Wiring for elevator -- Question

[positive]

We have some confustion about an elevator we plan on wiring. We have a single elevator with a nameplate amperage of 121 amps, 3 phase, 208 volt. Our foreman feels that this needs to be wired at 125%. I feel that since we are dealing with an intermittent usage motor, it only needs to be calculated at 85%.

So my calcuations are:

At 121 amps, at 85% = approx. 103 apms. I would size the wire at #3 copper THHN with a 110 amp breaker at main panel with a 110 amp shunt trip breaker in the elevator room.

Please advise me if this is correct. If not, please point me to the correct answer. You can e-mail me at this address:

Poselec@hotmail.com

Thank you,
Mike Gingerelli
Positive Electric Co.

 

[iwire]

Re: Wiring for elevator -- Question

This is two separate issues for motor loads, conductor size and Short circuit protection. You must remember that "Overload protection" for the motor and the conductors is from the motor overloads and not the breaker, much different from "normal" feeders.

For the conductor rating go to 2002 NEC 620.13(A) sends you to 430.22(A) and (E)

430.22(E) sends us to Table 430.22(E) which lists values of 85%, 90%, or 140% for passenger elevators depending on the rating of the motor, you will need to find this out from the supplier of the motor.

On the #3 copper with 103 amp load is that not a little small? 110 amps on #3 is from the 90C column and your breaker will not have terminals for that, #2 would be a better choice.

For the short circuit protection go to 620.61(D) which sends us back to 430 part IV

Table 430.52 lists the short circuit percentages of full load current which range from 150% up to 1100% depending on the motor type and the type of breaker or fuse.

The selection of the short circuit protection is based on this table not on 310.16

So your breaker might be 150 amps with #2 copper on it and still be code compliant.

In my opinion you will have trouble with starting a 121 amp motor with a 110 amp breaker.

[ March 13, 2003, 10:52 AM: Message edited by: iwire ]

 

[charlie b]

Re: Wiring for elevator -- Question

Another consideration is voltage drop due to starting current. It is a real problem if you have across-the-line starters. The motor will draw many times the nameplate current during its starting cycle. Even though this lasts a fraction of a second, it is enough to cause lights to flicker throughout the building. Over-sizing the conductors might alleviate this situation to some degree.

My company (we're design engineers) is presently assisting one party in a legal dispute between the building owners, the tenants, the electrical contractor, the elevator vendor, and the utility, because of just such a problem.

 

[jcormack]

Re: Wiring for elevator -- Question

First a comment - if this is an elevator moving people why would you even consider cutting corners. Over design for safety's sake.

Breaker must be at least 150 amp. You did not mention type of control - M-G set, VFD, two speed, single speed? Regen?? Most elevators will have 140-150% on start & regen on decel - don't wire for less than nameplate - I would suggest at least 125%. Controller terminals may only be rated for 60 deg. There are additional requirements in ANSI.

 

[iwire]

Re: Wiring for elevator -- Question

jcormack, I agree with you but I was giving him the code as I understand it.

The real question to me is why does the code allow us to cut corners on elevators? At least require us to size the conductors at 125% of name plate that would help start up currents, it makes no sense to me that just because it is not continuous duty it gets smaller wire. I would think the opposite, more starts need bigger wire. As Charlie pointed out if you follow the code you could or will still have serious voltage drop.

 

[positive]

Re: Wiring for elevator -- Question

This question was more theoretical than practical. We already installed 1/0 wire in 2 inch EMT due to distance and voltage drop with a 200 amp non-fuse disconnect in elevator room. Also a 150 amp shunt trip breaker in MDP.

I still feel the code would have allowed me to do it the original way. The question was about what the CODE allows... if anyone could answer this, could you please let me know.

Thank you,
Mike

 

[jcormack]

Re: Wiring for elevator -- Question

I don't install elevators on a regular basis, but have been involved in a few installations over the years - including some large hp "hoisting" applications that involved both personnel & equipment - I always tend to "over design" for safety and reliablity. Its not fun being on a hoist when a breaker trips & you have to worry about the brake holding or a conterweight casing "accending car". A typical DC contoller on a hoist will have a current (torque)limit of 200% - I never size wire according to load cycle.

 

[iwire]

Re: Wiring for elevator -- Question

The 110 amp breaker would not have worked as you where sizing it from 310.16 based on the reduced wire size, there is no code to allow a breaker smaller then the load served, the load is 121 amps not 103 amps, the 103 figure is just for conductor sizing.

I can not tell you if your 85% figure was correct unless you tell us if the motor is rated 5, 15, 30, 60 Minutes or continuous. Table 430.22(E)

[ March 13, 2003, 11:54 AM: Message edited by: iwire ]

 

[iwire]

Re: Wiring for elevator -- Question

The absolute minimum that it could be, assuming some info not given

121 amp 208 motor I will assume a 15 minute motor so table 430.23(E) will allow 85%

121 x .85 = 102.85 which will need # 2 copper as per 310.16 75C column, as you will not have 90C terminals, if you have 60C terminals you will need #1 copper

For the breaker I will assume a wound rotor motor with a inverse time breaker Table 430.52 will allow 250% of full load current
121 amp + 250% = 423 amps so the breaker could be up to 450 amps (roll up a size under 800 amps) but certainly not less then a 125 amp breaker to serve the 121 amp load, but as pointed out at least a 150 will be needed to start this motor The breaker size has no impact on conductor size for motor loads.

[ March 13, 2003, 01:02 PM: Message edited by: iwire ]

 

[charlie k.]

Re: Wiring for elevator -- Question

You also mention a 200 amp non fused disconnect at the equipment. Last elevators I did had to have fused disconnects lockable in the off position only. This was in Maryland, but you might want to check.