Internal Wire sizes in MCC Buckets

[PhaseShift]

We had an incident here recently where we had a size 5 480V starter bucket which had the wires between the bucket stabs and the line side of the breaker melted. Upon further investigation we found that the wires between the bucket stabs and the line side of the breaker were only parallel #4 which would be good for about 170A. Now I know that the MCP breaker in this bucket will not protect these cables from an overload, so my question is, what is supposed to protect these cables from an overload? Is it the motor overloads?

I would have thought that seeing this was a size 5 bucket the factory would have said o.k. the biggest motor that can be used with this bucket is 200hp which equates roughly to 240A or so, so lets size these wires to handle 240A at the minimum. This would be parallel #2's or greater.

Does anyone know the reasoning or design decisions that MCC manufacturers use for sizing the wires internal to these buckets when designing the MCC? I have seen several other cases where these wires seems much smaller than was needed for the motor load.

 

[jim dungar]

Wiring internal to an MCC is not sized per NEC tables. UL standards often allow conductor sizing to be done based on actual heat rise tests rather than simple look-up tables. The wiring in the MCC is probably closer to being in 'open air' than it is to being 'in conduit'.

 

[augie47]

NEC ampacities do not apply to these conductors. They are sized by the mfg. on engineering data. My experience has been that they are normally not of a THHN (Or standard building wire) insulation

 

[PhaseShift]

Should the overloads be capable of protecting these cables or is it hard to say?

After seeing the melted wire we went ahead and upgraded the size of these wires. Is there any issue with doing this?

 

[steve66]

Should the overloads be capable of protecting these cables or is it hard to say?

After seeing the melted wire we went ahead and upgraded the size of these wires. Is there any issue with doing this?

Yes, the overloads should protect these wires. I would double check the overload ratings, and the MCP ratings.

If the wires melted at the end, it might have been due to a bad connection.

 

[rcwilson]

Size 5 MCC stabs seem to fail after a while. Many users specify bolt-on bus connections for all Size 5 and larger MCC buckets.

Maybe your stabs got a little loose, making a higher resistance connection that overheated. The heat transferrred to the cable, causing the melting.

You should pull out the bucket and check the stabs.

Do an IR scan on the oeprating MCC buckets. It will pick up these problems before they fail.

 

[PhaseShift]

Yes, the overloads should protect these wires. I would double check the overload ratings, and the MCP ratings.

If the wires melted at the end, it might have been due to a bad connection.

The MCP circuit breaker is a 400A breaker however this is an instaneous only breaker so it will not protect any cables from overload. I have always understood that this was not the purpose of this type of breaker.

If others have said that MCC manufacturers do not conform to the same wire sizing guidelines as the NEC then how is the overload supposed to absolutly protect these cables on the line side of the breaker. Because it sounds like these cables and the overload come from two different design standpoints, and it may not guarentee that the overloads will protect these cables? At least that is not their intention.

 

[jim dungar]

If others have said that MCC manufacturers do not conform to the same wire sizing guidelines as the NEC then how is the overload supposed to absolutly protect these cables on the line side of the breaker. Because it sounds like these cables and the overload come from two different design standpoints, and it may not guarentee that the overloads will protect these cables? At least that is not their intention.

Part of the cable sizing that is done is the requirement that it must be protected by overload relays. This is one reason it is important to use a "listed combination" of devices when employing MCPs.

 

[steve66]

The MCP circuit breaker is a 400A breaker however this is an instaneous only breaker so it will not protect any cables from overload. I have always understood that this was not the purpose of this type of breaker.

I agree completely. But something went wrong, so it couldn't hurt to double check.

I also agree with Jim. If you use a listed combination of overloads and MCP's, one of those should give before the wire does.

Steve

 

[PhaseShift]

Part of the cable sizing that is done is the requirement that it must be protected by overload relays. This is one reason it is important to use a "listed combination" of devices when employing MCPs.

I also agree with you completely on the listed combination thing.

I just have management here who is trying to tell me that we sized the overloads too high and therefore did not protect these cables. I am trying to argue the point that it is not the intention of these overloads to protect these cables. Like you mentioned, they should protect them if everything is combination listed but if not it is not their job to do so.

He is also arguing that the breaker should have protected these on overload. I have explained to him several times the purpose of a MCP breaker.

 

[Jraef]

Size 5 MCC stabs seem to fail after a while. Many users specify bolt-on bus connections for all Size 5 and larger MCC buckets.

Maybe your stabs got a little loose, making a higher resistance connection that overheated. The heat transferrred to the cable, causing the melting.

You should pull out the bucket and check the stabs.

Do an IR scan on the oeprating MCC buckets. It will pick up these problems before they fail.

Excellent advice.

In fact, years and years ago there were no plug-in Size 5 buckets, Size 4 was the max before changing to fix-mounted with bolt-on bus connections. Manufacturers came up with stab-on Sz-5 because of consumer demand, but from the earliest days it was known by the engineers to be at the marginal edge of stab connection technology. The bus stabs need strong spring clips applying pressure to make the stabs have enough surface area contact with the bus bars. A tiny little bit of corrosion or a one-time motor jam can cause enough heat to soften those spring clips and the problem cascades to a failure.

And yes, the OL relay is the protection device for the cables in terms of long-time overload protection. You should check the settings and functionality of the OL relay. In spite o that, if the bad connection mentioned above were to happen, the OLR may not have detected it because although there would be heat in the stabs and conductors, the high resistance that is the very thing causing the heat will prevent the current from increasing down stream, so the OLR would not know there is a problem.

 

[Jraef]

The MCP circuit breaker is a 400A breaker however this is an instaneous only breaker so it will not protect any cables from overload. I have always understood that this was not the purpose of this type of breaker.

If others have said that MCC manufacturers do not conform to the same wire sizing guidelines as the NEC then how is the overload supposed to absolutly protect these cables on the line side of the breaker. Because it sounds like these cables and the overload come from two different design standpoints, and it may not guarentee that the overloads will protect these cables? At least that is not their intention.

MCC buckets ar built to and tested by the UL-845 standard, which DOES dictate that the OLR must protect the cables. The MCP is allowable ONLY in that specific circumstance BECAUSE the OLR is the cable protection so having a Thermal-Magnetic breaker would be redundant. It's not that you can't use a T-M breaker, there just is no need to.

As was mentioned earlier, this is probably not a design flaw (although I no longer have a copy of UL-845 to check their wire size selection); it's more likely this was a failure of some other sort.

 

[PhaseShift]

Excellent advice.

In fact, years and years ago there were no plug-in Size 5 buckets, Size 4 was the max before changing to fix-mounted with bolt-on bus connections. Manufacturers came up with stab-on Sz-5 because of consumer demand, but from the earliest days it was known by the engineers to be at the marginal edge of stab connection technology. The bus stabs need strong spring clips applying pressure to make the stabs have enough surface area contact with the bus bars. A tiny little bit of corrosion or a one-time motor jam can cause enough heat to soften those spring clips and the problem cascades to a failure.

And yes, the OL relay is the protection device for the cables in terms of long-time overload protection. You should check the settings and functionality of the OL relay. In spite o that, if the bad connection mentioned above were to happen, the OLR may not have detected it because although there would be heat in the stabs and conductors, the high resistance that is the very thing causing the heat will prevent the current from increasing down stream, so the OLR would not know there is a problem.

As far as the overload protection protecting these cables, I dont see it all adding up. As I stated earlier the OEM had parallel #4 from the stabs to the breaker which would have a capacity for about 170A. The motor connected to this bucket is a 200hp motor and therefore has overloads set for 230A based on the motor FLA. So when looking at this how do we say that overloads set for 230A (based on motor) will protect cable rated for 170A?

I guess the one caviet is what someone else mentioned about these cables possibly having a higher rating or ampacity, so you cant use a simple look up table. Maybe that is why I am not seeing the numbers make sense.

 

[PhaseShift]

As was mentioned earlier, this is probably not a design flaw (although I no longer have a copy of UL-845 to check their wire size selection); it's more likely this was a failure of some other sort.

There was indeed a problem with the load causing the motor to run high. We actually burnt up the motor as well, and then when looking through the bucket we noticed the wire problem. Although we were indeed running both the motor and cables over capacity I still am trying to see how the overload will protect these cables based on my previous post.

 

[jim dungar]

... someone else mentioned about these cables possibly having a higher rating or ampacity, so you cant use a simple look up table. Maybe that is why I am not seeing the numbers make sense.

That is correct, The NEC tables do not apply to the factory installed internal conductors of an MCC.

 

[jdsmith]

MCC buckets ar built to and tested by the UL-845 standard, which DOES dictate that the OLR must protect the cables. The MCP is allowable ONLY in that specific circumstance BECAUSE the OLR is the cable protection so having a Thermal-Magnetic breaker would be redundant. It's not that you can't use a T-M breaker, there just is no need to.

As was mentioned earlier, this is probably not a design flaw (although I no longer have a copy of UL-845 to check their wire size selection); it's more likely this was a failure of some other sort.

One thing I have heard is that if you order a size 5 bucket for a 150HP motor from the factory, they may size these wire jumpers per UL-845 but for a 150HP motor, not the 200 HP max that the bucket will support. My boss has told me to make sure we specify that wire jumpers in the bucket be sized to the maximum allowable motor HP for the bucket size.

Is this a real issue or just something somebody in my plant came up with years back? It seems like something manufacturers would reasonably try to do to save a buck on a bid.