Branch Circuit Protection for Derated Circuits

[600SL]

Not sure I understand Circuit protection for derated circuits.

I have two 240 volt circuits in a common 3/4 EMT. The EMT caries 4 hot's and 2 neutral's and each branch has several NEMA L15-20 receptacles rated at 20 Amp, 60°.

So technically the wire needs to be derated to 24 AMPS. But my question is does the circuit protection also need to be derated to 24 amps or can I use a 25AMP or 30 AMP breaker.

I see the NEC requires a CB 125% of the continuous load. I would like to plug in a 4800 (20Amps) watt heater. With the 20 amp CB I currently have It would probably blow the breaker and also be in violation of the 125% minimum.

I could also go to a 75° plug but I can only find those in large 50 amp welding receptacle's which I prefer to avoid.

 

[wyliesdiesels]

Im assuming u have an L14-20R as L15 is for 3phase 250v....

The whole point of derating is that the breaker protecting the wire needs to be sized at the derated value.

U didnt say what gauge and type of wire youre using....

The breaker needs to be sized based on the lowest ampacity device which is the 60* c ampacity rating because of the outlets...

So we need more info

what size and type(NM-b, MTW, THWN, etc?) of wire are u using?

 

[600SL]

Im assuming u have an L14-20R as L15 is for 3phase 250v....

The whole point of derating is that the breaker protecting the wire needs to be sized at the derated value.

U didnt say what gauge and type of wire youre using....

The breaker needs to be sized based on the lowest ampacity device which is the 60* c ampacity rating because of the outlets...

So we need more info

what size and type(NM-b, MTW, THWN, etc?) of wire are u using?

Correct that is L14-20R. This is single phase THHN 10 gage.

So at 60° 30 amp wire gets derated to 24 amps for which there is no breaker available. Sound like I' stuck with 20 Amps.

I can get a smaller heater and it will be adequate but for some reason they cost more.

Thanks

 

[alfredeneuman]

Regardless of wire size if you have 20 amp receptacles then by Code, you must use a 20 amp circuit breaker.

 

[wyliesdiesels]

The detated ampacity cant be more than the lowest rated device.

So youre doing the math wrong.

THHN is rated at 40a (90* c rating, which is used for derating purposes only).

4-6 CCC is derated to 80% of ampacity.

So #10 is derated to 32a.

The outlet limits ampacity to 60* c ampacity because of its temperature rating. So u could use a 30a breaker IF it was an L14-30. Unfortunately, the receptacle rating limits u to 20a. So youre right back where u started.

 

[600SL]

Regardless of wire size if you have 20 amp receptacles then by Code, you must use a 20 amp circuit breaker.

I'm not doubting this but I cant find it in the code. Also I see problems with it. Like say you had two 15 amp devices plugged into a two 20 amp receptacles on a single 20 amp circuit.

In that case I would think that the wire would need to carry 30 amps and the receptacles no more than 15 each. Also circuit protection should be 37.5 amps. I realize that is a typical situation for dedicated circuits but where is it in the code.

 

[600SL]

The detated ampacity cant be more than the lowest rated device.

So youre doing the math wrong.

THHN is rated at 40a (90* c rating, which is used for derating purposes only).

4-6 CCC is derated to 80% of ampacity.

So #10 is derated to 32a.

The outlet limits ampacity to 60* c ampacity because of its temperature rating. So u could use a 30a breaker IF it was an L14-30. Unfortunately, the receptacle rating limits u to 20a. So youre right back where u started.

Hmmmm

Sounds like what you are saying is the wire is the only part that gets derated and I can use the 90° table to do the rerating. The fact that the L14-30 is also a 60° device does not force me to use the 60° table for the wire.

I could use a 6-50R receptacle which I know to be rated to 75°, but an L14-30 would not be as ugly and require a box extender.

When you say the receptacle limits me to 20 amps. Does that mean that it limits me to a 20 amp CB or just a 20 amp load on that receptacle only.

 

[alfredeneuman]

I'm not doubting this but I cant find it in the code.

Table 210.21(B)(3) Receptacle Ratings for Various Size Circuits.

 

[wyliesdiesels]

Hmmmm

Sounds like what you are saying is the wire is the only part that gets derated and I can use the 90° table to do the rerating. The fact that the L14-30 is also a 60° device does not force me to use the 60° table for the wire.

I could use a 6-50R receptacle which I know to be rated to 75°, but an L14-30 would not be as ugly and require a box extender.

When you say the receptacle limits me to 20 amps. Does that mean that it limits me to a 20 amp CB or just a 20 amp load on that receptacle only.

There is many things at play here.

Yes, the wire is derated because of the number of CCC in a conduit.

But because u have a 60* c rated device, the ampacity of the circuit cant go over the 60* c rating of the wire.

If it was a 75* c rated device, it would be a different story. U could then use a 30a breaker as i said if it was an L14-30.

yes a 20a OCPD/CB is required because of the outlet being a single receptacle and rated for 20a. A duplex receptacle is consider 2 outlets so u can protect a 15a duplex with a 20a OCPD.

 

[600SL]

There is many things at play here.

Yes, the wire is derated because of the number of CCC in a conduit.

But because u have a 60* c rated device, the ampacity of the circuit cant go over the 60* c rating of the wire.

If it was a 75* c rated device, it would be a different story. U could then use a 30a breaker as i said if it was an L14-30.

yes a 20a OCPD/CB is required because of the outlet being a single receptacle and rated for 20a. A duplex receptacle is consider 2 outlets so u can protect a 15a duplex with a 20a OCPD.

Is there such a thing as a 75° rated L14-30. I have only seen them as 60°

 

[600SL]

Table 210.21(B)(3) Receptacle Ratings for Various Size Circuits.

Thanks

Table 210.21(B)(3) clearly states that for a 20 amp circuit receptacles have to be either 15 or 20 amps. For a 30 amp circuit they have to be 30 amps only. strange thing is that for a 40 amp circuit the receptacle can be either 40 or 50.

That means that this scenario is not workable without changing all L14-20 to L14-30. Then I still have the terminal temp issue.

Thanks for finding that for me.

 

[wyliesdiesels]

Hubbell

http://www.stayonline.com/detail.aspx?id=7071

Pass and Seymour

http://m.homedepot.com/p/Pass-Seymo...Outlet-Receptacle-Black-L1430RCCV3/100155901/

http://www.legrand.us/~/media/produ...gs Connectors 4 Wire Spec Sheet SF1704R1.ashx

What brand outlets do u have?

Thanks

Table 210.21(B)(3) clearly states that for a 20 amp circuit receptacles have to be either 15 or 20 amps. For a 30 amp circuit they have to be 30 amps only. strange thing is that for a 40 amp circuit the receptacle can be either 40 or 50.

That means that this scenario is not workable without changing all L14-20 to L14-30. Then I still have the terminal temp issue.

Thanks for finding that for me.

Whats weird about that particular code is there is no 40a receptacles.

if u switch to a 30a receptacle, then 60* c rated terminals wouldnt be an issue because theyre rated for 30a. #10 wire is rated 30a @ 60* c....your problem is solved!

 

[600SL]

Hubbell

http://www.stayonline.com/detail.aspx?id=7071

Pass and Seymour

http://m.homedepot.com/p/Pass-Seymo...Outlet-Receptacle-Black-L1430RCCV3/100155901/

http://www.legrand.us/~/media/produ...gs Connectors 4 Wire Spec Sheet SF1704R1.ashx

What brand outlets do u have?



Whats weird about that particular code is there is no 40a receptacles.

if u switch to a 30a receptacle, then 60* c rated terminals wouldnt be an issue because theyre rated for 30a. #10 wire is rated 30a @ 60* c....your problem is solved!

Thanks I was looking all over for that. I even checked Hubbell because I like there stuff. I have Leviton Outlets.

I'm starting to think more about this and its just getting too dumb. It means that my circuits that are running at most a 6 amp 1.5HP motor would be protected by a 30 amp breaker. I don't think its too bright.

I really need a dedicated circuit for these heaters.

But thanks for the chat.

 

[wyliesdiesels]

Well now that u mention motors, does your motor have an overload(red button) built in?

(It would have been helpful to know this in the beggining because motor circuits are wired differently than regular branch circuits...)

Breakers(OCPDs) for motor circuits arent sized to protect the motor or wire from overcurrents because of in-rush currents. Thats the job of the overloads, either built into the motor or in the motor starter. OCPD only protect against short circuit and ground fault conditions in motor circuits...

 

[600SL]

Well now that u mention motors, does your motor have an overload(red button) built in?

(It would have been helpful to know this in the beggining because motor circuits are wired differently than regular branch circuits...)

Breakers(OCPDs) for motor circuits arent sized to protect the motor or wire from overcurrents because of in-rush currents. Thats the job of the overloads, either built into the motor or in the motor starter. OCPD only protect against short circuit and ground fault conditions in motor circuits...

My milling machine is a 3 phase motor 1HP with a VFD I have a separate circuit breaker on that at 15 amps.

My Lathe is a 1.5HP 1 phase motor with a thermal reset button.

That's all I have on 240V right now but I do have 3 machines with 3/4 HP currently wired at 120V and can be converted to 240V. Two of these have a thermal protection switch. My bench grinder I don't believe has a thermal protection switch.