215.2(A)(1)(a) and 240.4(B) Interaction

[wwhitney]

I have a question about the details of how the 125% factor for continuous loads is incorporated into the NEC. [This is my second attempt at asking, as I apparently buried the lead in my first attempt.]

215.2(A)(1)(a) calls for the conductors supplying a continuous load to have an ampacity increased beyond the actual load current by 25% of the continuous load current. [And 210.19(A)(1)(a) is similar.] The limitation is entirely due to the behavior of OCPD; since for continuous loads regular OCPD may trip at below the nameplate rating. So to avoid this nuisance (that could turn into a safety hazard if a user repeatedly resets a tripping breaker), the OCPD should be upsized by 25% of the continuous load current.

However, that is already covered in 215.3. And 240.4 already requires conductors to be protected in accordance with their ampacities, so when the OCPD is upsized, the conductors have to be upsized. That means the requirement of 215.2(A)(1)(a) is generally a logical consequence of 215.3 and 240.4, and at best redundant. But 215.2(A)(1)(a) also prevents the use of 240.4(B) [next size up rule] to comply with 215.3.

Real world example: an EVSE with a 48A output current (e.g. Tesla Wall Connector with the proper internal setting) is a continuous load, so the OCPD protecting it should be 60A. #6 NM cable has an ampacity of 55A because it is limited to the 60C rating. 240.4(B) allows protecting this with a 60A breaker [when 55A ampacity is otherwise sufficient per other sections of the NEC.]

So what is the hazard of supplying a 48A continuous load with cable with an ampacity of 55A that is judged sufficiently well protected by a 60A breaker when the load is non-continuous? Is this an intentional prohibition, or just a side effect of the redundancy of 215.2(A)(1)(a)?

Cheers, Wayne

 

[kwired]

I have a question about the details of how the 125% factor for continuous loads is incorporated into the NEC. [This is my second attempt at asking, as I apparently buried the lead in my first attempt.]

215.2(A)(1)(a) calls for the conductors supplying a continuous load to have an ampacity increased beyond the actual load current by 25% of the continuous load current. [And 210.19(A)(1)(a) is similar.] The limitation is entirely due to the behavior of OCPD; since for continuous loads regular OCPD may trip at below the nameplate rating. So to avoid this nuisance (that could turn into a safety hazard if a user repeatedly resets a tripping breaker), the OCPD should be upsized by 25% of the continuous load current.

However, that is already covered in 215.3. And 240.4 already requires conductors to be protected in accordance with their ampacities, so when the OCPD is upsized, the conductors have to be upsized. That means the requirement of 215.2(A)(1)(a) is generally a logical consequence of 215.3 and 240.4, and at best redundant. But 215.2(A)(1)(a) also prevents the use of 240.4(B) [next size up rule] to comply with 215.3.

Real world example: an EVSE with a 48A output current (e.g. Tesla Wall Connector with the proper internal setting) is a continuous load, so the OCPD protecting it should be 60A. #6 NM cable has an ampacity of 55A because it is limited to the 60C rating. 240.4(B) allows protecting this with a 60A breaker [when 55A ampacity is otherwise sufficient per other sections of the NEC.]

So what is the hazard of supplying a 48A continuous load with cable with an ampacity of 55A that is judged sufficiently well protected by a 60A breaker when the load is non-continuous? Is this an intentional prohibition, or just a side effect of the redundancy of 215.2(A)(1)(a)?

Cheers, Wayne

Closed the last thread before I could respond, there I didn't necessarily look back to the OP to see what was really being asked, but yes the title sort of led me to the reply I made.

So what is the hazard of supplying a 48A continuous load with cable with an ampacity of 55A that is judged sufficiently well protected by a 60A breaker when the load is non-continuous? Is this an intentional prohibition, or just a side effect of the redundancy of 215.2(A)(1)(a)?

I don't see that a 60 amp breaker would be prohibited here. Minimum ampacity for non continuous load is 48 amps. Minimum ocpd would be 48 >>> 50 would be next up standard size. Where are you getting any kind of maximum from? I think you could supply this with 100 amp conductor and 100 amp OCPD if you want unless otherwise prohibited by an applicable article in chapter 4-7.

 

[kwired]

Closed the last thread before I could respond, there I didn't necessarily look back to the OP to see what was really being asked, but yes the title sort of led me to the reply I made.



I don't see that a 60 amp breaker would be prohibited here. Minimum ampacity for non continuous load is 48 amps. Minimum ocpd would be 48 >>> 50 would be next up standard size. Where are you getting any kind of maximum from? I think you could supply this with 100 amp conductor and 100 amp OCPD if you want unless otherwise prohibited by an applicable article in chapter 4-7.

Add: or by the listing instructions for the equipment in question

 

[wwhitney]

For the discussion, from the 2017 NEC:

215.2(A)(1)(a) Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

215.3 Overcurrent Protection. Feeders shall be protected against overcurrent in accordance with the provisions of Part I of Article 240. Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).

240.4(B) Overcurrent Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

Cheers, Wayne

 

[wwhitney]

I don't see that a 60 amp breaker would be prohibited here.

48A continuous load means 215.3 requires a 60A breaker (unless 100% rated). 240.4 requires that a 60A breaker protect conductors with an ampacity of 60A or more, except 240.4(B) allows the ampacity to be only 50.1A (if sufficient for the load), since 50A is the next lower breaker. If that were all, then "48A continuous load / 55A ampacity / 60A breaker" (which would have been a catchier thread title) would be allowed. And as far as I can see, it will work well and poses no particular hazard.

However, 215.2(A)(1)(a) requires 60A ampacity. This is the requirement I'm questioning. As far as I can see, 215.3 + 240.4 is sufficient protection, and the extra requirement imposed by 215.2(A)(1)(a) is excessive.

Cheers, Wayne

 

[kwired]

Is this your main question here?

So what is the hazard of supplying a 48A continuous load with cable with an ampacity of 55A that is judged sufficiently well protected by a 60A breaker when the load is non-continuous? Is this an intentional prohibition, or just a side effect of the redundancy of 215.2(A)(1)(a)?

 

[augie47]

I have a problem with your assumption:" The limitation is entirely due to the behavior of OCPD; since for continuous loads regular OCPD may trip at below the nameplate rating"
Taken at it's face value 215.2 & 210.19 state the ampacity of the conductor must have 125% of the continuous rating.
IF you have a continuous load of 48 amps, a conductor with a 60 amp rating is required.
So it is writen, so shall it be

 

[wwhitney]

I have a problem with your assumption:" The limitation is entirely due to the behavior of OCPD; since for continuous loads regular OCPD may trip at below the nameplate rating"

That is the only reason a 125% continuous load factor exists in the NEC. Every section that mentions it has an exception for 100% rated OCPD. So it's just an OCPD limitation, conductor ampacity itself is a continuous rating (per the Article 100 definition).

Taken at it's face value 215.2 & 210.19 state the ampacity of the conductor must have 125% of the continuous rating.

Absolutely they do. The point of this thread is whether that's unnecessary and overly conservative. Whether it enough to require the OCPD to be sized for 125% of the continuous load, and then the conductor ampacity selected to comply with 240.4.

Cheers, Wayne

 

[kwired]

That is the only reason a 125% continuous load factor exists in the NEC. Every section that mentions it has an exception for 100% rated OCPD. So it's just an OCPD limitation, conductor ampacity itself is a continuous rating (per the Article 100 definition).


Absolutely they do. The point of this thread is whether that's unnecessary and overly conservative. Whether it enough to require the OCPD to be sized for 125% of the continuous load, and then the conductor ampacity selected to comply with 240.4.

Cheers, Wayne

So your question is why does standard overcurrent devices require 125% factor for continuous loads?

 

[wwhitney]

Is this your main question here?

Yes, those are also the only questions in the OP.

Cheers, Wayne

 

[wwhitney]

So your question is why does standard overcurrent devices require 125% factor for continuous loads?

No, not at all. Let me try again succinctly:

"48A continuous load / 55A ampacity / 60A breaker" complies with 215.3 and 240.4, but not with 215.2(A)(1)(a). Why is this prohibited? It looks to me as if it should be allowed, and 215.2(A)(1)(a) should be changed/deleted so that it is allowed.

Cheers, Wayne

 

[kwired]

Yes, those are also the only questions in the OP.

Cheers, Wayne

So what is the hazard of supplying a 48A continuous load with cable with an ampacity of 55A that is judged sufficiently well protected by a 60A breaker when the load is non-continuous? Is this an intentional prohibition, or just a side effect of the redundancy of 215.2(A)(1)(a)?

There is no hazard. The rules require the conductor to be able to carry the load imposed on it. The rules require overcurrent device to protect conductor at it's ampacity, but do allow next size up as long as the load doesn't exceed conductor ampacity, until you get past the 800 amp level.

You still have same sort of things going on if you had 48 amp non continuous load and 50 amp OCPD.

 

[augie47]

To me, 240.4 simply takes into account the standard size of breakers.
Having the conductior rated to handle the load seems reasonable and going to the next standard size does not compromise the safety.

 

[wwhitney]

OK, are you both saying that "48A continuous load / 55A ampacity / 60A breaker" is fine, and that therefore 215.2(A)(1)(a) should be modified to not prohibit it? If so, what do you think of just deleting that section and replacing it with a fine print note referring to 215.3 and 240.4?

Cheers, Wayne

 

[Dennis Alwon]

I think the issue become T.310.15(B)(16). Once you allow what you are saying then the entire table because a mess. 240.4(B) already causes some issues for inspectors etc and perhaps that should be deleted but IMO the NFPA wants to keep the conductor size in line with the overcurrent protective device , at least for the most part...LOL

 

[wwhitney]

I think the issue become T.310.15(B)(16). Once you allow what you are saying then the entire table because a mess.

Sorry, I don't see the problem?

Cheers, Wayne

 

[Dennis Alwon]

What you are saying is that if I have a 100 amp load that is continuous I would need 125 amp overcurrent protective device but the conductor could be left at 100 amps.

Ex. # 3 on a 125 amp breaker @75C... Is this what you are saying?

How does that not change the ampacity ratings in T 310.16? It makes the table have a lot more discrepancies and inconsistencies. I think this is something the NEC does not want.

I understand your point that a #3 has no issues with 100 amps continuously- although I am not sre that is true but you don't have overload rotection as you do in motors to deal with this issue.

 

[wwhitney]

What you are saying is that if I have a 100 amp load that is continuous I would need 125 amp overcurrent protective device but the conductor could be left at 100 amps.

No, you'd still have to comply with 240.4.

For your example of a 100A continuous load on a 125A breaker, the standard breaker sizes are 100A, 110A, and then 125A. So what I'm saying is that it should be OK to use any conductor with an ampacity of over 110A; that would comply with 240.4(B). For 75C, you could use #2 Cu (115A) instead of going to #1 Cu (130A). Or #1/0 Al (120A) instead of #2/0 Al (135A).

A pretty small change.

I understand your point that a #3 has no issues with 100 amps continuously- although I am not sre that is true

Since #3 Cu (100A @ 75C) is allowed with a 100A continuous load on a 100% rated breaker (if they exist that small), then #3 Cu better be OK with 100A continuous. The Article 100 definition tells us the ampacity is a continuous rating.

Cheers, Wayne

 

[kwired]

with 100 amp continuous load minimum conductor ampacity must be 125%. #3 copper is only 100 amps, #2 copper is only 115 amps - you must use at least 1 AWG copper.

Overcurrent protection must be at least 125 amps. #1 copper ampacity of 130 - could actually put on a 150 amp breaker and is still in compliance with the rules here.

 

[kwired]

OK, are you both saying that "48A continuous load / 55A ampacity / 60A breaker" is fine, and that therefore 215.2(A)(1)(a) should be modified to not prohibit it? If so, what do you think of just deleting that section and replacing it with a fine print note referring to 215.3 and 240.4?

Cheers, Wayne

Ok I do see there may be some redundancy in 215.2 and in 215.3. Didn't study it hard enough to further comment though.

those sections are mostly about minimum conductor ampacity

240.4 is in overcurrent protection section and still needs to remain there.