Maximum Conductor / OCPD Rating

[saladhawks]

This is probably a silly question, but I am curious if someone can provide me a definitive code reference for the following situation:

Branch-circuit load of 70A continuous (type of load not listed in 240.3).

Per 210.19, ampacity of conductors sized to 125% of continuous load (no ampacity adjustments applicable) / 70A * 1.25 = 87.5A.

Per 210.20, overcurrent protection shall not be less than 125% of continuous load / 70A * 1.25 = 87.5A.

Is there a specific code reference that defines the maximum size of conductors and OCPDs utilized?

My contention is that such a reference does not exist and that obvious factors affecting the maximum conductor / OCPD would be:
- Cost
- Lugs
- Load Requirements
- Equipment Damage Curves

Am I missing something here????

 

[dpc]

There is no maximum size that I am aware of. As long as the conductors are protected, conductor and breaker size can be larger than required. However, at some point, the conductor has to be connected to the lugs at the other end. If you oversize the conductor too much, it may end up too large for the intended lugs.

 

[rbulsara]

Well...no restriction on maximum conductor size, but it not true for an OCPD.

It is one thing to provide a say 200A service and equipment and 200A sized wiring and OCPD for a 70A calculated load. But you can not protect a 100A rated lighting panelboard with a 200A OCPD. Look up NEC articles on panelboad overcurrent protection.

Other instances, are: LV transformer primary OCDP are limited to 125% of rated current max. The motor branch circuit protection in 430-152 are also maximum limits.

Also most UL listed HVAC equipment have maximum OCPD size listed on its nameplate or instructions, which you can not exceed.

In general, NEC does not permit underprotection of equipment or it is not the intent.

 

[dpc]

I agree with rbulsara - the OCPD requirement for the equipment must be met. This is often achieved at a fused disconnect local to the equipment.

Sorry if my earlier response created any confusion on this.

 

[saladhawks]

I agree about the OCPD requirement if the specific load is listed. I am specifically interested in loads that are not listed in 240.3 (motors, transformers, and HVAC equipment are all listed).

It seems to me that cost and lug requirements are the main factors governing maximum conductor and OCPD for non-listed equipment, not code.

 

[rbulsara]

You will use some listed device to feed your load...as I said there is no restriction has to how big feeder or equipment you provide, as long as it exceeds calculated load. But any equipment you use, can not be under protected...

What load you think are not "Code"?

 

[RalphChristie]

Does OCPD stands for overcurrent protective device? Sorry, I am not familiar with this term.

 

[rbulsara]

yes..

 

[RalphChristie]

Thanks for the clarification, rbulsara. I'm not a code expert and is not even under IEEE-jurisdiction, but your comments make sense. The same would apply in the IEC-world and should in other codes/standards.