Means of Disconnect

[djtazjr]

I have been studying about motors and sizing conductors also disconnect means and things, and something puzzles me.

I do not understand the purpose of putting a disconnect with or without fuses. The only difference to me is if you OCP is higher then you FLC of the motor then you use fuses at the disconnect. I really need help with understanding this so I can move on with studying.

Can anyone help me understand when fuses should be used compared if you just use a safety switch or even if you do have to use fused if you have an OCP that is the correct size. Too much in the head and ignorance is still there.

Thanks.

 

[bphgravity]

Re: Means of Disconnect

In many cases, an overcurrent device is incorporated into the means of disconnect, however in some cases this not required.

You may have a branch circuit serving a motor load in which the overcurrent device is at the point the branch circuit originates, however the means of disconnect is at the motor. In this case, a "fused" disconnect would not be required. On the other hand, the motor may be served from a feeder tap in which the branch circuit overcurrent protection and the means of disconnect are at the same point.

This is just one of the added benefits of using circuit breakers. They can act as switches, disconnects, and overcurrent devices.

 

[djtazjr]

Re: Means of Disconnect

What was read makes good sense to me. I was reading more and studying more of the NEC Article 430.24 just about feeder tap rule.

Thanks for the help. Cleared up much for me. Never would have thought about the way breakers are used as switches, disconnects, ect...

However, does distance have any result of putting a fused disconnect near the motor? And still, want if heat strips are on the same circuit?

 

[petersonra]

Re: Means of Disconnect

Originally posted by djtazjr:
However, does distance have any result of putting a fused disconnect near the motor? And still, want if heat strips are on the same circuit?

You typically want a disconnecting means near the motor. If the feed to the motor is a long distance, the OCPD will be at the source and a non-fused disconnect would be installed near the motor.

I'm not sure just what you are refering to about heat strips.

 

[charlie]

Re: Means of Disconnect

I am betting on the heaters for the overloads.

 

[petersonra]

Re: Means of Disconnect

Originally posted by charlie:
I am betting on the heaters for the overloads.

Which would be located with the starter and not with the disconnect (unless it was a combo starter).

This reminds me of a moderately humourous story. A long time ago I did a project where we sold the custoemr a bunch of combination starters located out on the equipment skids. There was maybe a dozen of them, probably a couple hundred HP total. We prewired the controls and pumps up so all the end user had to do was bring power into the starter.

The end user told the EC that he would provide a single power connection "in the vicinity" of the equipment and the EC would have to feed all the things that needed power.

The end users idea of "in the vicinity" was an MCC several hundred yards away. I think the EC ended up adding a small jbox with distribution block at each power drop. He was not thrilled.

 

[radiopet]

Re: Means of Disconnect

The code states that fuses can be used as the OCPD and Overload Protection. The fuses in the question I would think are more refering to the overload protection since they are sized to the Nameplate rating plus 115-125% and 130% 140 Max as listed by 430-32 and 430-34 based on table and it does say in 430-55 you can combine the both if listed for it and does meet 430-32.

The OCPD is based on 430-22 and 430-6(a)(1) which uses table 430-147-150 at FLC so if you combine the units they must be approaved and meet both requirements.