The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!

Featured:
   

2017 NEC and Related
2017 NEC
Now Available!

   
Recent Posts
Would ELV branch circuits be a good idea?
by LongRunner
Today at 07:42 AM
100% breakers?
by Texas_Ranger
Today at 06:13 AM
Forum Software Upgrade coming soon...
by Trumpy
01/14/17 07:11 PM
Faricating Guards out if Lexan?
by Trumpy
01/14/17 07:09 PM
Dangerous DIY Advice
by Potseal
01/12/17 09:52 AM
New in the Gallery:
Desk-mounted "power-board"
Shout Box

Top Posters (30 Days)
HotLine1 15
gfretwell 11
ghost307 10
sparkyinak 9
Webmaster 8
Who's Online
2 registered (frank, gfretwell), 363 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#98921 - 06/22/06 06:37 PM Ampacity for 6/3 Armored Cable
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
I have a customer who has a sub-panel fed by a 6/3 piece of AC. It's overcurrent protection is a double-pole 60AMP Homeline Square 'D' circuit breaker. One phase is operating at 53AMPS, the other at 43. On Tuesday, the circuit breaker tripped. I went there today and the breaker was RED-HOT. My question is, without running new #4 feeders to the sub-panel, can I upgrade to a double-pole 70 according to 310.16 using the 90º column?

Top
2014 / 2011 NEC & Related Books and Study Guides
#98922 - 06/22/06 07:17 PM Re: Ampacity for 6/3 Armored Cable
bot540 Offline
Member

Registered: 09/14/04
Posts: 177
Loc: Vernon Hills, IL
First of the wire must be of a type listed in the 90 degree column. Second the terminals of the breaker must be rated for 90 degrees. Also make sure that ambient tempature doesn't play a role. IE: does the cable run through a hot attic?

[This message has been edited by bot540 (edited 06-22-2006).]
_________________________
Jesus may have been a capenter,but God was an electrician.Genesis1:3

Top
#98923 - 06/22/06 07:58 PM Re: Ampacity for 6/3 Armored Cable
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
The cable DOES run through a suspended ceiling near the roof of the building, but not in or through any thermal insulation per 320.80 (A).

Top
#98924 - 06/22/06 10:57 PM Re: Ampacity for 6/3 Armored Cable
gfretwell Online   content


Member

Registered: 07/20/04
Posts: 9058
Loc: Estero,Fl,usa
The "red hot" is alarming to me. I wonder if you just had a bad breaker.
_________________________
Greg Fretwell

Top
#98925 - 06/23/06 02:13 AM Re: Ampacity for 6/3 Armored Cable
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
 Quote:
My question is, without running new #4 feeders to the sub-panel, can I upgrade to a double-pole 70 according to 310.16 using the 90º column?


No you can not use the 90 C column.

The reason is that none of your terminations are rated 90 C. The terminations will be 75 C at most.

However you could install a 2 pole 70 per 240.4(B) although the load on the conductors would have to remain under 65 amps.

I would install a new 70 amp breaker and look at balancing the load a little better.

With 53 and 43 amp readings you could try to balance it to about 48 amps each.

One other factor to consider is continuous loading.

With a 60 amp breaker the continuous load should not exceed 48 amps.

If you swap to a 70 amp breaker the continuous load could be 56 amps.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

Top
#98926 - 06/23/06 08:05 AM Re: Ampacity for 6/3 Armored Cable
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
A 60A circuit breaker carrying 53A should not be 'red hot'. Quite warm, perhaps, but not red hot.

I would check very carefully for damage to the bus stabs, or damage to the end of the conductor, both of which could cause a high impedance connection and localized heating. Was the breaker correct for the panel, or one that 'just happens to fit'?

If you have a high impedance connection, then it will heat and trip a new 70A breaker just as it did the 60A breaker.

-Jon

Top
#98927 - 06/24/06 02:55 PM Re: Ampacity for 6/3 Armored Cable
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
I was there today farting around during business hours. I balanced the loads (50AMPS and 48AMPS), but the new 70 AMP circuit breaker still got warm. Not red hot, but warm. The terminal connections to the breaker are solid, but I'll have to check for damage on the buss when I go back there later tonight (it's a bar so I do most of my work there when it is closed). And yes, the circuit breaker is the same as the manufacturer of the panelboard. 110.3 (B) was not violated.

Thanks to everyone for all of your help, and timely advice.

It's very much appreciated.

--Ron

Top
#98928 - 06/24/06 03:34 PM Re: Ampacity for 6/3 Armored Cable
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
 Quote:
If you have a high impedance connection, then it will heat and trip a new 70A breaker just as it did the 60A breaker.


Sure thats possible.

However by replacing the breaker you usually take care of any high impedance connection that is only affecting one branch circuit.

If the panel bus is causing the problem it will be quite evident when the breaker is removed.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

Top


ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals