How do I size a dedicated service for a 150HP motor?

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ajacobs710

Member
Location
Dallas, Tx
I have never built a dedicated service for a motor. I have now been asked to build two, one being 150HP and the other much smaller. How should I size the service? Use the branch OCPD sizing table in 430.52? Then would the service feeders have to be sized to the OCPD or the FLC x 125%? This is confusing to me combining the rules of 230 and 430 in one location.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
With more than one motor, refer to Art 430 part V (430.62) to size your Feeder SC-GF protection and Part II (430.24) to size your conductors.
With no othher loads, your service would be the same as this feeder, although they arte usually sized for added load.
{If applicable, be sure to address 250.24(C)}
 

templdl

Senior Member
Location
Wisconsin
Are we to assume that the voltage is 480v?
You don't size the feeder to the OCPD. You compute that load first, then size the feeder to carry the load. The feeder must carry the load. As such the feeder ampacity rating will be greater than the load.
Then and only then you will select the OCPD which will be sized to protect the feeder.
240.4 Protection of Conductors. Conductors, other than
flexible cords, flexible cables, and fixture wires, shall be
protected against overcurrent in accordance with their am-
pacities specified in 310.15, unless otherwise permitted or
required in 240.4(A) through (G).
(B) Devices Rated 800 Amperes or Less. The next higher
standard overcurrent device rating (above the ampacity of
the conductors being protected) shall be permitted to
used, provided all of the following conditions are met:
(3) The next higher standard rating selected does not ex-
ceed 800 amperes.

At this point you should have picked up on the fact that the OCPD is based on the feeder's rated ampacity and not the load itelf, that the OCPD's rating will most likely be greater than the feeder's rated ampacity.

(C) Devices Rated Over 800 Amperes. Where the over
current device is rated over 800 amperes, the ampacity
the conductors it protects shall be equal to or greater th
the rating of the overcurrent device defined in 240.6.
 

ajacobs710

Member
Location
Dallas, Tx
There are two separate services at different locations, each serving one motor. They are compressor motors at gas well sites so they are remote and there will be no other load. That is a great point about 250.24, I will be sure to bond the grounded conductor to the enclosure.

So are you saying the correct application would be to size the service to the feeder requirements for the motor?

150HP 3phase motor @ 480V= 180A x 125% cont. duty= 225A
(T430.248 x 430.24)

Are you saying you would size the service at 225A and size your service entrance conductors from that? (250Kcmil THHN= 290A x 82%= 237A, 110*F ambient temp)

What about motor starting? Use TD fuses and size them at 175% (225 x 175%= 393A) bringing my disconnect to 400A, and keep the feeder at the FLC ampacity? This makes sense for a motor circuit, but it being the only load on a service is throwing me.
 

ajacobs710

Member
Location
Dallas, Tx
Are we to assume that the voltage is 480v?<br>
You don't size the feeder to the OCPD. You compute that load first, then size the feeder to carry the load. The feeder must carry the load. As such the feeder ampacity rating will be greater than the load.<br>
Then and only then you will select the OCPD which will be sized to protect the feeder. <br>
240.4 Protection of Conductors. Conductors, other than<br>
flexible cords, flexible cables, and fixture wires, shall be<br>
protected against overcurrent in accordance with their am-<br>
pacities specified in 310.15, unless otherwise permitted or<br>
required in 240.4(A) through (G).<br>
(B) Devices Rated 800 Amperes or Less. The next higher<br>
standard overcurrent device rating (above the ampacity of<br>
the conductors being protected) shall be permitted to <br>
used, provided all of the following conditions are met:<br>
(3) The next higher standard rating selected does not ex-<br>
ceed 800 amperes.<br>
<br>
At this point you should have picked up on the fact that the OCPD is based on the feeder's rated ampacity and not the load itelf, that the OCPD's rating will most likely be greater than the feeder's rated ampacity. <br>
<br>
(C) Devices Rated Over 800 Amperes. Where the over<br>
current device is rated over 800 amperes, the ampacity<br>
the conductors it protects shall be equal to or greater th<br>
the rating of the overcurrent device defined in 240.6.

I am worried about the motor start up, sizing the OCPD per T430.52 would be normal for a motor, but sizing the OCPD of a service would be based off of load as you stated.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Allow me to post a hypothetical and see if it flies.
Lets say 15 HP + a 20 HP @ 480 v

The 150 HP would be 180 amps, the 20, 27 amps
The feeder would be sized 180 * 1.25 + 27 = 252 amps, for a 250 kcmil conductor

The 150 HP SC-GF protection could be a 350 amp fuse (next larger for 315 amp per 430.52)
The feeder SC-GF would also be a 350 amp fuse (350 + 27 ... next larger not permitted)

The service conductors could also be 250 kcmil based on the calculated load with a 350 amp main fuse.
If the system supply has a grounded conductor, a #2 grounded conductor would need to be brought to the service.
 

ajacobs710

Member
Location
Dallas, Tx
I am just confusing myself!

I am just confusing myself!

The SEC would need to be sized for the disconnect, the disconnect and motor feeder would be sized like normal for a motor, correct?
 

ajacobs710

Member
Location
Dallas, Tx
Allow me to post a hypothetical and see if it flies.
Lets say 15 HP + a 20 HP @ 480 v

The 150 HP would be 180 amps, the 20, 27 amps
The feeder would be sized 180 * 1.25 + 27 = 252 amps, for a 250 kcmil conductor

The 150 HP SC-GF protection could be a 350 amp fuse (next larger for 315 amp per 430.52)
The feeder SC-GF would also be a 350 amp fuse (350 + 27 ... next larger not permitted)

The service conductors could also be 250 kcmil based on the calculated load with a 350 amp main fuse.
If the system supply has a grounded conductor, a #2 grounded conductor would need to be brought to the service.

Thank you for sorting through my confusion! It is clear now, I don't know why I was making it harder than it needed to be. I certainly appreciate the help!
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
In your example, what we'd do usually is install a 400A CT can with a service rated pump panel, we use Allen Bradley. Size the wiring to the motor tables x 125% from the CT can all the way through the pump panel to the motor. Grounded conductor sized to the minimum, hit's the CT can and of course your pump panel as it's a service disconnect. Two ground rods with #4 bare to the pump panel generally, and we're done.
 

kwired

Electron manager
Location
NE Nebraska
Motors are one of the most common exceptions for higher setting of overcurrent devices being allowed on a conductor. This includes branch circuits, feeders and service conductors.
 

iwire

Moderator
Staff member
Location
Massachusetts
230.90 Where Required. Each ungrounded service conductor
shall have overload protection.

(A) Ungrounded Conductor. Such protection shall be provided
by an overcurrent device in series with each ungrounded
service conductor that has a rating or setting not higher than
the allowable ampacity of the conductor. A set of fuses shall
be considered all the fuses required to protect all the ungrounded
conductors of a circuit. Single-pole circuit breakers,
grouped in accordance with 230.71(B), shall be considered as
one protective device.


Exception No. 1: For motor-starting currents, ratings that
comply with 430.52, 430.62, and 430.63 shall be permitted.
 
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