Fischer-Price My First 3-Phase 4-Wire Service

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I'm sizing wires for my first 3-phase 4-wire High-Leg Delta. It's only a 100A service. Do I need to derate my service entrance conductors to 80% of their ampacity because in the entrance there are 4 wires in the conduit.

That would mean I'd have to go from #3 to #1. Is this accurate?

The grounded conductor does not have to be larger than any of the phase conductors correct? But should be at least he same size? These are for 3-Phase outlets that the customer plugs his machinery into.
 

LEO2854

Esteemed Member
Location
Ma
I'm sizing wires for my first 3-phase 4-wire High-Leg Delta. It's only a 100A service. Do I need to derate my service entrance conductors to 80% of their ampacity because in the entrance there are 4 wires in the conduit.

That would mean I'd have to go from #3 to #1. Is this accurate?

The grounded conductor does not have to be larger than any of the phase conductors correct? But should be at least he same size? These are for 3-Phase outlets that the customer plugs his machinery into.[/QUOT

You want to look at 310.16 table and you want to look at your turnimal ratings if you use the 75 deg colume you can use #3 copper.
 

Buck Parrish

Senior Member
Location
NC & IN
If you know that it is only a 100 amps. Then thier may be an electrical engineer involved. In most cases you will not have to derate with what you have described.
However thier are some variables which would make you have to derate.

You should tell the owner to get with an EE and describe all the equipment to him or her.
Then draw you a panel schedule to follow. This will be a great help to you.
Let the EE get involved if not all ready .He wants money, too;)
 

Smart $

Esteemed Member
Location
Ohio
I'm sizing wires for my first 3-phase 4-wire High-Leg Delta. It's only a 100A service. Do I need to derate my service entrance conductors to 80% of their ampacity because in the entrance there are 4 wires in the conduit.

That would mean I'd have to go from #3 to #1. Is this accurate?

The grounded conductor does not have to be larger than any of the phase conductors correct? But should be at least he same size? These are for 3-Phase outlets that the customer plugs his machinery into.
No derating necessary... the neutral does not count as a current-carrying conductor.

The grounded conductor needs to be the sized for the larger of the following:
  1. Maximum unbalanced neutral current. Add up line-to-neutral loads as calculated per Article 220 for each line. (This should give you two current values)
  2. The size of GEC required for the service as per 250.66.

If this service is only for 3? loads, #2 above would be the winner ;)
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
The neutral conductor does not needed to be counted as a current carrying conductor for derating purposes for this service.
 
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