Voltage drop cable increase then so is breaker

[hhsting]

I have loads of 133A so the cable sized #1/0 awg but due to voltage drop it is now up size to #500kcmil. So 500kcmil is provided.

Engineer is now telling me based on NEC 2017 section 240.4 less than 800A the breaker size should be 400A Not 150A.

I dont see anything in NEC 2017 that says breaker has to be 150A? I feel something is wrong but cant enforce 150A breaker

 

[ptonsparky]

Guessing you have continuous load. 133*1.25=166. That would mean a 200 Amp min CB to me.
Even if a 150 is adequate, it's not required when you have that size of wire. Issue being the transition of 1/0 to 500s. Those wires need to be protected at their ampacity.
IDK how the engineer came up with requiring a 400 for that load either.

 

[infinity]

With 500 kcmil the OCPD could be 400 amps but it doesn't have to be. What is the load?

 

[romex jockey]

Engineer is now telling me based on NEC 2017 section 240.4

which part of 240.4?

does your install meet 1,2,3 of 240.4 (B) ?

~RJ~

 

[jim dungar]

Why are you redesigning an engineered set of plans?

The NEC does not care about protecting a load, which is why it uses words like 'at least'. It is concerned with protecting conductors when it uses words like 'maximum'.

In your case the 400A breaker is more than the load and the conductors are protected, so the NEC is met.

 

[infinity]

In your case the 400A breaker is more than the load and the conductors are protected, so the NEC is met

Aren't there certain loads that have a maximum OCPD size? I'm thinking some loads limit the protection to 150%. Certainly a 133 amp load with 400 amp protection would exceed that.

 

[jim dungar]

Aren't there certain loads that have a maximum OCPD size? I'm thinking some loads limit the protection to 150%. Certainly a 133 amp load with 400 amp protection would exceed that.

Yes, branch breakers may have maximum sizing requirements, but not feeders. But this important information was not provided by the OP, so I provided a general answer.

 

[infinity]

Yes, branch breakers may have maximum sizing requirements, but not feeders. But this important information was not provided by the OP, so I provided a general answer.

I agree that's why I asked what the load is.

 

[hhsting]

With 500 kcmil the OCPD could be 400 amps but it doesn't have to be. What is the load?

The 500kcmil feeds panelboard with demand load of 133A.

Do u mean what the panelboard feeds? The panelboard itself feeds one other panel board, three booster pumps, supply fan in garage, sewage ejector pump, garage door, loading door, garage sump pump

 

[infinity]

The 500kcmil feeds panelboard with demand load of 133A.

Do u mean what the panelboard feeds? The panelboard itself feeds one other panel board, three booster pumps, supply fan in garage, sewage ejector pump, garage door, loading door, garage sump pump

Yes your OP simply stated that the load was 133 amps and not what it is. The 400 amp OCPD may or may not be properly sized based on the load (see post #6). Now that you've told us that it's a feeder for a panel as long as the panel is rated for 400 amp the engineer is correct. Per the calculated load you could use a 150 amp OCPD in the panel.

 

[jim dungar]

Per the calculated load you could use a 150 amp OCPD in the panel.

But you do not have to.

 

[kwired]

It can also be good design to protect OP's feeder with 150 amp OCPD from the perspective that if you intentionally wanted to achieve a certain amount of voltage drop, then installing a 400 amp OCPD will allow someone to add more load and increase voltage drop without the consequence of tripping the feeder OCPD.

 

[jim dungar]

It can also be good design to protect OP's feeder with 150 amp OCPD from the perspective that if you intentionally wanted to achieve a certain amount of voltage drop, then installing a 400 amp OCPD will allow someone to add more load and increase voltage drop without the consequence of tripping the feeder OCPD.

But that is beyond the NEC and is not in the purview of a plan review.

 

[kwired]

But that is beyond the NEC and is not in the purview of a plan review.

Yes, but might be good design in some instances. After all addressing voltage drop isn't exactly a general code requirement either but rather a suggestion in an informational note.

 

[wwhitney]

But that is beyond the NEC and is not in the purview of a plan review.

Some energy codes may put it under the purview of the plan review? Not sure about MD.

Cheers, Wayne

 

[augie47]

sting: Note the nature of the load was to a great degree the key element in your answer.
The prime purpose of your breaker is to provide protection for the conductor based in it's ampacity so your 500kcmil can be protected by a 400 amp breaker. At times, the breaker may also provide protection for the connected load as specified by the manufacturer or Code Section but that would be rare for a feeder. Once you let us know it was a feeder, then conductor protection became the prime duty of the breaker. As noted, concern beyond that is a design issue.

 

[infinity]

sting: Note the nature of the load was to a great degree the key element in your answer.

Also note that if it was mentioned in the OP then we wouldn't be a bunch of dogs chasing their tails. Note to OP try to provide as much information as possible.

 

[electrofelon]

Sometimes if you have much larger conductors for VD, it can be prudent to go up to that frame or equipment size for ease of terminations. I actually just did that where I needed a 250A feeder, but needed 2 sets of 205 amp conductors (250 AL) For VD. A 400A frame device had the lugs I needed while the 250 did not, and the cost increase was minimal. Also this get you around 250.122(B). OP didn't say the engineer said anything about this, bit it is possible we don't have the entire story.

 

[Seven-Delta-FortyOne]

If I’m reading this correctly, the engineer is mistaken. It’s a common error.

Conductors may be protected below their ampacity. Its done all the time for VD.

I can put #10 on a 15a breaker all day if I want.

You can put 500kcm on a 150 all day, if you wish too.

Unless there’s something I’m missing from your first post.

 

[Carultch]

I have loads of 133A so the cable sized #1/0 awg but due to voltage drop it is now up size to #500kcmil. So 500kcmil is provided.

Engineer is now telling me based on NEC 2017 section 240.4 less than 800A the breaker size should be 400A Not 150A.

I dont see anything in NEC 2017 that says breaker has to be 150A? I feel something is wrong but cant enforce 150A breaker

You do not necessarily *need* to increase the OCPD, due to voltage drop governing a change in size of the conductor. If anything, there may be reasons specific to the load that limit you to a maximum OCPD, which would prevent you from doing this. Or at least, insert a second OCPD close to the load in order to comply with the needs of the load. Some loads specify a maximum OCPD rating. You should check with the manufacturer or the documentation of a particular load, to see if you are allowed to increase the OCPD trip rating to far higher than the NEC's default size.

That being said, there are reasons you might want to increase the OCPD, such as having more capacity in the equipment to terminate your wires. I can't directly terminate a 500 kcmil wire in a standard 150A breaker. Not without a splice reducer to locally step it down to a size such as #1/0. And there is a good chance I would also need a junction box, to get more physical space to maneuver this giant wire and accomplish the splice/reduction. However, I can terminate 500 kcmil wire directly in a 400A frame size breaker, that has an adjustable trip setting or replaceable trip plug, to make its trip rating 150A. Or a 400A fused disconnect, with fuse reducers that enable me to insert 150A fuses.

Another abstract reason why one might want to increase the OCPD rating when curtailing voltage drop, is that it gets you around upsizing the EGC as required in 250.122(B). In your example, if I use a 150A trip rating, and I'm upsizing phase conductors from #1/0Cu to 500 kcmil Cu, then that is an upsize ratio of 4.716. The default #6 Cu ground wire, ends up getting upsized from 26.3 kcmil to 124 kcmil, which rounds up to #2/0 Cu. However, if I were permitted to use a 400A OCPD, then 500 kcmil Cu would qualify as "the minimum size with sufficient ampacity for the intended installation", and therefore require no upsizing at all of the #3Cu EGC.