Sizing AC CB

[iwire]

suppose the overload device in the unit fails?

How is that at all different from any other circuit we run? :-?

If the OCPD fails on any circuit all bets are off.

 

[scotteng]

This is one of the trickiest issues to understand. I get plan reviewers who sometimes don't grasp why the wire is smaller than the breaker size and they try to treat it as a standard branch circuit.

If you use the breaker in the panel as your MOCP and use a non-fused disconnect at the unit, size your wire to the MCA and breaker to the MOCP (or somewhere in between if you are trying to fit in a smaller frame size). I.e., if you have an MCA of 90 and an MOCP of 125, I will commonly size the breaker at 100A or 110A as this frame size can fit in most standard 225A panels.

If you are using a fused disconnect at the unit as your MOCP, then the breaker in the panel should be sized same as the fuse at the unit, but, the wiring between the panel and the unit is now a feeder (breaker is no longer the last overcurrent devices). The wire between must therefore be sized to the breaker (not to the MCA).

If you know your equipment MCA and MOCP, I would always choose to use the breaker as the MOCP and thus save on the wiring and conduit between by sizing to the MCA (not to the breaker). Careful though, if substitute equipment shows up with higer MCA values and you've already roughed in conduit and wire...bad news.

 

[peter d]

Money is never an issue.

That must be nice. For the rest of us though, money almost always factors into the equation. So in the case of A/C and motor branch circuits, I'll use the NEC guidelines over personal opinions.

 

[peter d]

Just knowing how to follow an instruction in a book don't make you an Electrician, Just a good follower and worker maybe.

I don't understand this comment at all....being able to understand and apply the rules of the NEC correctly is a vital part of our trade.

 

[neutral]

Fair enough. I will continue to protect downstream and not upstream. Everyone else can follow the code book if that?s what they are comfortable with.

 

[markstg]

This is one thing in the code book among many I disagree with, and ignore. I use the wire size needed to supply the current necessary for the load and use an OCPD that will protect the wire used. End of story.

So for the case stated, Min Circuit Ampacity 23A, you would have installed #12 or #10 AWG wire, and provided 20A or 30A OCPD respectively. What do you do when the OCPD trips on starting the A/C Unit?

 

[Dennis Alwon]

Fair enough. I will continue to protect downstream and not upstream. Everyone else can follow the code book if that?s what they are comfortable with.

Oh no , you can't start something and just walk away.

Let me ask you this.... Do you size your branch circuit fpr the maximum overcurrent protection allowed? Thus a unit that has a 20 amp nameplate-- would you run a #8 and use the max fuse of 35 amps?

Just curious

 

[neutral]

Well, I didn't mean to start anything. If I were to feed a load that was going to draw 20 amps from the supply I would most likely feed it with a #10 wire protected by a 30 amp breaker. Unless there were other factors to consider like distance of load from supply.

 

[augie47]

neutral, how do you handle "in-rush"? If that 15 hp motor draws 42 amps (nameplate RLA) lets say ? How do you wire for that ?

 

[neutral]

:smile: Rubber gloves? If I had a problem with in rush curent I would be looking at the load to see why the motor was taking so long to come up to speed.

 

[augie47]

:smile: Rubber gloves? If I had a problem with in rush curent I would be looking at the load to see why the motor was taking so long to come up to speed.

please allow me to ask the question in a different manner. If you were asked to install the circuit for a 15hp 3 phase 240 volt motor operating under normal everyday conditions and the motor had a 42 amp (FLA) nameplate, what size wire and breaker would you use.
Not a trick question...just everyday conditions./

 

[neutral]

Just off the top of my head without the actual job at hand I would say 50 amp breaker # 6 wires and the appropriate size motor starter (Allen Bradley)with overload protection. I think at this point i will get out of the ring throw in the towel.

 

[augie47]

don't jump out of the ring. there's always more than 1 point of view.

 

[don_resqcapt19]

Fair enough. I will continue to protect downstream and not upstream. Everyone else can follow the code book if that?s what they are comfortable with.

So am I to expect that you do not install services? The service entrance conductors are not protected from short circuits or ground faults and they are protected from overload conditions on the load end.

 

[infinity]

This is one thing in the code book among many I disagree with, and ignore. I use the wire size needed to supply the current necessary for the load and use an OCPD that will protect the wire used. End of story.

Neutral said this and is entitled to his opinion. He has not said that the rest of us are wrong, he just feels that he wants to go beyond the code requirements. Maybe it's time to lay off the guy. :wink: