Feeder taps 240.21(B)

[goldstar]

Went on a service call yesterday after receiving a call from a plumbing contractor that do work for. On the phone he explained that he was at a customers house where an instant hot water unit was installed by someone else and he was there testing to see what the hot water recovery time was, After bleeding the system of hot water about 10 minutes went by and then the sound of an explosion from the basement came. When he went down to see what the problem was he saw a sub-panel that went from gray to charcoal.

I went out to investigate and found the foillowing:

• There were two main lug panels; one was a 30 circuit SQ-D QO completely loaded the other was a 20 circuit
• Inside the 20 circuit panel were (2) 2-pole 30 amp breakers, (1) 15 amp 2-pole breaker and several 20 amp breakers
• the EC who installed the electric for the water heaters installed (2) 2-pole 50 amp breakers to feed the unit

Now, here's the problem, the feeder for both panels comes from a 300 amp main breaker that's about 20' away. The feeder is 350 KCM (underground) into a trough above the panels. Tapped on to the 350KCM are (3) 4/0 AL for the 30 circuit panel and #4's AL for the 20 circuit panel. Prior to this EC adding the two 50 amp breakers to the panel fed with #4's, the panels held and there were no problems. With respect to feeder taps and prior to the addition of the two 50 amp breakers was this a ligit tap installation ? The tap was less than 3'.

Long story-short, the #4's burnt up and separated inside the nipple to the trough. The 350's are still in tact and un-damaged.

If I'm called in to make the correction to this system IMHO, and from the standpoint of safety, I would install two 150 amp main breaker panels and re-make the taps in the trough. I would be using the 150 amp main breakers both as overload protection and for safety and convenience. Neutrals would float and EGC's bonded to the enclosure. Am I wrong or is this over-kill ?

 

[pete m.]

Do you know when and under what code edition this was installed? This has nothing to do with the taps but could have some importance with protection for the individual panels.

As I see it, the 4awg and 4/0awg aluminum taps are violating 240.21(B)(1) because they do not appear to meet all the conditions of this section.

Pete

 

[LarryFine]

Tapped on to the 350KCM are ... #4's AL for the 20 circuit panel. Prior to this EC adding the two 50 amp breakers to the panel fed with #4's, the panels held and there were no problems. With respect to feeder taps and prior to the addition of the two 50 amp breakers was this a ligit tap installation ? The tap was less than 3'.

Yes*, because the #4 taps were capable of more than 10% of the source OCP.

The additional load made the #4's no longer compliant with 240.21(B)(1)(1)a:

(1) Taps Not over 3 m (10 ft) Long. Where the length of
the tap conductors does not exceed 3 m (10 ft) and the tap
conductors comply with all of the following:

(1) The ampacity of the tap conductors is

a. Not less than the combined calculated loads on the
circuits supplied by the tap conductors, and

b. Not less than the rating of the device supplied by
the tap conductors or not less than the rating of the
overcurrent protective device at the termination of
the tap conductors.

*Whether it ever complied with b above depends on the 20-ckt panel's rating.

 

[goldstar]

Do you know when and under what code edition this was installed? This has nothing to do with the taps but could have some importance with protection for the individual panels.

As I see it, the 4awg and 4/0awg aluminum taps are violating 240.21(B)(1) because they do not appear to meet all the conditions of this section.

Pete

Not sure when it was installed Pete. I saw no inspection labels on either of the panels.

 

[goldstar]

Yes*, because the #4 taps were capable of more than 10% of the source OCP.

Interesting.

The additional load made the #4's no longer compliant with 240.21(B)(1)(1)a:
*Whether it ever complied with b above depends on the 20-ckt panel's rating.

How much can a 20 circuit Sq-D QO main lug panel be rated for ? 125 amps ? I'm guessing.

 

[mpd]

the original install is also a violation of 408.36

 

[S'mise]

#4 is good for 85A. I would think he would have landed them in a Main breaker panel with a 60a main

 

[Dennis Alwon]

Interesting.

How much can a 20 circuit Sq-D QO main lug panel be rated for ? 125 amps ? I'm guessing.

I have seen 200 amp 20 cir. panels. Usually they are 20-40 cir panels. A straight 20 cir. could be anything from 125 to 200 amps.

 

[goldstar]

I have seen 200 amp 20 cir. panels. Usually they are 20-40 cir panels. A straight 20 cir. could be anything from 125 to 200 amps.

I know Murry makes a 200 amp 20/40 but the lugs in the panel are capable of handling 4/0. In this case the lugs in the 20 circuit panel were only capable of handling #4's

 

[goldstar]

#4 is good for 85A. I would think he would have landed them in a Main breaker panel with a 60a main

It's only 65 amps if you use the 75 degree column. The main in this case was 300 amps. There was no other main protection beyond that

 

[goldstar]

the original install is also a violation of 408.36

Good catch

 

[goldstar]

Yes*, because the #4 taps were capable of more than 10% of the source OCP.

The additional load made the #4's no longer compliant with 240.21(B)(1)(1)a:
*Whether it ever complied with b above depends on the 20-ckt panel's rating.

The more I thought about this the more I think you were correct. This may have been a code compliant installation at one time but you would think that whoever installed this would have installed a smaller breaker panel so that no additional circuits (beyond those calculated for originally) could be added. It doesn't take rocket science to realize you shouldn't be installing two 50 amp breakers in the spare slots when the feeders are #4 AL.