Sizing electrical conductors

[SparkyJunior]

I wanted to know how to properly size electrical wiring for a distribution panel. Right now it is designed to be a 250A panel, and the overcurrent protection device is designed to be a 250A breaker. The trip setting of the breaker is currently designed to be 70A. The distribution panel currently feeds a 45KVA lighting transformer but more load is anticipated in the future, hence the larger breaker size. So my question is this.

Do I size my conductors based on the 250A rating of the breaker? Or..

Since breakers can only be loaded 80% of its value do I size the conductors for 80% of the 250A rating (200A)?

 

[VEBill]

I can't speak to such ground installations, but on airplanes the conductors are selected to be sufficiently large to open the breaker before they are overheated themselves - all confirmed as worst possible case.

Standby for other replies, probably directing you to National Eelectric Code.

 

[rbulsara]

Code minimmun conductor size is equal to the trip setting. That is 70A ampacity in this case with 70A trip.

However if you are installing for the future, and you intend to adjust the future trip settinng/rating plug to 250A then the condcutor needs to be sized for that ampacity regardless of 80% or 100% rating of the breaker.

80% or 100% rating of a CB (in the NEC world) only defines what is the maximum continuous load can be applied to that unit. The conductor ampacity still should be equal or more than the trip setting.

 

[MacGyverS2000]

In the same vein, are breakers rated to the wire gauge acceptable? For example, 12 Gauge wire supports 30A, so is that acceptable with a 30A breaker and a sub-24A power draw, or would you be forced to move to 10 Gauge?


Dan - Owner

http://www.Hi-TecDesigns.com

 

[dpc]

If the 30 A breaker is protecting the wire in question, it must be #10 AWG.

Also, in addition to the NEC requirements, UL-listed breakers must use conductors sizes as specified by UL, even if NEC seems to say a smaller size conductor is permissible.

UL testing is based on 60 deg C and 75 deg C insulation, depending on breaker size and type. So if 90 C insulation is used, it cannot be applied at full 90 C rating.

This is for molded case circuit breakers.

 

[rbulsara]

There are limitations in NEC world. NEC limits #14 AWG wire to 15A overcurrent prortection device, #12 to 20A and #10 to 30A. Rather those are the min. wires sizes for those breaker/fuse ratings.

To expand on what dpc said, wire guages #1 and smaller must use 60 deg. C rated ampacity from NEC tables unless terminatinos are UL (or equivalent) listed for higher temp. rating. See NEC article 110 for temperature limitations.

 

[controlsdude]

NEC only states that you have overcurrent protection rated for that size wire. If you have a circuit breaker at 70amp and a wire size rated for 250amps, I believe your ok with the NEC. I guess my only concern would be the physical size wire, would it still install into the 70amp circuit breaker? Or am I missing something here?

Or do you have a 250amp feed goes to a 250 amp circuit breaker. Then you have a 70 amp feed off of this panel to lighting? Is this what your descibing?

 

[MacGyverS2000]

j, It reads to me like the Sparky sized his panel with future upgrades in mind, i.e., he may be pulling up to 250A in the future. for now, he's only pulling 70A, so he put in a 70A breaker into a panel capable of supporting 250A.

I've been poring over the NEC guidelines (not UL, so YMMV) for a home project, and my understanding is what rbulsara described... if 70A-limit cable is run, then the breaker can be no larger than 70A, though thicker cable on a 70A fuse is fine. In Sparky's case, he would need to upgrade his wiring to 250A-capable if/when he decides to actually start using the 250A capabilities of that panel. On the same token, the draw on that line should be no more than 80% of the breaker's rated value, or 200A.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[dpc]

Regarding the reference to limiting load to 80% of the breaker rating. This applies to continuous load, which, per NEC, is three hours or more.

This is the flip side of saying that the circuit has to be sized for the sum of the non-continuous loads plus 125% of the continuous loads. If you've already taken 125% of the continuous load, you don't need to de-rate to 80%.

 

[MacGyverS2000]

Sorry, dpc, I should have been more clear on that one. Sparky mentioned this was for a lighting transformer, which I assumed would be active for at least a typical 8-hour workday (if not 24 hours as in a manufacturing environment).

Dan - Owner

http://www.Hi-TecDesigns.com