Protection of 600V 225A 1

[tulum]

Hey Folks...

I have been gone awhile... back now...

Here is a questions...

I have a proposed starter for a piece of mobile equipemnt (200HP 600V + 10kVA tz).

The starter consists of:
- 400A disconnect
- 350A J type fuses
- 350A contactor
- AB E3+ o/L
- GF Protection
- 225A outgoing cable connector
- 3/O cable from o/l to connector

I did not design the starter... My question is usually I size the fuses less than the cable connector size. In this case the fuses seem only to provide SC protection, are the O/L's providing the protection for the connector and the cable in the starter?

Regards,
TULUM

 

[davidbeach]

Yep, overload protection is provided by the overloads and short circuit protection is provided by the fuses. Just like any motor circuit.

 

[jraef]

Yes.
It's the same concept behind a Magnetic-Only (MCP) circuit breaker. In a motor starter circuit, the OL relay is providing closer running overload protection than a fuse or circuit breaker thermal trip ever could. So the fuse or CB just becomes the SCPD - Short Circuit protective Device, and the sizing rules can vary from what you would do in a non-motor starter branch circuit.

 

[tulum]

Thanks guys,

It turns out the overloads are not wired into the trip circuit. They are just there for monitoring purposes (smart overloads). The machine downstream will have it's own overloads.

So will the fuses adequately protect the cable and the plug?

Regards,
TULUM

 

[waross]

A short circuit may occur anywhere and so the fuses will only protect downstream equipment.
It is assumed that an overload will draw the same current from the point of supply to the motor. This current may be monitored anywhere between the point of supply and the final load.
respectfully

 

[jraef]

Yes, but the cables in this case, because they are going out of this cabinet and over to (presumably) another one with the motor starter, I don't think would be covered under that exception for motor starter circuits. I think this box is now essentially a feeder, not a motor starter per se. So now you have 3/0 cable being protected only by the 350A fuses. If you have a fault in between the two boxes, those cables would fry. I don't have my NEC tables with me here, but a few on-line sources show it at 301A under the best circumstances.

 

[waross]

Hi J
The point that I was trying to make is that a short circuit will draw enough current to trip instantaneous trips in a breaker or blow the fuses. Any current in the range between the load current of the motor and the fuse rating will most likely be the result of an overload and be disconnected by the local protection in the local starter. I understand that the conductors must be 125% of FLA as listed in the NEC but the fuses may be increased to allow motor starting. I am not aware of any limitation on the length of the conductors between the fuses and the motor starter. When I get home, I'll check my code.
respectfully

 

[waross]

Hi Tulum
Canadian code;
200 HP motor, 600 volts (575 rated) 192 amps. Table 44
10 KVA TX @ 600 volts 17 amps. (single phase, worst case)
Feeder size. (192A x 125%)+ 17 A = 257 amps.
CEC 28-110 Feeder conductors, and 28-106 Conductors - individual motors.
Ampacity of 3/0 power cable, 274 A Table 12A

Fuse rating (over current protection)
Time delay fuses.
Motor load 192 A x 175% = 336 A
Transformer load 17 A
Fuse maximum 353 A Table 29, rule 28-204(3)
This is the maximum for time delay type "J"s. fast acting type "J"s may be larger.

Your cable connectors may be a problem. Under rule 28-602, if the plug is used as a disconnecting means, The minimum, ampacity is the same as the conductors, 257 A.
However, your 400 amp switch would be the disconnecting device required by code. Is a 225 amp plug adequate, or should the plug be rated in excess of 257 amps? It could be argued either way.
respectfully

 

[tulum]

Thanks Waross... very valuable input.. thanks for your time and guidance.

One question though...eventhough the portable power cable is rated to table 12A (at 274A), would the cable inside the starter be rated to table 2 instead(or 200A for a 3/0)? and if so... is this still legal?

BEST REGARDS,
TULUM

 

[jraef]

Conductors INSIDE of a motor starter fall under different rules, i.e. UL or CSA (in the case of N. America). A listed controller will have had to pass testing to confirm suitability of conductor sizing. The code tables are related to field installations or where you want to make up your own controller from scratch and the components in the system have not necessarily been tested together.

The question still is, is the box in question a starter or a feeder? If the OL in this box is NOT connected to the contactor to open it in case of an over load, then IMHO this box is a feeder, not a starter, so the starter rules would not apply. But what do I know, I'm just some anonymous engineer at a keyboard... the real test will be how the AHJ sees it. If he accepts this "starter" box as a starter, then it's "legal".

 

[waross]

Hi J;
In agree with your comments re internal wiring.
I believe that tulum is in Canada. Under the Canadian code, the 350 amp fuses will be acceptable for this feeder.
The Canadian code allows such a feeder to be protected on the basis of the maximum fuse size allowed for the motor(s) plus the ampacity of other loads. Under these rules the maximum fuse size for this feeder is 353 Amps.
Overload protection may be provided at the equipment end of the feeder.
I respect your knowledge and usually find no fault with it, but in this case, J, I have the 2006 edition of the Canadian electrical code sitting beside me. The CEC and the NEC are quite well harmonized, but not perfectly so. This appears to be an area of variation between the codes.
I hope there are no hard feelings between you and I because of a disagreement between the NEC and the CEC.
If tulum is in the USA, then please accept my abject
appologies.
The rating of the 225 amp cable connectors is worrysom and frankly, I don't know. That may be up to the AHJ.
Respectfully

 

[LionelHutz]

I'm believing the transformer is seperate from the starter by it's own disconnect and protection?

In a motor starter the overload protects the cable and the fuses provide short circuit protection. BUT, the overloads must be wired to shut down the starter. This is not a motor starter because the overload can't shut down the motor. If the other overload is wired to trip this starter then it could be called a motor starter again.

Aso, I would have expected the plug to be rated for at least 240A (or 240+ TX current if TX is on same circuit). I took a quick look and didn't see anything specific to this but I'd think the plug must be rated to match the conductor requirements.

The internal conductors can usually follow CEC table 1 for sizing. Also, the manufacturer often has the required cable sizing specified in their UL or CSA file and if so then they can follow this regardless of the CEC tables.

 

[tulum]

Thanks everyone,


Waross, I am in Canada...thanks

Lionel...

Are you sure they can follow CEC Table 1 and not CEC table 2?

Jraef...I will run it by the authority as well... I just always like getting the opinion of engineering personnel such as yourself and Waross first... makes me look somewhat educated you know!

 

[waross]

Hi tulum;
Don't forget to check whether your cable falls under rule 4-040 and table 12A of the CEC.
respectfully

 

[LionelHutz]

Yes, Table 1 can be used for wires inside enclosures. We use wire current ratings very similar to Table 1 but dictated by UL.

If there is a CSA or cUL mark on the panel then the inspector should not be questioning what's inside. Depends on the inspector though.

 

[tulum]

Thanks,

Lionel...I would like to see this table... What standard is it in? There seems to be a big difference between outside cable size and the cable used inside the starter...

Regards,
TULUM