Thoughts on load.

[GBDamo]

I'm in the process of rewiring a large portion of a commercial premises. I know this business very well as have attended many times for maintenance/repairs.
Although the works are significant there was no new load being added.

I also know the sector so know how equipment is used, what is on, for how long and in conjunction with what else.

The site is TP&N TN-S its only one phase in use on a 100A fuse and a single phase meter.

An OSG load assessment would give a figure in the order of 260A.

My, better informed, assessment comes in at 140A peak and very intermittent load.

There are no visual signs of overload.

There is a lot of cycling equipment with high inrush and some constant loads that keep a realistic baseline figure around 60A (electric ovens, immersion, and heating)

Enough background?

Came in today to three new 10A AC/Heater units, two to replace two 5A units and one new unit, 20A of additional load.

The customer will pay for needed works but I don't want to rip them off.

My advice to them will be that although overloaded on paper the installation functions at the current load level so not overloaded in reality but is perilously close.

Any additional load is not encouraged but if you must then we need to get the other two phases on line and split the load up.

This will mean a new meter, isolator and TP board.

Does this seem a reasonable approach?

 

[Flanders]

We had a site where the customer wanted air conditioning to a site . After doing some calcs .there was a good chance of overloading the single phase 100 amp supply .customer did not want to upgrade supply and asked if we could just add them to his existing supply . Ok so we put it in writing that they would be responsible for any over loading of the supply. Now the ball was in their court they decided to upgrade the supply . They did save s bit of money on the upgrade as they are replacing the 100 amp single phase supply to 3 phase at 60 amps per phase. As opposed to the normal 100 Amps per phase

 

[James]

Ideally you would put a 3 phase board in.
If you want to keep it as existing, a single phase out from there to feed existing and a second single phase out to a new board.

If it is in the budget, moving all existing into a 3 phase board would be better as you can balance the phases better.

 

[DefyG]

You could run the heaters through current sensing relays so if total load approaches 100A they switch off!

 

[GBDamo]

Ideally you would put a 3 phase board in.
If you want to keep it as existing, a single phase out from there to feed existing and a second single phase out to a new board.

If it is in the budget, moving all existing into a 3 phase board would be better as you can balance the phases better.

My thoughts exactly, I've already split the top two floors onto their own boards so it wouldn't be too much of a ballache.

 

[GBDamo]

You could run the heaters through current sensing relays so if total load approaches 100A they switch off!

Which as the high current is likely when heating is needed they would be off when most needed, think of the savings though..

 

[UNG]

While you quote the existing load and the installation shows no signs of overload you don't say how long the installation has been operating at this load
Have you done any load analysis or monitoring while the plant is operating, I'm just curious as normally overloading normally causes voltage issues and I'm wondering how often the plant hits peak load and for how long

When a 3 phase supply is available it would make sense to use it, I'm guessing it is an expanding business so changing to a 3 phase supply will have to happen at some point so why not now.
While continuing to operate with the single phase supply consideration has to be given to how much disruption it would cause to the business if a failure occurred in the local distribution equipment because of continued operation above it's design load