ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Recent Posts
Look Ma! I Did It Myself!
by renosteinke - 12/04/23 02:57 AM
Scary Looking Receptacles
by renosteinke - 12/04/23 02:46 AM
Old Computers?
by gfretwell - 11/21/23 03:45 PM
Simplify MOSFET Test With Source Measure Meter
by gfretwell - 11/18/23 09:20 PM
New in the Gallery:
This is a new one
This is a new one
by timmp, September 24
Few pics I found
Few pics I found
by timmp, August 15
Who's Online Now
1 members (Scott35), 32 guests, and 17 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
#78031 08/09/01 06:29 AM
Joined: Aug 2001
Posts: 31
What is the NEC article that shows the back-up fuse req's for Motor Protection applications?

This is what I understand:

The back-up fuses are necessary only if the short circuit current is higher than the breaking capacity of the Motor starter/protector. This is also in addition to the branch protection fuses.

Or is the only reference in IEC 947-2?

Can someone elaborate on this?

Carl Lee Tolbert
Technical Support
AC Drives
Your source for the most practical automation products at almost-free pricing, delivered by 11 a.m., just by clicking a mouse.
PH: (770)844-4200
Stay up to Code with the Latest NEC:

>> 2023 NEC & Related Reference & Exam Prep
2023 NEC & Related Reference & Study Guides

Pass Your Exam the FIRST TIME with the Latest NEC & Exam Prep

>> 2020 NEC & Related Reference & Study Guides

#78032 08/09/01 08:04 AM
Joined: Jul 2001
Posts: 599
JBD Offline
I need you to clarify what you are referring to as "back-up" fuses.

In my experience this is a term used by fuse manufacturers to sell more fuses. The NEC allows the motor overload relay (OLR) to provide running current protection to the motor. The NEC allows the branch OCPD to be sized only for short circuit currents if you are using an OLR.

If you don't trust the OLR (there are lots of reasons not too), you you can size the branch OCPD small enough to also provide all/or some running current protection, thus providing "backup" protection to the OLR.

#78033 08/09/01 02:15 PM
Joined: Nov 2000
Posts: 2,148
I think that IEC rated equipment requires more protection to prevent equipment damage under fault conditions then does NEMA rated equipment.

#78034 08/09/01 02:29 PM
Joined: Aug 2001
Posts: 31
Thanks for the reply.

"Back-up fuses" are right from the IEC 947-2 that define their specs for MSP's (motor starter protectors.)

I guess the question should have been about the integration of IEC starters over NEMA and the affect it has on NEC.

The MSP's are rated to 100,000 amp short ckt breaking capacity under 6.3 FLA at 230 vac (the low end for ref.) If I wanted to use these beyond 6.3 amps I would need a back-up fuse based on their definition.

The trend currently (and will continue because of cost) is to use IEC versus the NEMA style. The MSP in conjunction with a contactor becomes an equivalent NEMA class starter.

Article 430-32 has the basic ol sizing guidelines for cd motors over 1 hp.

Interruptpting ratings; 110-9, 240-60, 240-83, and 240-86 are rather cryptic (lol, like most of the NEC sometimes.)

I just wanted some other viewpoints and opinions.

I do appreciate your post though.


Carl Lee Tolbert
Technical Support
AC Drives
Your source for the most practical automation products at almost-free pricing, delivered by 11 a.m., just by clicking a mouse.
PH: (770)844-4200
#78035 08/13/01 09:24 AM
Joined: Jul 2001
Posts: 599
JBD Offline
ctolbert - I didn't read enough of your original post to see you were referring to IEC terminology.

First IEC and NEMA can be used interchangeably if you use the same components. Correctly sized IEC contactors used with overload relays are applied the same as correctly sized NEMA devices.

In your case you are using a Motor Protective Switch (there are different names) which combines the branch short circuit protection and the motor overload protection into a single device. This device has different application considerations and it is not the same as a standard NEMA starter.

The need for back-up fusing is dependent on the manufacturer of your specific equipment not on the general idea. Square D's is rated 10KA for 5HP at 240V which is higher than their equivalent NEMA device. In some cases fuse sizes may be required to be smaller than if the back-up protection is a circuit breaker.

resqcapt19 - IEC defines two types of fault protection/coordination. Type 1 is what we consider normal for NEMA starters, replacement of some components (i.e.contacts) may be required. Type 2 coordination assures no damage to the contacts of the starter.

#78036 08/24/01 07:14 AM
Joined: Aug 2001
Posts: 31
OK, I found the information I was looking for. I wanted to thank everyone for their posts and share the information.

My concern was the use of MSPs' (Motor Starter Protectors) in motor branch circuits in the US and how they fit in with the NEC req's. The following is an outline of what I found. Hope you guys like it.

1. Short circuit/ground fault – excessive current condition that exceeds the circuit boundaries (can be 100’s of times greater than FLA.)
2. Overload – the over current condition that exceeds the normal full load current (FLA) of the motor (can be 6-10 times the FLA.)
3. Fault current- maximum amount of current available for a short circuit condition, restricted only by conductor size.
4. Breaking capacity/interrupting rating – maximum amount of current that a protection device can safely open or clear.
5. Withstand rating – the maximum amount of current that an enclosure can safely contain
6. Trip class – time in seconds measured for meltdown of heater element with 6 times the fla applied.

7. NEMA or IEC?
7.1. NEMA
7.1.1. NEMA has higher withstand ratings typically
7.1.2. NEMA has a standard sizing scheme that provides for interchangeability from the different manufacturers.
7.1.3. Designed with reserve capability.
7.1.4. Molded coils
7.2. IEC
7.2.1. IEC has no standard sizing guidelines and rating selection is evaluated into utilization categories.
7.2.2. Tape wound open coils
7.3. Same trip classes 10, 20, 30

8. 5 basic parts of a motor circuit
8.1. Disconnection
8.1.1. NEC part j
8.2. Short circuit protection
8.2.1. NEC part d
8.3. Controller
8.3.1. NEC part g
8.4. Overload protection
8.4.1. NEC part c
8.5. Motor
8.5.1. NEC part a

9. MSP’s in NA
9.1. Does not meet UL 489
9.1.1. Cannot be used for stand alone short circuit breaking
9.2. Can be used as “group motor rating” (UL/CSA)
9.2.1. Indicates that a single short circuit protective device can protect several branch circuits under the UL/CSA definition while using the standard sizing guidelines.
9.2.2. NEC 430-53 (sizing guidelines)
9.2.3. If the short circuit current exceeds the withstand rating of the MSP then fuses must be used in series to reduce this number (short circuit current) or the system must be redesigned to accommodate (NEC 110-10.)

10. MSP’s international
10.1. IEC 947-4-1
10.1.1. Recognized as a stand-alone short circuit protecting device.
10.1.2. Back-up fuses are only required when the fault current exceeds breaking capacity of the MSP.

11. Coordination levels
11.1. Type 1
11.1.1. No danger to personnel
11.1.2. No danger to equipment other than relay itself
11.1.3. Relay must be serviced or replaced
11.2. Type 2
11.2.1. No danger to personnel
11.2.2. No danger to other equipment
11.2.3. No danger to relay other than minor welding of contacts
11.2.4. Relay can be used without service or repair

The biggest issue is how the MSP is used overseas as a stand-alone short circuit breaking device....Just not in the US.

Thanks All,

Carl Lee Tolbert
Technical Support
AC Drives
Your source for the most practical automation products at almost-free pricing, delivered by 11 a.m., just by clicking a mouse.
PH: (770)844-4200

Link Copied to Clipboard

Tools for Electricians
Tools for Electricians

* * * * * * *
2023 National Electrical Code (NEC)
2023 NEC + Exam Prep Study Guides Now Available!
* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman


Member Spotlight
Posts: 44
Joined: July 2013
Top Posters(30 Days)
Popular Topics(Views)
313,287 Are you busy
239,195 Re: Forum
222,946 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5