Electric boiler

[ErikG]

I have an electric boiler, 480v 3 phase with a nameplate of 441A. For my feeder size to the unit I took 441x1.25% due to continuous duty. This equals 551a minimum conductor ampacity which is paralleled 300’s. (551/2=276a and 300 is good for 285a in the 75c column)

I have a 600a breaker available, an I able to use this or do I need to drop down to a 500 since that would be the next size up from the nameplate of 441A?

There is no mocpd listed on the unit.

 

[augie47]

You should be fine under the next size up rule on your conductors

 

[infinity]

If your minimum circuit size is 551 amps I don't see how you can use a 500 amp OCPD.

 

[ErikG]

If your minimum circuit size is 551 amps I don't see how you can use a 500 amp OCPD.

Minimum conductor ampacity would be 551a (441 nameplate amps x 125%= 551a). The boiler only draws 441a and therefore a 500a breaker is the next size up. Since I already have a 600a breaker though I would like to use it if acceptable.

 

[augie47]

According to Art 425 Part VII your boiler should have supplemental overcurrent protection so your branch circuit supplying the whole unit should quualify under 240.4(B) and the 600 amp breaker is allowed on conductors rated 570 amps.

 

[infinity]

Minimum conductor ampacity would be 551a (441 nameplate amps x 125%= 551a). The boiler only draws 441a and therefore a 500a breaker is the next size up.

What code section would allow the branch circuit OCPD for a continuous load to be smaller than the minimum conductor size?

 

[ErikG]

What code section would allow the branch circuit OCPD for a continuous load to be smaller than the minimum conductor size?

The load will only be 441A. The increase of 125% on the conductor size is because having a continuous load of 441A over time will increase heating on the conductors, and increasing the conductor size will help reduce heating. The ocpd is sized based on the actual load That it is protecting.

 

[infinity]

The load will only be 441A. The increase of 125% on the conductor size is because having a continuous load of 441A over time will increase heating on the conductors, and increasing the conductor size will help reduce heating. The ocpd is sized based on the actual load That it is protecting.

I understand your logic but I'm trying to think of a code section that would allow the smaller OCPD. Do you have one in mind?

 

[ErikG]

I understand your logic but I'm trying to think of a code section that would allow the smaller OCPD. Do you have one in mind?

The ocpd has to be sized to adequately protect your conductors…. You could put a 500kcmil on a 15a ocpd and there would be no harm or code violation.

 

[infinity]

The ocpd has to be sized to adequately protect your conductors…. You could put a 500kcmil on a 15a ocpd and there would be no harm or code violation.

That's not the question. The question is since this is a continuous load can the OCPD be smaller than the calculation provided by (nameplate load * 125% = 551 amps).

 

[augie47]

That's not the question. The question is since this is a continuous load can the OCPD be smaller than the calculation provided by (nameplate load * 125% = 551 amps).

Interesting !
I don't see anything in Art 240 that would make it a violation of that Section.. The breaker is providing protection for the conductors per 240.4.
That said, 424.3 says the branch circuit shall be sized as a continuous load and the branch circuit size is determined by the over-current device so that makes me think it has to be sized at at least 125%

 

[infinity]

That said, 424.3 says the branch circuit shall be sized as a continuous load and the branch circuit size is determined by the over-current device so that makes me think it has to be sized at at least 125%

I agree. Thanks for the code section. I was going by memory that there was a section that would not allow the mentioned 500 amp OCPD for this circuit.

 

[ErikG]

Awesome, I appreciate all of the input. I will use the 600A breaker that I have.