Generator load shedding

[brian ploskunak]

I have gas station that we are installing a permanent generator to run the whole store but not the car wash. What is the typical method for shedding loads? Shunt trip breakers or full voltage contactors?

 

[don_resqcapt19]

Brian,
Is there a single panel that serves all of the loads?
Don

 

[brian ploskunak]

There is a 3R 600 A I-line MDP which is a MLO. The MDP is back fed with a 600 A breaker for utility power and a 600 A breaker for generator power. Both of these breakers have factory installed key interlocks to prevent simultaneos operation. There are also (5) sub-feed breakers in the MDP for (5) panels and (1) of them is for the car wash. There is no ATS in this installation.

 

[steve66]

If the owner has to switch breakers to connect the generator, I would just let him/her shut off the breaker to the car wash.

Steve

 

[don_resqcapt19]

I agree with Steve. Just install an additional key interlock on the breaker that serves the car wash. Make sure that you check with the AHJ as to the use of key interlocks in place of a transfer switch. It should be ok, but some may have objections to it.
702.6
Don

 

[brian ploskunak]

The AHJ has no problem with the key interlocks which have become very popular here in South Florida. They are, however , been having a problem with a generator not big enough to include all the loads and that is why I was looking to shed the car wash load.

 

[kingpb]

If you are installing the generator new, why not size it for all the loads, and forget the load shedding?

If the Owner is willing to pay for a generator, I would convince him to install an ATS switch and set him up to do an auto transfer on power loss, and then return to normal service, without having to monkey around with making manual changes.

Do it right, do it once, and forget it.

 

[brian ploskunak]

The job is almost complete and the owner does not want a generator big enough for the car wash. They have not been putting in an ATS at gas stations because if they are hit by a hurricane and there is damage to the site and the dispensers, they do not want the generator coming on with no one there and causing another hazard.

Any thoughts on the best way to shed the car wash load?

 

[kingpb]

Thanks for the insight, interesting philosophy.

Sounds like Steve and Don had the best solution.

Good luck.

 

[sandsnow]

I agree with Steve. Just install an additional key interlock on the breaker that serves the car wash. Make sure that you check with the AHJ as to the use of key interlocks in place of a transfer switch. It should be ok, but some may have objections to it.
702.6
Don

Key interlocks or "kirk keys" (instead of a XFR sw) have to be bolted to the C/B itself with some kind of tamperproof fastener.
Not in the NEC verbatum, but our interpretation using 702.4 and 702.6

 

[iwire]

If the Owner is willing to pay for a generator, I would convince him to install an ATS switch and set him up to do an auto transfer on power loss, and then return to normal service, without having to monkey around with making manual changes.

Do it right, do it once, and forget it.

Sure whats a few more dollars.

At 600 amps that will be a major increase in price, I bet that the customer will not be as willing to part with their money as you are to spend it.

As far the opening question the cheapest automatic way out IMO would be a shunt trip kit for the car wash breaker.

How about just a phenolic label that says "Shut off the car wash breaker before starting generator."

 

[brian ploskunak]

I'll probably go with a shut trip if the inspector doesn't go for the "LABEL". Thanks for all the input!

 

[engy]

I'll probably go with a shut trip if the inspector doesn't go for the "LABEL". Thanks for all the input!

Is the shunt trip something the inspector can require? (I don't think so)
The generator is obviously not required for life safety.
Wouldn't it be purely a design decision how to drop the car wash load?

 

[iwire]

Is the shunt trip something the inspector can require? (I don't think so)
The generator is obviously not required for life safety.
Wouldn't it be purely a design decision how to drop the car wash load?

I was thinking all the same things.

702.5 Capacity and Rating.
An optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. Optional standby system equipment shall be suitable for the maximum available fault current at its terminals. The user of the optional standby system shall be permitted to select the load connected to the system.

Considering it is a manually connected / started generator I can't see how the inspector can ask for any load shedding device.

Today the owner wants to run the gas station without the car wash another day it may be the car wash without the gas station.

IMO the NEC very clearly gives the user the right to pick and chose loads at any time not just at the time of the installation.

 

[brian ploskunak]

I can't agree with you more. Unfortunately, some inspectors and plan review guys down here don't interpret it that way. They interpret it as the user can select which loads can be in a seperate generator panel. Hopefully they can clarify 702.5 better in 2008.

 

[benaround]

How would you wire the shunt trip for the carwash when power is from

same bus bars?

 

[iwire]

How would you wire the shunt trip for the car wash when power is from

same bus bars?

You could tap the generators line side.

Once the generator fires up it will pop the shunt on the car wash breaker.

Under 'normal' operating conditions the conductors between the generator and it's main breaker in the panel will be dead.

 

[benaround]

Bob,

You are quick,I was just coming back to edit my post. Tap the line side

of gen. feed was too easy. Hope they don't test gen. to often,with people in the car wash.

Thanks,

 

[motorcontrol]

If the owner is looking to install generator, why not consider parrell operation.
Inital investment is little more, but funding opportunities may offset the cost.
Funding Opportunity may vary from state to state but there are lots of funding opportunity from state to city level.
Our facility had installed 2 150kw induction generators and our are savings include not only kwh but demand charge (big chunck of change) and all the heat we recover is free (1.7mbh).
Our site is on the web. check it out: http://chp.nyserda.org
go to facility and select Arrow Linen
I think it is worth looking into....

DG CHP: Distributed Generation Combined Heat & Power.
Good Luck

 

[don_resqcapt19]

motorcontrol,
[FONT=&quot]You can't use induction generators for standby power and the controls to parallel an induction generator with the utility are much cheaper than those required for a synchronous generator.
Don
[/FONT]