tapping separate switchboard sections to feed a single load

[Tainted]

I have a main 208V 3-phase, 3000 amp switchboard with sections V, W, X, Y, and Z

I need to tap the switchboard to feed 3-phase chiller loads with a total of 2100MCA that is more than 25 feet away from the tap. This means that I need 8 sets of 3#500kcmils (full size 3000A copper feeder to comply with the tap rule). I will use this feeder to feed a new 3000A switchboard which will feed the chillers.

8 taps for phase A will be at section V
8 taps for phase B will be at section W
8 taps for phase C will be at section X

section Y (a 3000A switch) feeds sections V,W, and X

As you can see, I am proposing to tap at different switchboard sections to feed the new 3000A switchboard.
Is this code compliant or must the taps be from the same switchboard section?

 

[infinity]

So your conductors are 3000 amps and they're after the 3000 amp OCPD. That means that you do not have a tap and no tap rules apply which seems to be your goal since your conductor length is over 25'. So the question is can you connect each set of phase conductors in different sections of the switchboard?

 

[Tainted]

So your conductors are 3000 amps and they're after the 3000 amp OCPD? That means that you do not have a tap and no tap rules apply which seems to be your goal since you conductor length is over 25'. So the question is can you connect each set of phase conductors in different sections of the switchboard?

Correct, this is exactly the scenario. I'd like to tap each different section for each different phase

 

[Tainted]

Would also like to know another scenario,

what if it's:

(3) sets of 4#500kcmils at section V
(3) sets of 4#500kcmils at section W
(2) sets of 4#500kcmils at section X

 

[augie47]

You have a 3000 amp multi-section switchboard that has 2100 amps of available power ??

 

[Tainted]

You have a 3000 amp multi-section switchboard that has 2100 amps of available power ??

The total MCA of (16) chiller loads is 2100, however the total load required is 372.8kW. Realistically I think the real load is about 1035 amps instead of the full 2100 amps. MCA is to just size the wires. Not sure why the manufacturer don't give me kVA instead of KW though.

I want to take power from a existing 3000A multi-section switchboard and feed a new 3000A switchboard to feed this load.

 

[jim dungar]

Do you have room to add a fifth section containing lugs for all of the conductors?

 

[Tainted]

Do you have room to add a fifth section containing lugs for all of the conductors?

nope, absolutely no room in the electrical room

 

[wwhitney]

The total MCA of (16) chiller loads is 2100

Did you just add the MCAs of the 16 chiller loads? That will result in an extra 25% allowance for the largest motor in each of the 16 chillers, while your required feeder ampacity only need to include the extra 25% for the largest motor in the entire group of 16 chillers.

Cheers, Wayne

 

[Tainted]

Did you just add the MCAs of the 16 chiller loads? That will result in an extra 25% allowance for the largest motor in each of the 16 chillers, while your required feeder ampacity only need to include the extra 25% for the largest motor in the entire group of 16 chillers.

Cheers, Wayne

1050MCA for 8 chillers, so total is 2100.

Why do I have to do an extra 25%? I thought the MCA includes every safety factor and the required 125%. MCA value is from manufacturer

Are you saying I should size the circuit 125% of 2100?

 

[Tainted]

So your conductors are 3000 amps and they're after the 3000 amp OCPD. That means that you do not have a tap and no tap rules apply which seems to be your goal since your conductor length is over 25'. So the question is can you connect each set of phase conductors in different sections of the switchboard?

Let me put it this way:

Let's say I have (3) 3000A switchboard sections.

I tap (3) sections of the 3000A switchboards to feed a new 3000A switchboard.
Must the tap originate from a single section or is this ok?

 

[wwhitney]

1050MCA for 8 chillers, so total is 2100.

There's a product consisting of 8 chillers that comes with a single nameplate that lists a 1050 MCA? Or the 8 chillers each have individual nameplates?

What I'm saying is this: MCA is the ampacity you need for the branch circuit for a single unit. The manufacturer computes the MCA as basically 125% of the unit's largest motor plus 100% of everything else.

When you have a feeder for multiple units, you only need to use the 125% factor on the largest motor among all the units. So if you just add up 16 MCA numbers, the result includes an extra 25% of 15 motors that you don't need to include. I.e. 2100 may higher than you need.

An extreme example of the difference, using made up numbers, would be 16 units each with a 131 MCA, and that 131 MCA was computed as 125% * (100A compressor RLA) + 100% (6A fan FLA). In that case the MCA for the feeder for 16 units would be 125% * 100 + 15 * 100 + 16 * 6 = 1,721A. As opposed to the 2,096A figure you'd get by just taking 131 MCA * 16 units.

Now if these chillers are inverter-driven internally, then that generally changes the MCA math so that the largest motor is a small component, and the extra 25% hardly matters. I know very little about equipment of this size, so I don't know if that's very likely for your equipment.

Cheers, Wayne

 

[Tainted]

There's a product consisting of 8 chillers that comes with a single nameplate that lists a 1050 MCA? Or the 8 chillers each have individual nameplates?

What I'm saying is this: MCA is the ampacity you need for the branch circuit for a single unit. The manufacturer computes the MCA as basically 125% of the unit's largest motor plus 100% of everything else.

When you have a feeder for multiple units, you only need to use the 125% factor on the largest motor among all the units. So if you just add up 16 MCA numbers, the result includes an extra 25% of 15 motors that you don't need to include. I.e. 2100 may higher than you need.

An extreme example of the difference, using made up numbers, would be 16 units each with a 131 MCA, and that 131 MCA was computed as 125% * (100A compressor RLA) + 100% (6A fan FLA). In that case the MCA for the feeder for 16 units would be 125% * 100 + 15 * 100 + 16 * 6 = 1,721A. As opposed to the 2,096A figure you'd get by just taking 131 MCA * 16 units.

Now if these chillers are inverter-driven internally, then that generally changes the MCA math so that the largest motor is a small component, and the extra 25% hardly matters. I know very little about equipment of this size, so I don't know if that's very likely for your equipment.

Cheers, Wayne

I see what you mean, the manufacturer specifically gave me 1050MCA for (8) and I just doubled it to get 2100 for estimating purposes. I would still need a 3000A switchboard regardless because the tap length is going to be more than 25 feet.

still a little bit off topic here but I appreciate your input.

 

[wwhitney]

I see what you mean, the manufacturer specifically gave me 1050MCA for (8) and I just doubled it to get 2100 for estimating purposes. I would still need a 3000A switchboard regardless because the tap length is going to be more than 25 feet.

Do you have a manufacturer and model number for the chiller?

I have no sense of the physical scale of equipment at these current levels, but: if the feeder MCA comes out to under 2000A, your conductors coming out of the existing switchboard can land in 2000A rated equipment. Then if you can get the conductor length under 25ft, you can use 2000A of conductors under the tap rules; if they have to be over 25ft, you'll need 3000A conductors.

Cheers, Wayne

 

[Tainted]

Do you have a manufacturer and model number for the chiller?

I have no sense of the physical scale of equipment at these current levels, but: if the feeder MCA comes out to under 2000A, your conductors coming out of the existing switchboard can land in 2000A rated equipment. Then if you can get the conductor length under 25ft, you can use 2000A of conductors under the tap rules; if they have to be over 25ft, you'll need 3000A conductors.

Cheers, Wayne

cannot get it below 25 ft because of field conditions. The one I sent you is for 460V but they have the same one for 208V

 

[wwhitney]

The one I sent you is for 460V but they have the same one for 208V

The spec sheet here covers all the models:

WWM (31 Tons) | Aermec

Aermec’s first stackable modular unit is here. Up to 32 connectable units with two 15 HP circuits. The modules can be linked together side by side, back to back, and stacked to reach a capacity of 960 tons. Our modular system can save up to 60% of the floor space over the competition.

www.aermec.us

It says the 208V unit has an MCA of 129A and an MOCP of 175A. Unfortunately it doesn't give the compressor RLA to allow you to more sharply calculate the MCA for multiple units. [The 1050A figure you got for 8 units is from 8*129 = 1032A, apparently rounded up to the next multiple of fifty. So it is not sharp.] So really you need the manufacturer to give you the compressor RLA, or a picture of the actual nameplate.

But I think you can still make an inference that 16 units only need 2000A. Namely, the MOCP calculation is like the MCA calculation except that you can use 225% (worst case) of the largest motor load, instead of the 125% used in the MCA calculation. I'm not clear on whether you can then bump up to the next standard size breaker or not, but worst case for this question you can. That means the MOCP calculation before bumping up was at least 151A, so the largest motor is at least (151-129) = 22A. Which means that taking 16*MCA is oversizing by at least 25% * 15 * 22 = 82A. Which means the MCA for a feeder for 16 units will be at most 16*129 - 82 = 1,982A.

As to your original question about switchboard connection, sorry I have no idea. : - )

Cheers, Wayne

 

[Tainted]

The spec sheet here covers all the models:

WWM (31 Tons) | Aermec

Aermec’s first stackable modular unit is here. Up to 32 connectable units with two 15 HP circuits. The modules can be linked together side by side, back to back, and stacked to reach a capacity of 960 tons. Our modular system can save up to 60% of the floor space over the competition.

www.aermec.us

It says the 208V unit has an MCA of 129A and an MOCP of 175A. Unfortunately it doesn't give the compressor RLA to allow you to more sharply calculate the MCA for multiple units. [The 1050A figure you got for 8 units is from 8*129 = 1032A, apparently rounded up to the next multiple of fifty. So it is not sharp.] So really you need the manufacturer to give you the compressor RLA, or a picture of the actual nameplate.

But I think you can still make an inference that 16 units only need 2000A. Namely, the MOCP calculation is like the MCA calculation except that you can use 225% (worst case) of the largest motor load, instead of the 125% used in the MCA calculation. I'm not clear on whether you can then bump up to the next standard size breaker or not, but worst case for this question you can. That means the MOCP calculation before bumping up was at least 151A, so the largest motor is at least (151-129) = 22A. Which means that taking 16*MCA is oversizing by at least 25% * 15 * 22 = 82A. Which means the MCA for a feeder for 16 units will be at most 16*129 - 82 = 1,982A.

As to your original question about switchboard connection, sorry I have no idea. : - )

Cheers, Wayne

haha this is ok...

Do you know why the equipment is rated 23kW but MCA is way too high per module?

 

[wwhitney]

Do you know why the equipment is rated 23kW but MCA is way too high per module?

I believe that as HVAC numbers rated input power and cooling capacity are for a standard set of conditions (outdoor temp, water flow rate, etc). While as an electrical design number, MCA is going to be a conservative number for worst case operating conditions.

Cheers, Wayne

 

[synchro]

I need to tap the switchboard to feed 3-phase chiller loads with a total of 2100MCA that is more than 25 feet away from the tap. This means that I need 8 sets of 3#500kcmils (full size 3000A copper feeder to comply with the tap rule). I will use this feeder to feed a new 3000A switchboard which will feed the chillers.

Could you have the chillers fed from two lower rated switchboards, and where each of these switchboards is fed by its own section in the main switchboard? That way you could have 4 sets of 3#500 kcmils from each of the two sections, if that would work out.

 

[petersonra]

I think we should not use the word tap since these are just normal feeder conductors.

Could you have the chillers fed from two lower rated switchboards, and where each of these switchboards is fed by its own section in the main switchboard? That way you could have 4 sets of 3#500 kcmils from each of the two sections, if that would work out.

Nice idea but then they would be taps and they are not close enough to meet the tap rules.

Even if there is no room for a new switchboard near the existing one perhaps there is room for some fused disconnects. Then you could potentially tap to them and then feed to the chillers.