not if the load served is more than 55 amps. We have seen this occasionally. Heater element with the fan at 1.25 can be 56 or 58 amps. #6 NM is no good in that case
 
augie47

augie47

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Tennessee
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State Electrical Inspector (Retired)
Dennis Alwon said:
Not if the load served is more than 55 amps. We have seen this occasionally. Heater element with the fan at 1.25 can be 56 or 58 amps. #6 NM is no good in that case
Click to expand...

Yep :grin: Happens two or three times a month in this area.
SE has been a solution, but '08 Code shoots that down now.
 
Dennis Alwon

Dennis Alwon

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augie47 said:
Yep :grin: Happens two or three times a month in this area.
SE has been a solution, but '08 Code shoots that down now.
Click to expand...

Nc is revisiting this SE issue and will rule on it by Jan. 1, 2009. It is interesting because I have heard of at least 2 jurisdictions around here that are ignoring the rule. Not a good think but there seems to be a bit of a rebellion going on here. :)
 
D

dnem

Senior Member
Location
Ohio
Dennis Alwon said:
dnem said:
The Romex fly in the ointment is 334.80.
Even if Romex, the 60?C column is 55a. . 240.4(B): one up, 240.6(A): 55 bumps up to 60amps and the breakers are 60a, so it's good.
Click to expand...

Not if the load served is more than 55 amps. We have seen this occasionally. Heater element with the fan at 1.25 can be 56 or 58 amps. #6 NM is no good in that case
Click to expand...

Heater element with the fan at 1.25 can be 56 or 58 amps will not have a max 60a OCPD.

As a general course to take for all installs, I would agree with you. . You should check the load served which will be FLA on the nameplate. . But in this specific case it will not be an issue.

harley123064 said:
Name plate rating on a furnace calls out for max overcurrent protection on circuit 1 to be 60 amps max on circuit 2 to be 30 amp max.
Click to expand...

There's no way that a nameplate with a max 60a OCPD spec will also contain a FLA more than 55a. . Motor nameplates never set the max OCPD and the FLA that close together. . I would expect a max 60a OCPD to have a FLA of 30-40a.
 
Dennis Alwon

Dennis Alwon

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Location
Chapel Hill, NC
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Retired Electrical Contractor
dnem said:
Heater element with the fan at 1.25 can be 56 or 58 amps will not have a max 60a OCPD.

As a general course to take for all installs, I would agree with you. . You should check the load served which will be FLA on the nameplate. . But in this specific case it will not be an issue.



There's no way that a nameplate with a max 60a OCPD spec will also contain a FLA more than 55a. . Motor nameplates never set the max OCPD and the FLA that close together. . I would expect a max 60a OCPD to have a FLA of 30-40a.
Click to expand...
I have to disagree. If the heater elements were a full 10KW then you would have 52amps with the 125% and if the motor were another 4 amps the load would be higher then 55. Now why would a load with the 125% included of 56 amps not be on a 60 amp breaker? Of course it would. I have done it many times and it is perfectly legal and I have seen units so labeled.
 
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dnem

Senior Member
Location
Ohio
Dennis Alwon said:
I have to disagree. If the heater elements were a full 10KW then you would have 52amps with the 125% and if the motor were another 4 amps the load would be higher then 55. Now why would a load with the 125% included of 56 amps not be on a 60 amp breaker? Of course it would. I have done it many times and it is perfectly legal and I have seen units so labeled.
Click to expand...

First, the OP is talking about a furnace. . Are you calculating the furnace heater to run 3 hours continuously ?

Second, this furnace specs 2 circuits. . Are you thinking that a 10kw 42a furnace heater is on the same circuit as the furnace motor ?
 
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ce2two

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where it rarely rains .
ce2two

ce2two

#6 THHN relative to romex (nm) is good for 75 amps max. section 310-16 ,90c/194 f.............used this for the longest time ........correct me if i'm wrong?????????? #8 ,55amps, #10 ,30 amps.....
 
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dnem

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Location
Ohio
ce2two said:
#6 THHN relative to romex (nm) is good for 75 amps max. section 310-16 ,90c/194 f.............used this for the longest time ........correct me if i'm wrong?????????? #8 ,55amps, #10 ,30 amps.....
Click to expand...

You gotta build your circuit according to the weakest link.
Here's your links:
1] lug
2] wire
3] raceway

1] Your breaker lug with a #6 is rated for 60?C.
110.14(C)(1)(a) is telling you that certain lug sizes should be assumed to be 60?C unless marked otherwise. . 110.14(C)(1), "..... Unless the equipment is listed and marked otherwise, .....". . If marked otherwise, you can use it as marked. . 75?C is very very common marking for smaller lug sizes. . And all common SquareD, Siemens, CutlerHammer, + GE breakers are listed for 75?C.
The lug on your load equipment is also most likely rated for 75?C.
So your lug amps with the #6 attached is 65a.

2] With Romex your wire is THHN rated at 90?C.
#6 THHN is 75a.
Apply all correction factors such as ambient correction from the bottom of T310.16 and over 3 conductors from T310.15(B)(2)(a).
I think it's safe to say that this installation doesn't have any correction factors so the #6 wire is 75a.

3] The raceway is Romex.
334.80 limits Romex to 60?C.
So your #6 Romex amps are 55a.

1] lug is 65a.
2] wire is 75a.
3] raceway is 55a.
You gotta build your circuit according to the weakest link and that's 55a.

For choice of breaker, you can "one-up" [240.4(B)].
"One-up" for 55a gives you a 60a breaker [240.6(A)].

Is the connected load 55a or less ? . If your load is continuous it can not be greater than 44a [44 x 1.25 = 55, 210.19(A)(1)]. . If the connected load is larger, then the wire is too small to remain. . It has to be replaced with larger conductors.
 
D

dnem

Senior Member
Location
Ohio
celtic said:
:-?

You have picture, link, documentation or anything that shows NM has THHN inside it?
Click to expand...

I don't think the conductors themselves are technically THHN because I believe you need to rely on the Romex jacket as the nylon. . The insulation is thermoplastic and 334.80 confirms that it is 90?C wire so that gives it the double Hs. . Maybe the individual conductors should be called THH.
 
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