VFD Panel Breaker sizing & its feeder sizing

[kevlarster]

A 100hp, 40hp, & 10hp motor are controlled from a single panel with 3 VFDs in it. The VFDs are rated for 150% of the motor HP. Therefore the VFD ratings total 285 amps. (motors FLA x 1.5)

The 3 motors are intermittent duty and will never operate simultaneously.
The incoming breaker in the panel is rated at 400A, but is SET for 280A.

My question is; Do I have to feed this panel (via 3p Breaker in MCC) with a 400A, or can I use a 300A breaker? I've been told that it depends on whether or not the panel breaker's trip setting is readily accessible or not.

Any help is greatly appreciated.
Kevlar8)

 

[Zifkwong]

You are protecting the system with the 300 Amp circuit breaker. The 400 Amp circuit breaker which is set to 285 Amps is additional protection.

I do not recall of any standard mentioning any exceptions which have exceptions because the protection settings are accessible.

 

[iwire]

Take a look at 430.2

 

[kingpb]

A 100hp, 40hp, & 10hp motor are controlled from a single panel with 3 VFDs in it. The VFDs are rated for 150% of the motor HP. Therefore the VFD ratings total 285 amps. (motors FLA x 1.5)

The 3 motors are intermittent duty and will never operate simultaneously.
The incoming breaker in the panel is rated at 400A, but is SET for 280A.

My question is; Do I have to feed this panel (via 3p Breaker in MCC) with a 400A, or can I use a 300A breaker? I've been told that it depends on whether or not the panel breaker's trip setting is readily accessible or not.

Any help is greatly appreciated.
Kevlar8)

If the panel main is 400A, then use a 400A breaker as the feeder main, and run a set of 500KCMIL to the panel, and forget about it. Running less than the 400A to the panel is doing the Owner/Client a dis-service.

 

[petersonra]

If the panel main is 400A, then use a 400A breaker as the feeder main, and run a set of 500KCMIL to the panel, and forget about it. Running less than the 400A to the panel is doing the Owner/Client a dis-service.

How do you figure that reducing the owner's cost is necessarily doing him a disservice? The OP indicated that "The 3 motors are intermittent duty and will never operate simultaneously."

This would indicate to me that the maximum load the thing will ever see is from the 100HP motor.

I am a big fan of not painting one's self into a corner regarding future uses, but in this case, the cheap way seems like it might make sense.

added: Incidentally, (as Don indicated previously) 430.2 indicates you should size the conductors based on the rated input of the drives, and not 150% of the motor current. I think the maximum ampacity required is 125% of whatever the 100HP drive is rated for plus 100% of what the other two drives are rated for in accordance with 430.24.

 

[ramsy]

..can I use a 300A breaker? I've been told that it depends on whether or not the panel breaker's trip setting is readily accessible or not.

Don't use adj.trip breaker, if referring to NEC 240.6(B-C)

(B) Adjustable-Trip Circuit Breakers. 
The rating of adjustable-trip circuit breakers having external means 
for adjusting the current setting (long-time pickup setting), not meeting 
the requirements of 240.6(C), shall be the maximum setting possible.

(C) Restricted Access Adjustable-Trip Circuit Breakers. 
A circuit breaker(s) that has restricted access to the adjusting means 
shall be permitted to have an ampere rating(s) that is equal to the adjusted 
current setting (long-time pickup setting). Restricted access shall be 
defined as located behind one of the following:
(1) Removable and sealable covers over the adjusting means
(2) Bolted equipment enclosure doors
(3) Locked doors accessible only to qualified personnel

 

[kingpb]

How do you figure that reducing the owner's cost is necessarily doing him a disservice? The OP indicated that "The 3 motors are intermittent duty and will never operate simultaneously."

That's this week, and next week the 10Hp motor goes out and somebody decides he wants to change it to a 40Hp motor.

Then two weeks later, another wiz bang gets the big idea, that they can increase production by 0.001% by running the 100Hp and one 40Hp at the same time. Oops, can't do that, because it was decided to only install 300A worth of cable and breaker to a 400A panel. Everybody looks around, points the finger at the poor slob that made that decision, and in unison declare "you dumb-ass" (from the 70's show).

That's just one scenario, I'm sure there are others.

We work on large scale projects where if the panel is rated 4000A, we provide 4000A worth of feeder. If it's a 1200A MCC, we provide a 1200A feeder. The incremental cost on a $500M-$750M project when electrical portion is only $60-$90M is not even discussed. So, my perspective may be slanted.

Carry on.....

 

[petersonra]

I would not disagree that it is probably best to put in feeders for general use at their maximum sizes based on the switchgear being used, but for a specific piece of equipment it is a different thing.

Keep in mind everyone gets in the act when this line of logic gets used. Three inch water lines get bumped to four. Waste lines increase in size. The next size air compressor gets bought. The building gets bigger. Next thing you know you have added 20% to the cost of the project.

 

[iwire]

We work on large scale projects where if the panel is rated 4000A, we provide 4000A worth of feeder. If it's a 1200A MCC, we provide a 1200A feeder.

No.

You don't provide anything, you specify.

The customer provides the $.

I have sat in enough job meetings seeing the customers blood boil when they realize every part of the job was over done 'just in case.'

All I am suggesting is that to make a blanket statement that a 400 amp panel always gets a 400 amp feed without at least putting a little thought into it is unfair to the customer.

The person footing the bill should get to see alternate prices and options.

 

[kevlarster]

Thanks Bob, This is helpful. Kevlar

 

[kevlarster]

Thanks, This is very helpful...Kevlar

 

[kingpb]

No.

You don't provide anything, you specify.

The customer provides the $.

QUOTE]

Provide in the specifcation/consulting arena means "furnish and install", at least to our contracts/legal group. Yes, we do a lot of EPC projects, so we do "provide" equipment. In my sepcifcations, I am also going to tell the Contractor he shall "provide" the equipment, which in a court of law means furnish and install.

This does bring up an interesting point, in that it should be made clear to the Contractor, prior to bidding, that the feeders to panels shall be fully rated, if so desired, otherwise this would give the flexibility to provide the minimum needed for the load. I need to check our specs.

 

[iwire]

[Provide in the specifcation/consulting arena means "furnish and install", at least to our contracts/legal group. Yes, we do a lot of EPC projects, so we do "provide" equipment. In my sepcifcations, I am also going to tell the Contractor he shall "provide" the equipment, which in a court of law means furnish and install.

I am quite familiar with the terms.

You....your firm, the firm you work for, do not donate that equipment.

What I am pointing out is that you seem to have forgotten that you are spending some one else's money.

And if indeed your firm is actually purchasing and providing the equipment you stand to make more money by using more expensive equipment.

If you stick with your blanket statement that it is always better to provide larger feeders than to consider each case on it's own merits you are not IMO providing a good service to your clients.

Heck we don't even need engineers if we just oversize / over spec each and every item.

I should find my picture of a 225 amp 208Y/120 panel supplied with a 9 KVA transformer.

Not my install, but it's been working fine for at least 20 years and the customer has no interest in having us 'correct it'.

It is after all up to the person paying the bill.

 

[kingpb]

If you stick with your blanket statement that it is always better to provide larger feeders than to consider each case on it's own merits you are not IMO providing a good service to your clients.

I went back and read all my posts, and fail to see where I made that "blanket" statement. I believe what I said has been taken out of context. My reference was to the types of projects I'm involved with, which happen to be large power generating facilities. In this industry, it is considered unacceptable to run a feeder that is less then the equipment rating. The incremental cost reduction to the project would be considered insignificant in the overall scheme of things, and unacceptable to the O&M guys.

We are currently working on design of a $5Billion project where the electrical may be $600M. Sorry if I don't see the need to justify saving even $50,000 in cable, which would amount to 0.001% of the project cost. Now, not needing to have an additional 3200A low voltage swithgear line-up, would be worth, at least a short discussion.

 

[iwire]

I went back and read all my posts, and fail to see where I made that "blanket" statement. I believe what I said has been taken out of context.

Here is what I took to be a blanket statement

We work on large scale projects where if the panel is rated 4000A, we provide 4000A worth of feeder. If it's a 1200A MCC, we provide a 1200A feeder.

Is this regardless of the actual loads and conditions of the job?

We are currently working on design of a $5Billion project where the electrical may be $600M. Sorry if I don't see the need to justify saving even $50,000 in cable, which would amount to 0.001% of the project cost.

That is a big project....good for you, I am sure it is an interesting one.

What percentage of folks here on this board would you say work on projects even 1/1000th of that size?

Most electrical projects would jump at the chance to save $50 K and I bet you would too if it was your $50,000.00.

I am not trying to say your not entitled to your view.

What I will always point out is that I install equipment and work on equipment in the real world and I often see money wasted on over design.

On your type of project that may not be an issue.

I can however think of dozens of over blown services and feeders we have installed for more average projects.