power infrastructure 1

[Mefi01]

Hello,

I am designing the power infrastructure for a factory that has around 20 CNN, Welding , drilling machines etc ( with loads between 4A to 50A) and based on the loads I have, the max amperage demand is around 400A with a diversity of 50%. with this in mind, one can supply these through 250A distribution board. However, I am worried about the inrush current which can use the main 250A MCCB to trip. How can I know if the inrush current would cause me a problem here?

 

[itsmoked]

With the impossible dealings with 90% of power companies who will soak the customer after torturing them for possibly years to upgrade a service, I would make it abundantly clear to the customer that they should opt for a considerably larger service than they think they need now.

I would not consider anything less than 400A in your example. Places with the tool mix you're describing could easily double their load requirement if business changes or they switch to bigger, faster, more powerful machines. Retrofitting in the future would cost ten times more than the initial installation. In fact, that's the way to present this to the user, 'they can buy future power infrastructure for better than 10 cents on the dollar now'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Even if you have far better luck with the utility than Keith’s experiences you should still leave plenty room for growth. A future project that needs just a bit more capacity than you left will have people using your name in vain.

Back when I was doing building electrical system design I ran into lots and lots of places that hadn’t been left with enough room for growth. Only once did I ever a complaint about having built in too much capacity; and I had to agree. The owner’s rep I was working with at the time kept wanting more and more capacity. He had previously forced the cooling plant way beyond what the ME thought necessary. Eventually I figured out the electrical system as I had it was already capable of dumping more heat into the building than the cooling system could remove, never mind people or solar gain. At that point he quit pushing but wouldn’t go for a reduction either. So one that had too much capacity against lots found to have inadequate capacity. Err on the side too much spare capacity rather than the other way.

 

[itsmoked]

The place I just finished with added 9 CNC mills, a CNC lathe, and an 80kW coolant recycler to their existing stable of 16 CNC lathes, 10 surface grinders, and 12 CNC mills. We had to run conduit a loooooong way to support some of the new stuff. I kept thinking how nice it was that they'd divvyed up their 1600A 480V service into half a dozen panels around the facility. All the subpanels were supplied with conduit buried in the concrete. I can't imagine how much that would cost to upgrade...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

That brings up another good point. Conduit pipe is cheap compared to the cost of installing it. Think about a future load increase before installing the minimum required conduit size for the load expected today. Think even harder about it when burying conduit under a concrete floor.

 

[waross]

I identified with your suggestions to plan for increased load in the future.
The customer said that he would "never add more load", and "the checks in the mail" and "I'm from the government and I'm here to help you."
One of my best/worst was a series of cabins across a hillside at a diving resort.
30 Amps was more than adequate for each cabin. No appliances, just lights. A gas fired on-demand water heater in each cabin.
No, we will never ever use an electric on-demand water heater here.
Fast forward a year. The gas on-demand heater in the furthest cabin dies. (The manager's cabin) Business is down and money is short.
An electric heater is so much cheaper. The resort mechanic installed it.
I am trying to remind the manager of the promises that an electric heater would never be used.
I'm getting a "Deer in the headlights" look, and an attitude of "It worked for a year with no problem. Now the lights almost go out and the breaker trips and takes out three cabins when I use the new water heater. What did you do wrong?"

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

An option I have found useful in the water heater example is to wire a 240 volt heater to 120 volts. It goes from 4500 watts to 1500 watts. It just takes longer for the tank to recover, which will not be noticed if you do not run the tank out.