Coordination Conflict: Adjusted Conductor Ampacity vs. OCPD Sizing for High-Fill Raceway

A

AkulaEE

Member
Location
New York
Occupation
Electrical Engineer
Hey everyone,

I'm looking for some consensus on a coordination issue regarding NEC 240.4 and conductor derating in a high-fill application.

I want to be clear, I do have an assumption and I think the correct path, but want to not say it to prevent any biases in the responses.

The Setup:

  • Load: 15A Constant Load.
  • OCPD: 30A 2-pole breaker.
  • Conductors: 26 Current-Carrying Conductors (CCC) in a single raceway.
  • Wire Type: XHHW (using 75°C terminations/limitations).
The Calculation:

We are applying a diversity derating process that results in a 70% adjustment factor. Starting with a conductor base ampacity of 35A:

35A times 0.70 = 24.5A(Adjusted Ampacity)
The Dilemma:

The adjusted ampacity (24.5A) is sufficient to carry the 15A constant load (meeting the 125% requirement of 18.75A). However, the 30A breaker now exceeds the calculated allowable ampacity of the conductor.

The Questions:

  1. Does 240.4(B) (the "Next Standard Size" rule) allow the use of a 30A OCPD here since 24.5A is not a standard rating, or does the fact that the OCPD is significantly higher than the adjusted ampacity create a violation?
  2. In an industrial environment under engineering supervision, would you downsize the breaker to 25A to stay closer to the wire’s adjusted "worth," or is the 30A OCPD considered compliant as long as the load doesn't exceed 24.5A?
  3. Are there specific concerns regarding the "Small Conductor" rules in 240.4(D) if this happens to be 10 AWG?
I’d appreciate your take on how you’d handle this during a plan review or site inspection.

Thanks,

AkulaEE

Electrical Engineer
 
A number of questions.
!) what permits you to protect a 15 amp load with a 30 amp OCPD?
2) is the location of the conductors a wet location?
3) what is the nature of the load as related to the restrictions in 240.4(B)?

25 amp is a standard OCPD in Table 240.6(A).
 
Is the conductor only XHHW or is it XHHW-2? As Don asked is this wet or dry location? XHHW is a 90°C conductor in a dry location.
 
don_resqcapt19 said:
A number of questions.
!) what permits you to protect a 15 amp load with a 30 amp OCPD?
2) is the location of the conductors a wet location?
3) what is the nature of the load as related to the restrictions in 240.4(B)?

25 amp is a standard OCPD in Table 240.6(A).
Click to expand...
Don, thanks for the reply.

Responses below:
1) The load in question has larger starting amps. And has been requested by the system manufacturer
2) The conductors will be in a dry environment.
3) It is a fixed electric heating load

For the OCPD I want to give as much head room as possible as well. since the wire is derated to 28A, I think the next size up will work on this. I could be wrong, and wanted to see what people here would think.
 
AkulaEE said:
Don, thanks for the reply.

Responses below:
1) The load in question has larger starting amps. And has been requested by the system manufacturer
2) The conductors will be in a dry environment.
3) It is a fixed electric heating load

For the OCPD I want to give as much head room as possible as well. since the wire is derated to 28A, I think the next size up will work on this. I could be wrong, and wanted to see what people here would think.
Click to expand...
Some what surprising that that a heating load has a large starting current.
XHHW in a dry location is a 40 amp conductor, so the adjusted current at 70% is 28 amps.
The next size up always works as long as the adjusted and or corrected ampacity of the conductor equals or exceeds the load, as long as the circuit does not supply multiple receptacles, and as long as the next higher standard size does not exceed 800 amps.
 
AkulaEE said:
1) The load in question has larger starting amps. And has been requested by the system manufacturer

3) It is a fixed electric heating load
Click to expand...
Is it JUST heaters or is it heaters and a motor? Heaters don't have inrush current on startup (technically they do but it is marginal at best so it is ignored). What they do have is increased current draw when they increase in temperature (due to the nature of their operation). As their temperature increases, their resistance decreases, which increases their current draw.

That's probably the reason for the 30A OCPD if it is purely a heater load. They sized it for the current draw of the heaters at max temperature.
 
AkulaEE said:
Starting with a conductor base ampacity of 35A:
Click to expand...
That would be fairly unusual. Just a note, the ampacity is the lesser of (a) the table value at the termination temperature rating, no derating applied and (b) the table value at the cable inulation temperature rating, with derating applied.

So #10 Cu with 90C insulation and 75C terminations and 70% derating would have an ampacity of the lesser of (a) 35A and (b) 40A * 70% = 28A.

AkulaEE said:
Does 240.4(B) (the "Next Standard Size" rule) allow the use of a 30A OCPD here since 24.5A is not a standard rating, or does the fact that the OCPD is significantly higher than the adjusted ampacity create a violation?
Click to expand...
No, because 25A is a standard rating, so 240.4(B) only permits using 25A. But if the ampacity is actually 28A as above, then 240.4(B) would permit 30A if the circuit meets the restrictions listed there.

AkulaEE said:
Are there specific concerns regarding the "Small Conductor" rules in 240.4(D) if this happens to be 10 AWG?
Click to expand...
No, 240.4(D) limits the OCPD to 30A for #10 Cu absent one of the exclusions, but that requirement is satisfied for all the cases under discussion.

Cheers, Wayne
 
AkulaEE said:
.

The Setup:

  • Load: 15A Constant Load.
  • OCPD: 30A 2-pole breaker.
  • Conductors: 26 Current-Carrying Conductors (CCC) in a single raceway.
  • Wire Type: XHHW (using 75°C terminations/limitations).
Click to expand...
26 ccc is at 45.% not 70. (21-30)

#10 xhhw-2 cu @ 90c =40
40 x .45 =18 amps
15*1.25 = 18.75 (continues load)
I missed the number 10 so I will read again in case of change in ccc.
 
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