Does 240.4(B) trump 334.80

Status
Not open for further replies.

mj63

Member
I have an install where I need a 60 amp circuit to a new electric furnace in an unfinished basement.
I want to install a #4/3 wg aluminum SER cable to the furnace.

Article 338 says to comply with article 334 for install which says for ampacity to use the 60 degree column which limits me to 55 amps for the #4SER.

240.4(B) says I can go the the next higher breaker, provided 3 conditions are met which my install meets.
Does 240.4(B) trump 334.80?

I also would like to strap the SER to the bottom of the joist in the unfinished basement and I believe that Article 334.15(C) says that it is legal.

Am I on the right track here?

Thanks in advance
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
240.4(B) does not change the ampacity of the conductors. It simply says that conductors of a given ampacity may be _protected_ by the next standard size breaker.

In your case you have conductors of 55A ampacity. The maximum allowed load that you are allowed to put on these conductors is 55A (non continuous) or 55 * 0.8 (continuous). As long as you meet these load limits you may protect these conductors with a 60A breaker.

-Jon
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Unfortunately many of these furnaces that use to have a minimum cir of 52 amps or so now have a 56 minimum ampacity . It stinks. You get over 55 and the next size is needed. The same for copper #6 NM, which we always used, is no longer any good for a 10 kw unit.

BTW, I have heard there is a very good chance that SE cable is going to go back to the 75C rating in 2011. We'll see.
 

OTT2

Senior Member
Location
Orygun
Unfortunately many of these furnaces that use to have a minimum cir of 52 amps or so now have a 56 minimum ampacity . It stinks. You get over 55 and the next size is needed. The same for copper #6 NM, which we always used, is no longer any good for a 10 kw unit.

BTW, I have heard there is a very good chance that SE cable is going to go back to the 75C rating in 2011. We'll see.

This catches a lot of electricians off guard. They are used to providing a #6 AWG to the furnance assuming it will be adequate for the load, not always so though!!

Don't forget to look at the label on the furnace for MCA and max OCP.

Quite often the funace installers do not mark the label so you may have to look near the heating elements to find the size of heat package installed.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I do not understand ..... or did you mean 65 amps? :confused:

The older 10kw furnaces were rated as having a min. cir ampacity as 52 or 54 amps. In this case a number 6 nm cable was sufficient. Now the furnaces have designed their units such that the same 10 kw furnaces are rated at 56 amps. Now the #6 is no longer any good. We have to pipe or go up to a #4.
 

amptech

Senior Member
Location
Indiana
Another thing to be aware of is a lot of HVAC equipment manufacturers prohibit the use of AL conductors to feed them. I learned this the hard way several years ago when running AL SER to a Lennox unit.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Another thing to be aware of is a lot of HVAC equipment manufacturers prohibit the use of AL conductors to feed them. I learned this the hard way several years ago when running AL SER to a Lennox unit.


Couldn't you just splice on a short piece of copper to the Al?
 
Status
Not open for further replies.
Top